Cowpea powdery mildew is one of the most common and damaging diseases affecting leguminous vegetables. It is prevalent in both northern and southern vegetable-growing regions of China. During the disease's peak season, it causes a significant number of yellowing and dying leaves on the lower and middle parts of the plant, leading to reduced yield and poor quality of the produce. This disease can cause substantial economic losses for farmers.
Symptoms of cowpea powdery mildew primarily affect the leaves. The fungus grows on both sides of the leaves, as well as on the petioles and stems. As the fruit matures, the mycelium gradually disappears. In the early stages of infection, small, round powdery patches appear on the leaves. These spots eventually merge into larger patches, causing the leaves to wither or fall off.
The pathogen responsible for cowpea powdery mildew is *Erysiphe polygoni* DC., a member of the Ascomycota fungi. The sexual stage produces dark brown, spherical ascocarps with filament-like appendages on their surface. The asexual stage involves *Oidium* spores. The conidiophores are short and branched, producing a series of conidia. The earliest conidia form at the top, and they are colorless, elongated-oval, measuring 14 to 17 micrometers in size.
This disease has a broad host range, infecting various legumes as well as other crops such as cabbage, celery, rapeseed, mustard, and tomatoes. Powdery mildew is a biotrophic parasite that cannot survive on dead plant tissue. In northern China, it overwinters mainly through the formation of ascospores, but in our region, it rarely forms thick-walled structures. The primary source of infection comes from conidia produced on other crops or weeds in the field. These spores easily detach and spread via wind. When conditions are favorable, they germinate, penetrate the host’s epidermal cells, and grow within the plant, producing new conidia and continuing the cycle of infection.
Powdery mildew is drought-tolerant and thrives in humid conditions. While excessive rain may inhibit its development, wet weather and poor air circulation still favor the disease. Plants under water stress, especially those lacking sufficient soil moisture, are more susceptible. High planting density, poor ventilation, and excessive nitrogen fertilizer application also contribute to the spread of this disease.
To manage cowpea powdery mildew, it is recommended to:
1. Choose resistant varieties suitable for local conditions.
2. Implement proper cultural practices, including good ventilation, adequate light exposure, and timely watering during dry periods. After flowering, apply balanced fertilizers, avoiding excessive nitrogen and adding phosphorus and potassium to prevent early plant aging.
3. Apply fungicides early. Spray before the onset of symptoms or when the first signs appear, focusing on protecting the fruits. Options include: 70% thiophanate + 75% chlorothalonil (1:1) diluted 1000–1500 times; 30% copper oxychloride + 65% mancozeb (1:1); 80% mancozeb WP diluted 500 times; agricultural anti-120 water agent at 200 times; 50% protection work wet powder at 1000 times; 20% triazole copper emulsion at 2000 times; 6% Le Bifeng wettable powder at 1000–1500 times; 12.5% speed Poly WP at 2000–2500 times; 25% demulsification oil at 4000 times; 40% Fuxing at 8000 times. Repeat applications every 7–15 days, and stop spraying 7 days before harvest.
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