Extension Plant Pathologist
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 disease was recorded in North Carolina as early as 1954, but has become a serious concern over the last 10 years. The disease usually starts on plants in low, poorly drained areas of the field especially after excessive rains. The pathogen can be dispersed in soil, with surface water following drainage patterns, and via splash dispersal from soil to foliage. During conditions of heavy wind and rain, the pathogen can be distributed over entire fields and cause extensive losses within a few days.
The pathogen has a wide host range including pepper, tomato, eggplant and most cucurbits (e.g., cucumber, squash, pumpkins, watermelon and muskmelons). Seed has been reported to be a source of inoculum, but transplants and contaminated water or soil are the most common sources of initial inoculum. P. capsici is heterothallic, meaning that the two mating types (e.g. "A1" and "A2" strains) must be present to form persistent resting structures called oospores. 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. The pathogen can persist from one year to the next on host debris or alternative hosts and soilborne oospores. Oospores can germinate in the spring to form structures called sporangia that release zoospores. Zoospores are single-celled spores that can swim in the soil water and lodge on host roots. Once a plant is infected, more sporangia are formed and zoospores released. Amount of rainfall or frequency of irrigation has a dramatic influence on final disease incidence.
The disease initially occurs as a crown rot characterized by a black lesion just above the soil line. Affected plants wilt and progressively die. Diseased 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. Large black lesions form along stems, circular yellow lesions on the leaves, and cream-colored, moist, yeast-like appressed growth on fruit.
In summer squash the disease appears as a crown rot, often moving from the top of the crown to the root. Wilt may also occur. However, in most cucurbits the fruit rot phase of white mycelial growth is the most common sign. Crown rot has not been observed in watermelon and is uncommon in cucumbers, muskmelon and winter squash. Highly susceptible crops include summer and zucchini squash, pumpkins, winter squash and watermelon; cucumber and muskmelon are slightly less susceptible.
There is no complete management program to reliably control Phytophthora blight in peppers and cucurbits. If inoculum is present in the soil and a susceptible pepper/cucurbit variety is grown, the disease can spread rapidly under conditions of heavy rainfall, despite growers' best management practices. To reduce risk of Phytophthora blight, a complete integrated management program needs to be adopted with an emphasis on water management. The following practices should be considered:
The decision to use fungicides can be a difficult one and depends on the history of the field and personal management decisions. Grower experiences have shown that under heavy disease pressure, fungicides have not proven effective at limiting economic losses. This could be due to two problems: 1) the fungicide was not present in sufficient concentrations during the onset of disease due to soil/weather conditions or due to timing and placement of application and 2) Phytophthora strains resistant to the fungicide may be present in the field.
During a 1997 survey, infected plants were collected from 12 fields in North Carolina and one field in New Jersey. Three quarters of the fields sampled contained isolates that were resistant to Ridomil Gold, and insensitivity ranged from 11 to 80% within the fields. A total of 161 isolates were evaluated and 57% were found to be resistant to Ridomil Gold. Resistant isolates grew in the presence of both metalaxyl (Ridomil) and mefenoxam (Ridomil Gold). This dramatic shift in populations of P. capsici from sensitivity to resistance has occurred in pepper fields in the last 3 years in North Carolina.
Ridomil Gold EC is registered for pre-plant and early post-plant application to control Phytophthora blight in peppers. Ridomil Gold EC 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 EC application.
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 a resistant population of the pathogen will likely benefit
from a fungicide program. If rotation is not practiced in fields with
a history of disease or if resistant populations are known to be present,
the benefit of a fungicide program is uncertain. Some research evidence
suggests Ridomil Gold provides benefit where resistant populations are
present. In all cases, the focus should be on water management, to minimize
conditions conducive to the initiation and spread of this destructive
North Carolina Cooperative Extension Service personnel.
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Recommendations of specific chemicals are based upon information
on the manufacturer's label and performance in a limited number of trials.
Because environmental conditions and methods of application by growers
may vary widely, performance of the chemical will not always conform to
the safety and pest control standards indicated by experimental data.