Phytophthora blight, caused by Phytophthora capsici, is one of the most destructive pepper and cucurbit diseases in North Carolina and many other regions of the US. The pathogen has a wide host range including pepper, tomato, eggplant and most cucurbits (e.g. cucumber, squash, pumpkins, watermelon and muskmelons). Recently, snap bean was added to the list of hosts attacked by P. capsici. The disease was recorded in North Carolina as early as 1954, but has become a serious concern over the last 10 years. Intensive vegetable cropping, short crop rotations, heavy rainfall and flooding in areas of NC have led to severe outbreaks of the disease (Fig. 1). Under ideal environmental conditions including heavy wind and rain, the disease can result in complete crop loss prior to the first harvest.
Symptoms vary depending on the affected crop, the specific host tissue(s) involved (i.e. roots, crowns, leaves, fruit) and the timing of symptom development in relation to plant maturity. For example, seedlings may die rapidly from lesions on the lower stems while mature plants may develop crown rot, fruit rot and foliar lesions. The disease is most likely to occur at the soil line or on the lower 12 to 18 inches of the plant where water often pools or splashes. The most common symptoms regardless of the host plant are crown rot and fruit rot. On peppers, the disease causes a black lesion just above the soil line (Fig. 2). Affected plants wilt and progressively die (Fig. 3). In squashes and pumpkin, the crown is extremely susceptible because plant architecture favors water accumulation in this area (Fig. 4).
Fruit rot is also very common, especially if fruit are in direct contact with soil as in most cucurbits. Lesions often form dark, concentric rings as they develop (Figs. 5 & 6). Such rings are generally absent from pepper fruit where rot appears as moist, cream-colored appressed growth (Fig. 7). It is important to remember that decay takes a few days to develop; fruit that are infected in the field may be symptomless at harvest, but rot a few days later. This can result in substantial losses during transit to market.
Fruit of known hosts are highly susceptible to the disease, but crown tissue susceptibility varies widely among hosts. For example, crown rot is very common on pepper, but uncommon on tomato and eggplant. In cucurbits, crown rot is very common on squash and pumpkin, but virtually absent on cucumber, cantaloupe and watermelon. In contrast, fruit rot is common on all these hosts.
Foliar lesions are less common, but can occur, especially when conditions are particularly favorable (e.g. extended periods of heavy rain and warm days). The classic leaf lesion caused by P. capsici is fairly circular with a tan margin and brown, necrotic center. Foliar lesions are most common in pepper and squash (Fig. 8), less common in cantaloupe, rare in cucumber, watermelon and eggplant, and we have not observed them in tomato.
Phytophthora blight is caused by the fungus-like organism Phytophthora capsici. P. capsici is an oomycete, which is a group of organisms sometimes referred to as "water molds" that are more closely related to brown algae than to fungi. P. capsici is a soilborne pathogen which produces several types of spores, enabling it to spread throughout the field, and to persist in the field between crops. Sporangia (Fig. 9) are short-lived asexual spores that can spread by wind and water. This pathogen can also form sexual structures called oospores. These spores allow the pathogen to survive in soils for many years and therefore short crop rotations (2-3 years) with non-susceptible hosts may not have a dramatic impact on disease in a susceptible crop.
Rainfall, soil saturation, and temperatures between 75° to 85°F favor the development of Phytophthora blight. The disease usually starts on plants in low, poorly drained areas of the field especially after excessive rains. Once plants are present in a field and the environmental conditions are favorable, oospores germinate to form structures called sporangia (Fig. 9) that release zoospores. Zoospores are single-celled spores that can swim in the soil water and lodge on host roots or stems, initiating infection. The pathogen can be dispersed in soil, with surface water following drainage patterns, and via splash dispersal from soil to foliage. Once a plant is infected, more sporangia are formed and zoospores are released repeatedly throughout the season. Infected plants enable the pathogen to produce more inoculum that is splash-dispersed by wind-driven rain to the upper parts of neighboring plants including stems, leaves and fruit. Amount of rainfall or frequency of irrigation has a dramatic influence on the level of final disease.
Transplants and contaminated water or soil are the most common sources of initial inoculum. P. capsici survives between susceptible crops as oospores or mycelium in infected tissue, or on weed species such as the Carolina wild geranium (Geranium carolinianum) and black nightshade (Solanum nigrum). In a previous survey, around 14% of fields surveyed in North Carolina harbored both mating types. If oospores form, the pathogen is likely to persist for years in the soil even in the absence of host plants.
Diagnosis of Phytophthora is typically based on symptoms and detection of distinctively shaped sporangia (Fig. 9) on infected plant tissues. Clinical diagnosticians should look for sporangia that are mostly papillate, but in some cases they appear to be semipapillate, and occasionally have two or three apices. In addition, they are also caducous (i.e., having a short stem). Although some molecular approaches have been developed for P. capsici detection that utilize P. capsici-specific PCR primers, diagnostic laboratories do not usually employ these techniques.
There is no complete management program to reliably control Phytophthora blight in peppers and cucurbits. Strategies for management of the disease include cultural techniques, utilization of resistant varieties, and fungicide applications. To reduce risk of Phytophthora blight, a complete integrated management program needs to be adopted.Cultural controlThe following practices should be considered:
1. Rotate away from susceptible crops for a minimum of two years, preferably four years. Grain crops are most suitable (e.g., corn and small grains) but crucifers are also good options. Sweetpotatoes, potatoes and tobacco are also not susceptible to this pathogen. This disease is particularly severe in pepper fields following a cucurbit crop;
2. Avoid poorly drained soils and low lying areas;
3. Do not allow soil build-up at the headlands of fields but create drainage ditches to ensure maximum soil-surface drainage from furrows;
4. Break up hardpans and plow-pans by subsoiling to increase soil drainage;
5. Always plant peppers and bush-type cucurbits on dome-shaped ridges or beds that are as high as possible and do not allow planting depressions that collect water to form near plants. Note: Vining cucurbits planted on a bed or ridge will develop a portion of their fruit in furrows where water accumulates, and therefore fruit rot, is more likely;
6. Avoid excessive overhead irrigation;
7. Enter infested fields last and clean equipment before moving to other fields;
8. Maintain sufficient surface crop residue through the use of no-till production or application of small-grain mulches to the bed and furrows to limit splash dispersal and surface water movement. Note: No-till production in early spring can increase the incidence of the root and crown rot phase of the disease if soilborne inoculum is present but will limit secondary spread;
9. Evaluate irrigation water to ensure it is not contaminated with the pathogen (a bioassay is possible; further information can be obtained from the NCSU Plant Disease and Insect Clinic) and avoid drainage of water from infested fields back into irrigation sources.
Host resistanceHost resistance is becoming an integral component in Phytophthora blight management for commercial production of peppers in NC. Host resistance offers a better possibility of reduced disease severity and crop loss; a few moderately resistant bell pepper varieties are commercially available (Table 1). These cultivars possess resistance to the crown rot phase of the disease, yet lack resistance to the foliar phase, which often occurs in western NC. Host resistance in cucurbits exists, but currently no cultivars are marketed claiming resistance to Phytophthora blight.Table 1. Pepper lines that offer resistance or tolerance to Phytophthora crown rot*.
(as determined by seed supplier)****
green blocky bell pepper, large to extra large, thick walls
BLS 1, 2, 3; PVY Tobamovirus
sweet, green to red bell pepper; mid-season maturity, dark-green extra-large fruit; high quality, high yielding
TMV, Phytophthora capsici
Sweet, blocky bell pepper; extra large; early set, cold tolerant
BLS 1,2,3,5; tolerant to CMV and Phytophthora capsici
** Moderate resistance offers 50 to 80% control compared to a highly susceptible line; Good resistance offers above 80% control. There are no commercial lines with excellent resistance.
*** Modified from: Vegetable Cultivar Descriptions for
Notes: Seminis does not offer any claims about Phytophthora resistance in Aristotle, although multiple years of research in NC has shown moderate levels of resistance to crown rot. Paladin offers the highest level of resistance but does not meet market demands for all market segments.Chemical controlShould a fungicide program be implemented? On farms without a history of disease and practicing good crop rotation, a fungicide program may not be necessary. Farms that have a history of the disease, that can employ rotation, and do not have fungicide resistant populations of the pathogen will likely benefit from a fungicide program. While fungicides cannot be the sole method for preventing the disease, they can provide some degree of disease control, especially when used in combination with other management practices. In all cases, the focus should be on water management, to minimize conditions conducive to the initiation and spread of this destructive disease.
Conventional chemistries recommended for control of Phytophthora blight include mefenoxam (Ridomil Gold, Ultra Flourish), mixes of mefenoxam and copper hydroxide (Ridomil Gold/copper), famoxadone plus cymoxanil (Tanos), zoxamide plus mancozeb (Gavel), or copper plus maneb. Many of these chemicals are also labeled for suppression of the fruit phase of Phytophthora blight. One consideration is that different products have different preharvest intervals (PHI). A product with a PHI of 5 days (i.e. maneb) cannot be used when harvests are done more than once per week. Another important consideration is fungicide resistance management, as pathogens may develop insensitivity (resistance) to certain chemistries if these products are used too frequently. For instance, the phenylamide class of fungicides, specifically metalaxyl and the newer chemistry mefenoxam (Ridomil Gold), has been used by many growers for controlling various Phytophthora diseases. Metalaxyl and mefenoxam applications are short-term solutions to a long-term problem as populations of P. capsici have developed resistance in many regions where peppers and cucurbits are grown. Because only sensitive populations of the pathogen are controlled by these chemistries, observed control failure documented in some fields is likely due to the development of insensitive P. capsici populations. Sexual recombination between the two mating types can also generate fully insensitive isolates.
Ridomil Gold is registered for pre-plant and early post-plant application to control Phytophthora blight in peppers. Ridomil Gold is labeled for use on cucurbits to control Pythium damping-off, but not Phytophthora blight. Thus, the manufacturer makes no claim of efficacy against this disease in cucurbits. Specific recommendations of rates and timing for Ridomil Gold can be obtained from the label. In the case of peppers, placement of Ridomil Gold in the root zone is extremely important. Ridomil Gold applications should be incorporated or moved into the zone by irrigation. Ridomil Gold/Copper or maneb plus copper can be used as foliar sprays to limit the foliar phase of the disease. The maneb plus copper combination is a protectant spray and must be applied before conditions conducive to Phytophthora prevail. Growers may want to consider up to three maneb/copper applications on 7-10 day intervals starting 2 to 3 weeks after the last Ridomil Gold application.
There are several new chemistries available (or in the process of becoming available) that have been shown to be effective at reducing plant mortality caused by Phytophthora blight. Newly labeled products containing cyazofamid (Ranman), fluopicolide (Presidio), and mandipropamid (Revus) have shown to be very effective at reducing Phytophthora blight severity, although Ranman is not currently labeled for use on pepper. However, even the most effective products typically only reduce disease levels by 40-60%, which may not be commercially acceptable. Specific recommendations of rates and timing for each product should be obtained from the label. Check the NCSU pepper foliar fungicide recommendations for help on schedules and rates: http://www.ces.ncsu.edu/fletcher/programs/plantpath/
____________________________________________________The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by North Carolina Cooperative Extension Service is implied.**May 2008