Chile Pepper Diseases
Circular 549
Natalie P. Goldberg, Extension Plant Pathologist
College of Agriculture, Consumer and Environmental Sciences, New Mexico State University
Table of Contents
New Mexico is rapidly growing in its reputation for producing some of the finest chilies in the world. The hot, dry summer climate is ideal for the production of many chile pepper varieties and is especially suitable for the production of "hot" chiles. The producing of peppers, however is not without problems. Chile peppers (Capsicum spp.) are susceptible to several diseases which can cause excessive losses, both in quality and quantity.
Plant disease occurs when some external factor disrupts the "normal" growth and development of the plant. Many different parasitic and non-parasitic disease agents can cause disease in peppers: parasitic diseases are caused by infectious disease agents that, can spread rapidly from one plant to another under the right environmental conditions causing an epidemic; non-parasitic diseases are caused by abiotic disorders and are related to nutrients, chemicals, and the environment. This publication will address the most common problems associated with chiles grown in New Mexico. In addition to some specific control measures for particular diseases, there is a check list of general control practices for growing disease-free chile.
Parasitic Diseases (caused by infectious disease agents)
Wilt and Root Rot Diseases Three common soil fungi cause severe wilt and root rot diseases in chile. These fungi can cause similar symptoms, most notably severe wilting and plant death, and at times may be confused with one another. Often accurate diagnosis of the disease depends on isolating and identifying the causal agent; however, there are some differences in symptoms, expression and knowing of these differences can be helpful in diagnosis and control.
Phytophthora Root Rot. Phytophthora root rot, also called "chile wilt," is caused by the soil-borne fungus, Phytophthora capsici. This fungus is a serious pathogen on peppers worldwide, but the disease is particularly widespread in furrow-irrigated fields in the southwestern U.S.
Conditions for disease: This fungus causes a problem when soils are excessively wet, either from over - irrigation, heavy rains, or both. Disease outbreaks usually occur in heavy soil or in low spots in the field where water tends to sit for long periods of time. It is not uncommon to see fields where diseased plants are grouped together in particular areas of the field, in specific rows, or at one end of the field, while the remainder of the plants may be healthy and show no indication of disease. When disease occurs in particular rows, this often indicates excessive irrigation and the spread of infective spores by the irrigation water. In addition, chile plants grown next to tall trees or buildings may become diseased due to shading which causes high humidity and slow drying, favoring activity by the fungus.
Symptoms:Symptoms of Phytophthora root rot usually occur in the late summer and early fall during periods of heavy rainfall and warm nights, and when the foliage is dense and plants become crowded. The first symptom of infected plants is severe wilting. Within a few days, infected plants collapse due to root and stem infections and die, turning straw-colored (fig. 1). In many cases, plants are defoliated. Diseased plants removed from the field exhibit symptoms of severe root rot, such as discolored, dead roots from which the bark sheds easily (fig. 2).
Phytophthora capsicialso causes diseases, called spots, blights, and fruit rots on the leaves, stems, and fruit. Above - ground infections generally occur during the summer rainy season at elevations above 3500 feet. Fungal infections result when rain splashes infested soil onto water-soaked stems and leaves. Fruit become infected when wet humid conditions persist for several days. (For more information on fruit rots, see following section on fruit rots.)
Control:Excessive soil moisture triggers and intensifies the crown and root rot stage of this disease. If possible, avoid poorly drained, heavy soils. Cultural practices that reduce the length of time that soil remains saturated will help reduce disease incidence. These practices include proper field leveling (laser leveling with a slight slope will remove low spots and help water move down the rows), planting in raised beds, irrigating every other row when plants are immature, shorter irrigation periods, and shorter row length.
Phytophthora capsicisurvives in crop debris and soil as oospores, which can persist for over two years in the absence of the host. Research indicates that 3-4 year crop rotations out of peppers and other susceptible hosts such as tomatoes and alfalfa can reduce residual populations of the fungus in the field. Suggested rotational crops include lettuce, cabbage, onions, and small grains, such as wheat, barley, and oats.
Chemicals have not proved very effective in controlling this disease. Metalaxyl is registered for use on peppers to control root and crown rots; however, once a plant is infected, metalaxyl will not cure a plant of the disease. There are no chemicals recommended for the above-ground spots and blights.
While chile breeders continue to strive for cultivars tolerant to Phytophthora capsici, there are, as yet, no cultivars highly tolerant to this disease.
Verticillium Wilt. Verticillium dahliae is a soil-borne fungus that occurs worldwide and causes diseases on a diverse group of plants. Susceptible hosts include several weed species and many crops grown in New Mexico, including chile, cotton, alfalfa, melons, and ornamentals. Like Phytophthora root rot, Verticillium wilt is primarily a problem in temperate climates.
Conditions for disease: Verticillium dahliae survives in soil and crop debris as specialized structures called "microsclerotia." These structures enable the fungus to tolerate extreme environmental conditions and lie dormant in the soil for many years in the absence of a susceptible host. In the presence of moisture, root exudates of susceptible plants stimulate microsclerotia germination. The fungus directly penetrates the roots and subsequently moves through the root cortex to the water-conducting xylem vessels. The xylem becomes plugged with the fungus, leading to above-ground disease symptoms.
Symptoms: Symptoms of Verticillium wilt are highly variable, depending on the susceptibility of the host, the aggressiveness of the pathogen, and the environmental conditions, especially related to air and soil temperature and available nutrients. Early symptoms include yellowing of lower leaves and plant stunting (fig. 3). As the disease progresses, excessive yellowing and leaves shedding may occur. This fungus is restricted to the internal vascular tissues of the stems and therefore causes no rot of the roots or the crown. However, varying degrees of browning may occur in the xylem tissue (fig. 4). As the water-conducting tissues are plugged by the fungus, the plant will wilt due to water stress. Infected plants may recover at night for a few days before permanent wilting and death occurs. This disease may occur sporadically throughout the field, thus creating a poor stand.
Control: There are no adequate control measures known once Verticillium wilt appears in the field. Control strategies targeted at avoiding the disease are most effective. Although Verticillium dahliae has a wide host range, it occurs in races (genetically and often geographically distinct groups of pathogens that may infect certain plant species), so all isolates of the fungus may not attack all hosts. Observations of the disease indicate that Verticillium wilt tends to be most severe in fields where chile is planted continually. Although research is lacking on race specificity of V. dahliaein New Mexico, observations of the disease in Arizona indicate the population is capable of infecting cotton as well as chile, thus avoiding cotton fields with a history of the disease may help reduce disease incidence. Crop rotations where chile is not planted more than once every 3 - 4 years is recommended. Using barley or other small grains may help reduce the population of microsclerotia in the soil and thus reduce the incidence of disease when a susceptible host is planted.
Soil fumigants containing chloropicrin have effectively controlled Verticillium wilt in many crops, but they may not be economically feasible in chile. To date, there are no known chile cultivars tolerant to Verticillium dahliae.
Rhizoctonia Root Rot. Rhizoctonia solaniis a common soil fungus that infects a large number of vegetables and agronomic crops. This pathogen causes root rot of mature plants as well as seedling disease (described in detail in the following section).
Conditions for disease: Rhizoctonia solani infection is thought to occur in early spring during the seedling stage of growth. If environmental conditions are less than optimum for the fungus, the plant may continue to grow while infected. Plants infected with Rhizoctonia root rot have reduced vigor compared to non-infected plants. In the summer when the plant is mature, stresses such as excess heat, drought, and fruit set cause infected plants to wilt. This disease is most severe in fields that are planted in chile year after year, although it should be noted that other crops such as cotton and alfalfa are also susceptible to this fungus. Additionally, this fungus has a tremendous capacity for saprophytic growth and can survive in the soil indefinitely in the absence of a host plant.
Symptoms: Rhizoctonia solani attacks plants on the lower stem near the soil. As the fungus moves up and down the stem the tap root rots, and the developing lesions turn reddish-brown. These reddish-brown lesions are a diagnostic characteristic for this disease. Diseased plants often produce an abundance of secondary roots above the rotted tap root, but wilting and death of plants scattered throughout the field are the most noticeable symptoms of Rhizoctonia root rot. Wilted plants may recover at night unless high temperatures prevail. In addition, providing adequate soil moisture may prolong the life of infected plants. Once a plant is infected, however, the vigor is greatly reduced and production is poor.
Control: Specific control measures (other than seed treatments explained in the following section) are not usually recommended for Rhizoctonia root rot. There are no resistant chile varieties.
Seedling Disease
Seedling disease, commonly called "damping off," can be caused by a number of soil-borne fungi such as Rhizoctonia solani, Phytophthora capsici, Pythium spp., and Fusarium spp. Damping off occurs when seeds or young seedlings are attacked by these pathogens. Seeds attacked by these fungi usually fail to germinate. Seedlings can be damaged in two ways: the roots may rot causing the seedling to wilt and die quickly, or the seedling may be attacked on the stem at the ground line, causing the seedling to collapse (fig. 5). Although both seeds and seedlings can be attacked by these fungi, direct-seeded chile tends to be more susceptible than chile transplants. When fields suffer from severe damping off, regardless of the pathogen or the growth stage of the plant attacked, the final result is poor stand development.
Seedling diseases usually develop during cold, wet periods in the spring after planting. In addition to damping off, these conditions delay seedling growth, which keeps the plants susceptible to attack for a longer period of time. Excessive irrigation prior to emergence increases disease severity. Damping off can also be a problem in raising transplants in the greenhouse, where high humidity and frequent overhead watering can favor seedling diseases. Raising transplants in sterile soil or potting mix is recommended.
Additional causes of seedling loss include poor seed quality, improper planting depth, high salt concentrations in the seed bed; strong winds, over - irrigation, severe nutrient deficiencies or toxicities, pre- and post-plant herbicide applications, and insects. In order to prove that fungal pathogens are involved in seedling disease, it is necessary to eliminate these other non-parasitic problems. Observing the distribution pattern of seedling loss in the field is extremely helpful in accurately diagnosing the problem. For example, if the disease is occurs in rows or in a specific area of the field, the cause is most likely a non-parasitic disorder ( discussed later in this publication). Alternatively, if the disease is scattered throughout the field, pathogenic fungi are the likely cause. The earliest symptom of damping off is the failure of seedlings to emerge (or seedling death) for a short distance in the seed bed. These infected locations then enlarge, creating a scattered disease pattern in the field. Good observations of symptoms and disease patterns in the field are essential for diagnosis; however, the only positive method of determining the cause of seedling disease is isolating the fungus from diseased tissue.
In order to prevent seedling diseases, plant only high - quality seed or transplants, and avoid fields and seed beds that are poorly drained. Seed treatment with fungicides such as thiram or captan helps protect seeds from pre-emergence damping off.
Leaf Diseases
Parasitic leaf diseases can be caused by fungi, bacteria, or viruses (in a separate section). Three leaf infecting pathogens have been found on chile in New Mexico.
Bacterial Leaf Spot. This bacterial disease, caused by Xanthomonas campestrispv vesicatoria, occurs worldwide and has been found periodically in most of the chile-growing areas of the state. Disease epidemics can occur during particularly favorable environmental conditions (overhead irrigation or heavy rainfall) and can result in significant crop losses.
Conditions for disease:This bacterium survives in seed, on infected crop debris in the soil, and in weeds. Outbreaks of bacterial spot usually occur in July and August during periods of warm temperatures and humid, wet weather. While all above-ground plants parts can be infected by this bacterium, fruit infections tend to be the most damaging from an economic stand point. The bacterium enters the leaves and stems through stomata or wounds, and enters fruit through wounds as well. It is spread from one plant to another by splashing water (either from overhead irrigation or rain), wind, or plant-to-plant contact. Disease severity depends on the level of resistance within the cultivar and the conduciveness of the environment (moisture and temperature).
Symptoms: Bacterial leaf spot appears as circular to irregular, water-soaked lesions on leaves and stems (fig. 6). As the spots age, they become purplish-gray with a black center and are surrounded by a narrow yellow halo. Infections tend to appear more frequently in the lower canopy, where infected leaves become ragged and eventually turn brown and fall from the plant. Severe infections can actually result in defoliation of the plant. Flower infection results in severe blossom drop. The disease appears on the fruit as small, roundish, raised, dark, scabby lesions (fig 7).
Control: Initial control of bacterial leaf spot relies on planting resistant varieties and only disease-free seed and transplants. Purchase only high-quality seed which has been screened for the presence of the pathogen. Seed treatment in a 20% bleach solution for 40 minutes is recommended in areas with a history of bacterial spot. Use 1 gallon of water per lb of seed and be sure to agitate the seed continually while soaking. After washing, air dry promptly.
Check seed sources for cultivars resistant to Xanthomonas campestrispv vesicatoria. Crop rotation is also important in reducing the occurrence of epidemics.
Copper-based chemicals such as copper hydroxide, copper sulfate, and copper ammonium carbonate can be effective in controlling disease. These materials are best applied with a spreader sticker as foliar sprays prior to the rainy season. These sprays can also be applied when the disease threatens to spread in the field; however, effectiveness of these sprays depends on dry weather. It is also important to understand that some strains of this bacterium are resistant to copper-based chemicals, which may not be effective in controlling disease caused by these strains of the bacterium. For specific chemical information, see the pesticide reference in the appendix.
Bacterial leaf spot is capable of infecting many solanaceous weeds, including nightshade and ground cherry; thus, controlling weeds near chile fields is important to control the disease.
Cercospora Leaf Spot. The fungus responsible for this disease, Cercospora capsici, is active during the same environmental conditions that favor bacterial leaf spot. In fact, these two diseases, along with Alternaria sp., are often found together on infected leaves. Alternaria sp. is generally considered to be a secondary pathogen that is taking advantage of tissue weakened by these other pathogens. As such, Alternaria sp. is rarely thought of as a leaf pathogen, but it does cause a fruit rot and is discussed in the fruit rot section of this publication.
Conditions for disease:Cercospora leaf spot survives in or on seed, and as tiny black stromata in old affected leaves in the soil. Infection occurs by direct penetration of the leaf. Cercospora sp. spores require water for germination and penetration of the host; however, heavy dew appears to be sufficient for infection. The disease is most severe during periods of warm temperatures and excessive moisture (either from rain or overhead irrigation). The fungus is spread by splashing water, wind, and leaf-to-leaf contact. Unlike bacterial leaf spot, this pathogen does not infect fruit.
Symptoms: Lesions created by Cercospora capsiciare roughly circular in shape and may be yellowish at first, quickly turning to gray or white a few days after infection; subsequently to dark brown with a reddish margin. A clear to yellowish halo may appear around the reddish margin (fig. 8). The diseased spots usually dry and fall from the leaf, leaving conspicuous holes. Severely infected leaves turn yellow and drop from the plant.
Control: The same control practices are used to control bacterial leaf spot are helpful in controlling Cercospora leaf spot. Chile cultivars vary in their susceptibility to Cercospora capsici, but 'Sandia' appears to be one of the most susceptible.
Powdery Mildew. Powdery mildew is a common disease on many types of crops, yet the disease is fairly uncommon in New Mexico on chile. However, when environmental conditions are favorable chile has been attacked by this pathogen. The fungus that causes the disease is Leveillula taurica(although the asexual stage of the fungus, Oidiopsis taurica, is typically found).
Conditions for disease: The disease is favored by warm temperatures (from 65-95o F). Although high humidity favors germination of spores, infection can occur during periods of high or low humidity. The fungus reproduces rapidly under favorable conditions. Wind-disseminated spores cause secondary infections, which help spread the disease. The fungus predominately infects leaves, but it can occasionally be found attacking fruit. The disease is most severe on older leaves just prior to fruit set, but can occur at any time throughout the season if environmental conditions are favorable. Severe infections early in the season can result in heavy yield losses.
O. tauricahas a wide range of hosts, including, cotton, onion, tomatoes, and weeds such as sowthistle and groudcherry. The fungus survives between chile crops on other agronomic hosts and weeds. The amount of inoculum that survives each year depends environmental conditions.
Symptoms:The primary disease symptom is the presence of a white, powdery, fungal growth that covers the lower leaf surface. The upper leaf surface of infected leaves may show a yellow or brownish discoloration and, in some cases, the fungus may actually sporulate on the upper leaf surface. The edges of infected leaves eventually roll upward, exposing the fungus. Infected leaves will drop prematurely from the plant, exposing the fruit to the sun, perhaps causing sunscald on the fruit.
Control: Due to the wide host range of this fungus, sanitation practices (removing and destroying infected crop debris and weed control) in and around chile fields are not always sufficient to control the disease. Additionally, most chile cultivars do not possess high levels of tolerance to this fungus. Because of the these factors, control usually depends on chemical sprays. Chemicals registered for use on peppers to control of powdery mildew contain sulfur -- for example, Thiolux®, Microthiol Special®, and sulfur dusts (sulfur dusts can be used on organically grown chile). The effectiveness of these sprays is depends on early detection and thorough application coverage. When conditions are highly favorable for the fungus, these sprays may provide only partial control.
Fruit Rots
Fruit rot results from infection by fungi, bacteria, or abiotic disorders (see abiotic diseases for information on non-parasitic fruit disorders). Some viruses also cause fruit damage, however, the rot that results from these infections is usually caused by secondary fungi or bacteria. Parasitic organisms can attack fruit directly (primary causal agent) or can attack tissue weakened either by another organism or by environmental factors (secondary causal agent). Fruit rots can occur in the field or postharvest.
Phytophthora Pod Rot. The causal agent of Phytophthora root rot, Phytophthora capsici, also attacks fruit, causing a disease known as Phytophthora pod rot.
Conditions for disease:This disease attacks fruit in the field under high rainfall and high humidity conditions, typical of a southern New Mexico summer rainy season. In these conditions, the fruit becomes watersoaked and susceptible to attack. Phytophthora capsici is splashed from the soil to the fruit and infection occurs when the fungus directly penetrats the skin. Fruit lesions usually occur on the ends of the fruit where water and fungal spores tend to accumulate.
Symptoms:Infected pods shrivel (fig. 9) and rot and white mold (mycelium of the fungus) develop inside the pod. Once inside the fruit, seeds become infested with the pathogen.
Control:The same control practices used to control Phytophthora root rot can help to control Phytophthora pod rot. Fungicide sprays are not effective.
Black Mold. Several fungi cause black mold on chile fruit (fig. 10); however, the most commonly associated organism is Alternaria spp. Black mold is a disease of mature red fruit and may be seen in the field or post-harvest. Field infections occur during periods of excessive moisture late in the season. Excessive fertilization, late irrigations, and early freezes add to the severity of the disease. Black mold may develop on harvested fruit that are not stored in a dry location prior to processing.
Alternaria sp., as well as other black molds, can also cause secondary infections on fruit that has been attacked by primary fungi, bacteria or viruses.
Black mold is controlled obtained by harvesting red chile as soon as possible and avoiding late-season irrigation and fertilization. Harvested chile should always be stored in a dry location.
Anthracnose (Ripe Rot). Anthracnose caused by Colletotrichum spp. occurs worldwide wherever peppers are grown. Although the disease is rare in New Mexico, it can be a problem in fields with overhead irrigation. The fungus persists in infected seed, crop debris, and alternate hosts. Infection occurs during periods of excess irrigation or rain on immature pods; however, the symptoms are normally not expressed until the pod becomes mature and completes its final color change.
Symptoms:The symptoms first appear as small, watersoaked lesions that expand rapidly. Fully expanded lesions are sunken and range in color from dark red to tan to black (fig. 11). As the infection progresses, buff to salmon-colored spores appear either scattered or in concentric rings within the lesions. Because this disease attacks immature fruit, infection takes place in the field; however, post-harvest symptom development is common.
Control: Using clean seed and rotating crops are the two most important control practices. Additionally, fungicide sprays may be helpful for control during favorable environmental conditions (check pesticide recommendation appendix for registered material).
Bacterial Soft Rot. Soft rot is primarily a post-harvest disease, although occasional field infections have been observed. This disease is caused by the soil-borne bacterium Erwinia carotovora pv. carotovora. Infections occur during rainy weather when soil containing the bacterium is splashed onto susceptible fruit. High moisture content in the fruit predisposes them to infection by this pathogen. The bacterium enters the plant through any wound, particularly those created by insects. Harvested fruit are infected through the stem end, where crevices tend to hold moisture. Tissue around the infection site begins to soften and eventually turns to a watery mass (fig. 12). Fruit infected in the field tends to collapse and hang on the plant like a water-filled bag. When the contents leak out, the outer skin of the fruit dries and remains attached to the plant. Postharvest infections are particularly damaging on fresh market peppers, because as infected fruit rot in the box, the disease will eventually spread to all fruit in the container.
Field infections are best controlled by controlling insects that create wounds for infection. Postharvest decay is reduced by picking fruit when it is dry, avoiding injury during handling, and cool storage. If fruit is washed after harvest, the water bath should be chlorinated and the fruit should be dried as quickly as possible to avoid encouraging disease development.
Viruses
Plant viruses are extremely tiny particles of nucleic acid that have the ability to cause disease in plants. These particles are not living organisms, as they carry out no metabolic processes such as respiration or digestion. However, viruses are considered parasitic infectious disease agents because they reproduce inside their host plant and can be spread from one plant to another by vectors, such as man, agricultural machinery, propagation tools, insects, nematodes, and fungi.
Control of virus diseases is difficult. No chemicals are available that are effective in controlling viruses; thus control strategies are designed to prevent infection. While there are several economically important virus diseases of chile, this publication will address the most common virus diseases in chile in New Mexico. However, because new viruses are rapidly being discovered, this section does not include all virus diseases that infect peppers.
Beet Curly Top Virus. Beet curly top virus (BCTV) was first identified in 1899 and to date remains the most important virus disease of many crops including peppers, melons, beans, tomatoes, spinach, and ornamentals. This virus is transmitted by the beet leafhopper (Circulifer tenellus). Curly top continues to be successful because it occurs in many strains, and because it infects a wide range of perennial and annual plants. This disease is common in home gardens as well as commercial fields.
Conditions for disease:Leafhoppers build up in high numbers on tumbleweeds and other summer annuals. They survive the winters on winter annuals such as mustards. The disease is most severe when rainfall is abundant in the fall and winter, as this supports the growth of the winter annuals. The leafhoppers migrate into cultivated fields and home gardens during the height of the growing season. The virus is then spread from the leafhopper to susceptible plants as the insects feed on the plants.
Symptoms: Peppers of all ages are susceptible to infection by BCTV; however, the disease is more severe when young plants are infected. Infected seedlings exhibit yellowing, curling, and twisting of the foliage. Seedling infection often results in death. The first symptom exhibited when older plants are infected is stunting. As the disease progresses, the symptoms consist of vein clearing, curling, twisting, and puckering of the leaves. In time, the leaves become leathery and stiff (fig. 13). The roots of infected plants gradually die. Infected plants are severely stunted and produce little or no fruit. Early-season infection often causes in plant death. Plants alive at the end of the season are easily detected in the field,as they are quite stunted and yellow (fig 14).
Control:Because BCTV exists in several strains that vary in host range and in their ability to cause disease, complete resistance is difficult to obtain. However, some cultivars are tolerant of some strains of the virus and can provide some disease control. Controlling weeds and volunteer plants from past susceptible crops is important in reducing the reservoir of virus in the field. If possible, chile should not follow other susceptible hosts such as tomatoes, beets, beans, potatoes, and spinach in rotation. Additionally, disease losses are reduced by planting for a thick stand and thinning stunted plants as they become noticeable. Research indicates the severity of curly top in peppers is reduced when the distance between plants is reduced. Insecticide sprays have not proven effective in preventing leafhoppers from transmitting BCTV.
Home gardeners can protect their plants by providing partial shade early in the season. Shading is beneficial in helping control curly top because leafhoppers generally will not feed in shady locations. Shade may be provided by fine mesh cages, which prevent leafhoppers from getting to the plants, or by other plants or structures. Cages should be provided that are large enough to keep the foliage from touching the cage material. The cages should be removed later in the season as the plants become less susceptible to the virus.
Tomato Spotted Wilt Virus. Tomato spotted wilt virus (TSWV) has been found periodically in most chile growing areas in New Mexico and, if allowed to go unchecked, has the potential for causing significant crop losses. The disease affects late-maturing pods, reducing yield of processing chile.
This virus occurs throughout temperate and subtropical regions and infects a diverse group of plant species from tomatoes and peppers to peanuts, lettuce, pineapple and many ornamentals. The virus is transmitted from diseased to healthy plants by thrips. The virus overwinters in perennial weeds, most importantly field bindweed and curly dock.
Symptoms: Symptoms of TSWV are numerous and varied, but the disease is most commonly recognized by the symptoms on the fruit. Both green and red fruit can be infected. Infected green pods display small, off-colored spots. The presence of several of these spots will cause the pod to be rejected for processing. Red fruit exhibit patches of yellow that never turn red. Other fruit symptoms include chlorotic and necrotic spotting, concentric ring patterns, and distortion (fig. 15). Foliage symptoms include general mosaic (fig. 16), chlorotic ring spots, and deformation. In some cultivars, the shoot terminals die and leaves fall from the plant. When new growth develops, it is severely distorted. Plants infected at an early age are stunted severely. All of these symptoms are not necessarily present on all plants, and symptom development appears to be most closely linked to cultivar.
Control:Because of the wide host range, including many perennial ornamentals and weeds, it is extremely difficult to eradicate this disease. Efforts to control thrips has had little effect on controlling TSWV. While eliminating the disease may not be possible, the incidence and severity of the disease may be reduced by removing all infected plants, rotating crops with non-hosts, and controlling perennial weed hosts. Additionally, growers should check seed companies for the availability of "newly developed" resistant or tolerant cultivars.
Pepper Mottle Virus. Pepper mottle virus (PMV) is an aphid-transmitted disease found in chile fields in New Mexico every year. Symptoms produced by this virus are reminiscent of 2,4-D herbicide injury. Infected plants exhibit misshapen leaves, which become quite puckered, and light and dark patches on the foliage give the plant a mottled appearance. Additionally, the fruit is small and misshapen. The overall effect of the disease is stunted plants with reduced yield. The disease usually occurs in late summer or early fall and reduces the yield of red chile.
This virus is not transmitted through seed and is killed inside infected plants when the plants freeze at the end of the season. However, the virus is able to persist because aphid vectors carry the virus from infected peppers to native, perennial, solanaceous weeds such as Datura sp. (jimson weed) and Solanum elaeagnifolium(silverleaf nightshade).
The incidence of PMV is depends on the population of aphids and susceptible weed hosts. Typically, severely infected fields are within a few miles of infected weeds. The best control is to avoid weed-infested locations, or control the weeds within several miles of chile fields. Control of the aphid vector is not effective in controlling the disease. No chile cultivars are resistant to pepper mottle virus.
Alfalfa Mosaic Virus. Alfalfa mosaic virus is an aphid-transmitted virus with a wide host range, including alfalfa, tomatoes, lettuce, and potatoes. The primary source for transmission to chile is alfalfa. This virus causes mild stunting and whitish, blotchy leaves and is sometimes referred to as "calico virus" (fig. 18). In some cases, fruit may be distorted.
Alfalfa mosaic is only a problem when chile follows alfalfa in a rotation, or when chile is planted adjacent to alfalfa. The best control, therefore, is to avoid planting chile in these locations.
Cucumber Mosaic Virus. Like beet curly top virus and alfalfa mosaic virus, cucumber mosaic virus has a wide host range, including chile, and exists in numerous strains that vary in their ability to cause disease. This virus is transmitted by aphid species.
Symptoms: Symptoms of this disease are quite variable depending on the infecting virus strain; however, most plants exhibit some degree of "shoestringing" (narrowing of the leaves) in addition to stunting, yellowing, and whitish spotting of the leaves. Fruit may be small and distorted.
Control: Crop rotation that avoids other susceptible hosts such as tomatoes, lettuce, and cucurbits is recommended to help control the disease. Many ornamental plants used in urban landscaping are also susceptible to the disease and serve as alternative sources of the virus. When possible planting away from urban areas can reduce the incidence of the disease.
Tobacco Mosaic Virus. Tobacco mosaic virus (TMV) is one of the most common and widespread of all the plant viruses. This virus infects numerous plant species, including vegetables and weed species such as mustards, nightshades, and jimson weed. TMV persists and remains infectious for many years in dried crop debris. TMV is readily transmitted by mechanical means, such as hands, cutting tools and other equipment--another factor that contributes to its success. Because of the ease with which this virus is transmitted, it can become a severe problem during greenhouse and field cultivation. Furthermore, the virus is also seed-borne.
Symptoms:Symptoms of this disease vary depending on the host and strain of the virus. The most common symptoms on peppers are raised bumps and mottled areas of light and dark green areas on the foliage (fig. 19). Fruit ripens unevenly and is reduced in size.
Control: In most cases, fields planted by direct seed have fewer problems with tobacco mosaic than fields planted with transplants. This is primarily due to reduced seed handling in comparison with greenhouse-grown transplants; however, it is important to start with disease-free seed. Fields with a history of TMV should be avoided. Because several varieties of chile possess some resistance to this virus, check for resistant varieties adapted to your growing region.
Mixed Virus Infections. Several of the above-mentioned viruses may be found together in infected plants, said to have mixed infections. Mixed infections make diagnosising the particular viruses involved difficult, as symptoms become numerous and quite variable. In some cases, mixed infections are more severe than infection by single virus. General control practices used for single virus infections, such as sanitation, weed control, crop rotation, and host tolerance are also useful for controlling mixed infections.
Pepper Geminiviruses. In recent years, researchers have identified several whitefly-transmitted geminiviruses that infect peppers. Among these "new" viruses are chino del tomate virus, serrano golden mosaic virus, Sinaloa tomato leaf curl virus, pepper mild tigre virus, and Texas pepper virus. These viruses all have similar symptoms but are biologically and genetically distinct. The common symptoms are stunting, curling, or twisting of the leaves, bright yellow mosaic, distortion of leaves and fruit, and reduced yield. Most of the geminiviruses have fairly restricted host ranges among economic crops, but most also infect weeds such as, Daturaspp. and Malva parvifolia (cheeseweed).
These viruses are transmitted by the whitefly, Bemisia tabaci, and are not known to be transmitted either mechanically or by seed in nature. B. tabaci, the sweetpotato whitefly, exists in several biotypes that are indistinguishable morphologically, but that vary genetically and with respect to host preference. In the 1980 a researcher in Arizona discovered an important biotype, identified as the 'B' biotype (a.k.a Bemisia argentifolii). The 'B' biotype whitefly differs from the original 'A' biotype in that it can survive on more hosts and possesses a degree of resistance to commonly used insecticides. These two characteristics have enabled the whitefly to build large populations in greenhouses worldwide, and in the field in the Caribbean and the sunbelt states of the US. Studies indicate both biotypes are efficient in transmitting geminiviruses.
Many of these "new" viruses have not yet been identified in the U.S., but are common in pepper-growing areas of Mexico. Some uncharacterized geminiviruses have been identified in tomatoes in Arizona and peppers in Texas. To date, none of these geminiviruses have been identified in New Mexico. However, because New Mexico is near Arizona, Mexico, and Texas and the generally favorable environmental conditions in New Mexico's chile growing regions, it is not unreasonable to believe that if geminiviruses are not already here, they soon will be.
Control of geminiviruses is difficult once plants become infected. Management strategies include removing of diseased plants and rotating crop with non-host plants. Additionally, controlling perennial weeds that may help the virus and the whitefly survive through the winter is important in reducing the potential for disease outbreaks.
Nematodes
Root-Knot Nematode. Root-knot nematode, Meloidogyne incognita, causes damage on peppers worldwide and may be particularly troublesome in warm, sandy soils. Damage can also be severe in greenhouse production of transplants, particularly when unsterilized soil is used.
This microscopic, nonsegmented worm is free-living in the soil as a juvenile. When the nematode infects a host, the adult becomes sedentary, feeding in one location in the root. The female nematode becomes pear shaped and reproduces inside the host tissue, laying her eggs in a gelatinous matrix either inside the root or on the root surface (depending on her location within the root). The eggs are released into the soil and may hatch immediately or overwinter until the temperature warms in the spring.
Root-knot nematode has an extremely wide host range, attacking over 2000 plant species, including chile, cotton, cucurbits, alfalfa, tomatoes, and many ornamentals.
Conditions for disease: Damage due to nematodes is generally restricted to soils that are at least 50% sand. Damage by root-knot nematode may be sporadic in the field and is generally associated with sandy areas that may correspond to old drainage systems. In heavy soils, even if plants are infected there may be little or no yield loss. Soil temperatures between 60 and 80_ F favor nematode development.
The degree to which losses occur depends largely on the age of the plant when it is attacked. When plants are infected in the seedling stage, the result is often complete loss of production. However, when mature plants become infected, yield loss will depend on the extent of the damage to each particular plant.
Nematodes move slowly through the soil under their own power; however, they can easily be moved by anything that moves soil. Thus, nematodes are disseminated by irrigation water, field equipment, shoes, and infected transplants.
Symptoms:The primary symptom or sign of root-knot nematode is the formation of galls or knots on the root system (fig. 20). These knots form as a result of the nematode's feeding and residing in the root tissue. The knots range in size from 1/8 to1" in diameter, but tend to be smaller on peppers than on many other hosts. Damage to infected plants results from the inability of water and nutrients to move up through the knots, resulting in above-ground symptoms similar to those caused by many root-infecting pathogens or environmental factors that reduce water uptake by the plant. Infected plants are generally stunted and have fewer, small, pale green, or yellowish leaves (fig. 21). The plant may wilt during periods of high temperatures. Infected plants produce fewer fruit of poor quality.
Control:Transplants should be grown in soil that has been sterilized or in soilless potting mixes. The best nematode control is by soil fumigation. While the EPA registration on nematicides is constantly changing, currently Telone II® and Vapam® are soil fumigants registered for use on chile. It is important, before using any of these materials, to check the label for use and restrictions and follow instructions carefully. Other control methods include late-season (late fall and early winter) cultivation of infested fields. While no chile cultivars are totally resistant to root-knot nematodes, current research efforts continue to search for better varieties.
For detailed information on root-knot nematode, see New Mexico State University - Cooperative Extension Guide H-241.
Abiotic Diseases (caused by non-infectious disease agents)
Blossom-End Rot. Blossom-end rot is a fruit disorder associated with inconsistent watering and a calcium deficiency. Other factors contributing to the occurrence of disease include root pruning, excessive soil salinity, and heavy applications of nitrogen fertilizers.
Symptoms:The disease typically first appears as a small, water-soaked, light-brown spot on the blossom end of immature fruit. The diseased area enlarges and becomes sunken and leathery in appearance (fig. 22). Infected fruit often matures prematurely. The sunken lesions caused by this disorder are prime locations for attack by secondary microorganisms, particularly fungi and bacteria. Thus, the diseased tissue may appear black or soft and watery.
Control:This disease is best controlled by providing sufficient water to prevent water stress. Soil amendments such as lime or calcium fertilizations have been effective in reducing the incidence of disease in some situations, but successful results are sporadic. Likewise, the application of foliar sprays of calcium salts have given erratic results.
Sunburn. Sunburn occurs on pepper fruit that is exposed directly to intense sunlight. This type of damage often results when shaded fruit is suddenly exposed to the sun. Defoliation or prolonged wilting caused by other diseases, such as Verticillium wilt, root-knot nematode, and bacterial leaf spot, or by salt and wind injury, can contribute to problems of sunburn.
The affected area is light-colored, soft and wrinkled. The damaged tissue eventually turns whitish-tan and papery in texture (fig. 23). Sunburned skin is often subsequently attacked by secondary fungi and bacteria, which then contribute to further decay.
Salt Injury. Excessive salts in the soil or irrigation water can become problematic in some chile fields. Likewise, heavy application of fertilizers, especially if placed too close to seed or transplants, can cause similar injury. Plants of all ages are susceptible to salt injury; however, mature plants may be more resistant to serious injury. The damage on young seedlings can be devastating as plants are severely stunted or, in many cases, killed (fig. 24). Seedling injury can result in sudden loss of the stand. Mature plants exhibit symptoms of burned root tips, marginal leaf necrosis, and wilt. Additionally, the hypocotyl may become desiccated, and severely affected plants may shed their leaves.
Symptoms often develop after light rains, which can wash salts into the root zone. Although it is not possible to completely eliminate problems due to excess salts, the damage may be minimized by a few cultural practices. Furrow-irrigated plants should be planted on the side of the bed, as furrow irrigation tends to push salts to the center of the row. Conversely, drip-irrigated plants should be planted in the center of the bed as drip irrigation will tend to push salts to the sides of the row. Regardless of the irrigation method, enough water should be applied with each irrigation to help leach the salts down into the soil. Other control to reduce seedling damage methods include planting in pre-irrigated beds and capping the row.
Wind Injury. Excessively strong winds can damage chile plants in several ways. Damage can result from rapid desiccation of the foliage. When this occurs, leaves may wilt and eventually dry up. Severe wind injury may lead to dead limbs as a result of desiccation. Wind injury may also result in physical damage to the foliage from cracks or tears that occur during wind storms (fig. 25). This damage can be intensified with blowing sand, which adds to the damage by the formation of necrotic spots. Damaged tissue may turn reddish-brown, chlorotic, or necrotic as the plant responds to the injury. Additionally, plants may snap off at the soil line where callus tissue forms as a result of wind whipping the plant back and forth.
Windbreaks planted around the edge of field or in a few rows within the field may provide protection for plants growing in high wind areas.
Hail Injury. Severe hail can result in pock-marked foliage and fruit (fig. 26), similar to injury created by wind-blown sand. The damaged tissue may become chlorotic or necrotic in response to the injury.
Herbicide Injury. Herbicide injury can occur on chile plants as a result of spray drift from target weeds near the chile, or from the use of contaminated spray containers. Even a small amount of the chemical can cause severe injury.
The type of damage that occurs is depends somewhat dependant on the herbicide. Two of the more common herbicides that damage chile are 2,4-D and Paraquat®. 2,4-D and related compounds such as 2,4,5-T and MCPA cause severe distortion of the foliage (fig. 27). Developing leaves become narrow with raised veins. Older leaves are also distorted and the leaf margins may become wavy. Leaves receiving a high dosage of the chemical develop into filiform leaves (i.e., leaves composed almost entirely of midrib, with little tissue development). Premature flower drop and the development of adventitious roots along the lower stems are also symptoms of 2,4-D injury. Paraquat® injury appears as small, circular white-to-tan lesions on affected leaves. Numerous lesions on one leaf may coalesce, causing irregular large, necrotic areas and eventual leaf drop.
Nutrient Deficiencies and Toxicities. This is one of the more difficult problems to diagnose, as symptoms caused by nutrient deficiencies or toxicities are similar to symptoms caused by many other abiotic and biotic (parasitic) diseases. Additionally, plants often suffer from deficiency of more than one element. Plants stressed from nutrient problems are also more susceptible to other disease organisms, and these pathogens may interfere with the diagnosis of the primary problem.
Plant nutrients may be required in relatively large amounts (major elements such as nitrogen, phosphorus and potassium) or in very small amounts (minor elements such as iron, boron, and zinc). Thus, determining individual nutrient problems is depends on the plant's need for each element. In many cases a plant tissue analysis run in the laboratory may be required to determine nutrient deficiencies and toxicities. However, diagnosis of some nutritional problems may be possible from the plant's appearance. For example, nitrogen-deficient plants grow poorly and are pale green or yellow in color, while plants lacking sufficient iron have young leaves that are severely chlorotic, but the veins remain green (interveinal chlorosis). Toxicities related to nutrients result from an excess of the chemical and often result in burning of the roots or foliage.
Figures
Fig. 1. Phytophthora root rot - field symptoms.
Fig. 2. Phytophthora root rot - root symptoms.
Fig. 3. Verticillium wd plilt - infecteant.
Fig. 4. Verticillium wilt - vascular discoloration.
Fig. 5. Seedling disease (damping off).
Fig. 6. Bacterial leaf spot - foliar symptoms.
Fig. 7. Bacterial spot on fruit.
Fig. 8. Cerscospora Leaf Spot
Fig. 9. Phytophthora pod rot on green chile.
Fig. 10. Black mold (Alternaria sp.) on red chile.
Fig. 11. Anthracnose on pimento fruit.
Fig. 12. Bacterial soft rot on pimento fruit.
Fig. 13. Beet Curly Top Virus - foliar symptoms.
Fig. 14. Beet Curly Top Virus - field symptoms.
Fig. 15. Tomato Spotted Wilt Virus - fruit symptoms.
Fig. 16. Tomato Spotted Wilt Virus-foliar symptoms.
Fig. 17. Pepper Mottle Virus.
Fig. 18. Alfalfa Mosaic Virus.
Fig. 19. Tobacco Mosaic Virus.
Fig. 20. Root-knot Nematode - galls on roots.
Fig. 21. Root-knot Nematode - above ground symptoms.
Fig. 22. Blossom-end rot.
Fig. 23. Sunburn on fruit.
Fig. 24. Salt injury on seedlings.
Fig. 25. Wind injury.
Fig. 26. Hail injury.
Fig. 27. 2,4-D herbicide injury.
Summary
Many different infectious and non-infectious diseases can cause problems in the production of chile peppers. While some of these diseases have specific control practices, several general control strategies apply to many and should be used in a overall disease management program. These strategies include:
- Choose cultivars carefully. When possible, select varieties resistant to diseases that commonly occur in your area. Also, choose varieties that are well adapted to your growing region.
- Purchase high-quality seeds or transplants. Starting with good, quality plant material is an important step in establishing a good stand of chile. High-quality seeds will have a high germination rate and will establish faster, helping reduce problems associated with seedling disease. Be sure transplants are free of diseases, such as root-knot nematode before planting in the field.
- Select the best field possible. Avoid fields with a history of disease problems or that possess other negative characteristics, such as poor drainage or, high salt content or are close to trees, buildings, or undesirable crops.
- Use crop rotation. Never plant chile back-to-back in the same field. Chile produces best and has the fewest disease problems when planted no more than once every 3-4 years. Rotations with crops such as corn, sorghum, and small grains help prevent build-up of many chile disease agents. If possible, avoid planting chile where crops susceptible to chile diseases, such as cotton, alfalfa and other vegetables, were planted the year before. In addition, allowing a field to be fallow periodically can reduce a buildup of some pathogens.
- Plant on raised beds. This cultural practice is important for controlling many diseases favored by wet soils. Raised beds provide better drainage. Cultivating so that soil is thrown to the center of the row throughout the growing season can reduce the incidence of chile wilt. Also, plant beds according to the type of irrigation method to help reduce the effect of excessive salts.
- Provide plants with an even supply of water. Avoid drought and flood situations that stress plants, making them susceptible to diseases such as wilt and blossom-end rot. Irrigate deeply to leach salts from the root zone, but try to avoid having water stand in the field for longer than 6 hours.
- Control perennial weeds. Perennial weeds serve as reservoirs for many plant pathogens--particularly plant viruses. Removing alternate hosts can help reduce not only the amount of the pathogen present each year, but also the resident insect population that is capable of transmitting the disease.
Acknowledgement: The author expresses sincere appreciation to Dr Emroy L. Shannon and Dr. Donald Cotter, Professors Emeritus of New Mexico State University and Dr. Richard B. Hine, Professor Emeritus of The University of Arizona, for their contributions to this manuscript.
APPENDIX A
Pesticide Information for the Chemical Control of Chile Diseases
This information is based on the 1995 Crop Protection Chemicals Reference Guide and other 1995 chemical reference material. This information is meant only as a reference of currently registered products, but do not stand as recommendations by the author, the New Mexico Cooperative Extension Service or New Mexico State University. Inclusion or exclusion of a pesticide in the following product list does not constitute a recommendation or condemnation of use in chile pepper fields by New Mexico State University. Constant changes in pesticide labeling sat he federal and state level makes it virtually impossible to produce a current product list of registered pesticides. Although every attempt has been made to validate the information in the list, it is the legal responsibility of producers, crop consultants or applicators to read the entire product label to check for registered use. Consult your area extension service or crop advisor for additional information on selecting appropriate pesticides.
| PRODUCT | DISEASE | RATE | NOTES |
| Basic Copper 53 (ai: basic copper sulfate) |
Anthracnose Bacterial Leaf Spot Early/Late Blight Cercospora Leaf Spot |
3 to 4 lbs/acre | • Treat early before disease is severe. • Repeat every 7 to 10 days as needed. |
| Blue Shield DF Blue Shield WP (ai: cupric hydroxide) |
Bacterial Leaf Spot | 2 to 3 lbs/acre | • Spray every 7 to 10 days as needed. |
| COC WP (ai: copper oxychloride) |
Bacterial Leaf Spot Damping Off |
BLS - 3 to 4 lbs/acre DO - 4 to 5 lbs/acre |
BLS - • Spray every 5 to 10 days as needed. DO - • For use in greenhouse or cold frame production of transplants. • Spray on soil before seedlings emerge, repeat every 4 to 7 days until transplanting. |
| Copper-Z 4/4 (ai: copper sulfate anhydrous) |
Bacterial Leaf Spot Cercospora Leaf Spot |
31 to 72 fl. oz./acre | • Spray before disease becomes severe. • Repeat at 7 to 10 day intervals. |
| Liquid Sulfur Six (ai: sulfur) |
Powdery Mildew | 3 to 4 pints/8 gallons of water | • Do not apply through any type of irrigation system. • Apply at first sign of infection at repeat at 3 week intervals. |
| Maneb 75 DF Maneb 80 (ai: maneb) |
Anthracnose Phytophthora Blight Ripe Rot Cercospora Leaf Spot |
1.5-2.0 lbs/acre | • Begin when disease threatens. • Repeat at 7 to 10 day intervals. • Do not apply more than 12.8 lbs Maneb 75 DF/acre/season. • Do not apply more than 12 lbs Maneb 80/acre/season. •Do not apply within 7 d of harvest |
| Microthiol Special (ai: sulfur) |
Powdery Mildew | 3 to 10 lbs/acre | • Apply at early leaf stage and repeat every 14 days as needed. |
| Nemacur 15% Granular (ai: ethyl 3-methyl-4-(methylthio)phenyl (1-methylethyl)- phosphoramidate) |
Nematodes | 10 to 14.7 oz./1000 ft of row (or 9 to 13.3 lbs/acre on 36-inch rows) |
• For Non-Bell peppers only. • RESTRICTED USE PESTICIDE • NM special local need registration [sec 24 C] • Apply in a 12 inch band over row at planting and incorporate into soil. • Use high rate for fields with heavy nematode infestation or for fields with a history of nematode problems. |
| Ridomil 2E (ai: metalaxyl) |
Pythium spp. Phytophthora Root and Crown Rot |
4-8 pts/treated acre | • Apply before plants become infected. Will not cure infected plants. •Apply at planting follow with 2 applications of 4 pts/acre at 30 day intervals. • May cause yellowing of pepper leaves. • Do not use in greenhouses. • Do not apply within 7 days of harvest. • Do not apply more than 12 pts/acre/season. |
| Ridomil Copper 70W (ai (s): metalaxyl and copper hydroxide) |
Pythium spp. Phytophthora Root and Crown Rot |
Soil - 4 pt/ac Foliar - 2.5 lbs/acre |
• Apply to soil at planting followed by another soil application 30 days later. • Follow with foliar applications at 10-14 day intervals. • Do not apply within 7 days of harvest. • Do no apply more than 3 lbs metalaxyl/acre/year |
| Telone* C-17 (ai(s): 1,3-dichloropropene and chloropicrin) |
Soil-borne diseases Nematodes |
Mineral soil - 10.3 to 17.1 gal/acre Muck or Peat soil - 27.4 to 41.1 gal/acre |
• RESTRICTED USE PESTICIDE • Broadcast application. • Mineral soil = sand, sandy loam, silt and clay loam |
| Telone* II (ai: 1,3-dichloropropene) |
Soil-borne diseases Nematodes |
Mineral soil - 9-18 gal/acre Muck or Peat - 24-36 gal/acre |
• RESTRICTED USE PESTICIDE • Broadcast application. • Mineral soil = sand, sandy loam, silt and clay loam |
| Thiolux (ai: sulfur) |
Powdery Mildew | 3 to 5 lbs/acre | |
| Vapam (ai: metam sodium) |
Soil-borne diseases Nematodes |
Entire field - 50-100 gal/acre Rows/Beds - 75-100 gal/acre (1 1/2 - 2 pt/100 sq. ft.) Drip line - 25-75 gal/acre |
• Apply with injector blades spaced 5" apart and 4' deep. • Immediately roll ground after application. • Follow application with a light irrigation or tarp to prevent volatilization of gases. |
For further reading
H-657: Diseases and Other Disorders of Pecan in New Mexico
https://pubs.nmsu.edu/_h/H657/
Circular 538: Onion Diseases in New Mexico
https://pubs.nmsu.edu/_circulars/CR538/
H-169: Using Insectary Plants to Attract and Sustain Beneficial Insects for Biological Pest Control
https://pubs.nmsu.edu/_h/H169/
H-242: Tomato Spotted Wilt Virus
https://pubs.nmsu.edu/_h/H242/
To find more resources for your business, home, or family, visit the College of Agricultural, Consumer and Environmental Sciences on the World Wide Web at pubs.nmsu.edu
Contents of publications may be freely reproduced for educational purposes. All other rights reserved. For permission to use publications for other purposes, contact pubs@nmsu.edu or the authors listed on the publication.
New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating.
August1995 Las Cruces, NM
