Common Disease of Cucurbits: Powdery Mildew
Powdery mildew is a widespread disease that affects cucurbits, such as melons, pumpkins, and cucumbers, in both field and greenhouse environments across the world. The two most common fungi responsible for causing cucurbit powdery mildew are Podosphaera xanthii and Erysiphe cichoracearum.
Impact on Yield
The presence of powdery mildew can lead to a decrease in the quantity and quality of the yield. This can be due to smaller or fewer fruits, a shorter harvest period, or premature senescence of infected leaves. Fruits affected by powdery mildew may become sunburnt, ripen too early or incompletely, resulting in poor storability, low soluble solids (which affects flavor), and discolored handles.
- Stress from the disease can cause imperfections on the fruit rind, such as speckling, raised indentations, and oedema.
- Increased Susceptibility to Other Diseases
- Plants infected with powdery mildew become more susceptible to other diseases, including gummy stem blight.
The powdery mildew fungus is a parasite that requires a living host to survive. It cannot exist without one.
The primary source of initial infection is believed to be airborne spores (conidia) that can travel long distances from other affected crops.
Powdery mildew develops rapidly under favorable conditions. The time between infection and symptom appearance is usually short, ranging from 3 to 7 days. The fungus can produce a large number of spores in a short period.
Conditions that favor infection include a dense plant canopy, low-intensity light, high nitrogen fertilization, and high relative humidity. However, infection can occur even at relative humidity levels below 50%. The optimal temperature range for disease development is between 68-80°F, while temperatures of 100°F or above inhibit powdery mildew growth.
Symptoms and Signs
Identifying symptoms of powdery mildew is relatively straightforward compared to other diseases. The fungus creates noticeable patches of whitish mycelium, resembling talc, on both the upper and lower surfaces of leaves, petioles, and stems.
In the initial stages, powdery mildew manifests as pale yellow spots on older leaves, stems, and petioles. As the white, fluffy mycelium continues to grow and produce spores, the affected areas take on a powdery appearance.
Over time, the affected leaves become dull and chlorotic, exhibiting a yellowing color. They may also show signs of wilting, and eventually, they turn brown and acquire a papery texture. In addition, yellow spots may develop on the upper leaf surfaces, opposite to the powdery mildew colonies.
Implementing an integrated approach involving multiple practices is crucial for controlling powdery mildew. Regular scouting for symptoms in cucurbit plants is necessary to detect early infection and initiate appropriate treatment. The following control strategies are recommended:
1. Cultural Control Methods
- Maintain healthy and vigorous plants through proper nutrition and suitable sanitary conditions to reduce susceptibility to powdery mildew.
- Ensure proper crop spacing to allow for free air circulation.
- Choose planting sites with good air movement and no shade.
- Physically separate successive cucurbit plantings to prevent older plants from becoming a source of infection for the main crop.
- Provide adequate field and greenhouse ventilation.
- Minimize movement between infected and non-infected areas.
- Practice field sanitation by removing affected plants and debris.
- Utilize resistant varieties.
- Avoid overhead irrigation to keep foliage dry.
2. Chemical Control Method
Fungicides are essential for effectively controlling powdery mildew, a common disease. For successful control, it is crucial to target both the upper and lower leaf surfaces, as the fungus tends to develop more on the lower surfaces. Recommended fungicides include systemic or translaminar ones that can reach the lower leaf surface. However, using the same systemic fungicides repeatedly can result in the development of resistance in powdery mildew fungi. To tackle this resistance, it is advisable to alternate between different fungicides throughout the crop season.
Recommended Fungicides for Powdery Mildew Control in Cucurbits:
- ABSOLUTE® 375SC: Mix 10ml per 20l of water
- EXEMPO CURVE® 250SC: Mix 15ml per 20l of water
- EXPLORER® 3 SL: Mix 10ml per 20l of water
- MEGAPRODE LOCK® 525WP: Mix 30g per 20l of water
- MILESTONE® 250SC: Mix 10ml per 20l of water
- JUPITER® 125SC: Mix 15ml per 20l of water
- RANSOM® 600WP: Mix 15g per 20l of water
- TOMAHAWK® 250EC: Mix 10ml per 20l of water
- CHARIOT® 500SC: Mix 20ml per 20l of water
- When spraying fungicides, consider adding INTEGRA at a rate of 3ml per 20l of water. INTEGRA acts as a sticker, spreader, and penetrant, enhancing the efficacy of the fungicide.
- Timely control of the disease is crucial to reduce and prevent losses associated with powdery mildew infection.
- Fungicides should be reapplied every 1-2 weeks.
- Providing proper nutrition to plants helps boost their immunity against infections.