Managing Anthracnose Disease in Cucurbits

Introduction to Anthracnose Disease

Anthracnose disease, caused by the pathogen Colletotrichum spp., is one of the top ten plant pathogens globally. This fungus affects nearly all crops, including grains, vegetables, and fruit trees. The disease can devastate crops if not managed properly.

Symptoms of Anthracnose in Cucurbits

Cucumbers: Anthracnose symptoms in cucumbers start as light yellow, water-soaked spots near the leaf veins. These spots quickly expand into round or irregularly shaped brown to dark brown lesions. The center of the lesions often develops holes, leading to a ragged, tattered appearance of the affected leaves. In severe cases, entire necrotic spots may fall out, leaving holes in the leaves. Anthracnose can occur alongside other diseases like downy mildew and fusarium wilt, particularly affecting older and mature leaves. Unlike downy mildew, anthracnose lesions are not limited by leaf veins and often appear on veins as well.

Melons: In melons, anthracnose symptoms can be similar to those in cucumbers but with some differences. The lesions are typically irregular and can resemble downy mildew but are distinguished by their jagged, tooth-like edges and radial star-shaped patterns. On melons, anthracnose is more noticeable on the fruit, presenting as sunken, circular spots with a lighter center and darker edges. Over time, these spots may crack.

Watermelons: Watermelon anthracnose lesions are often round and perforated but can also be irregular. They share similarities with watermelon downy mildew but are differentiated by their serrated or spiked edges and radial patterns. Anthracnose is less common on watermelon fruits compared to cucumbers but can still cause significant damage.

Occurrence and Spread of Anthracnose

Pathogen Characteristics: The pathogen Colletotrichum spp., also known as Colletotrichum capsici, Colletotrichum coccodes, and Colletotrichum orbiculare, affects a wide range of crops. Unlike specialized pathogens like downy mildew, Colletotrichum species do not strictly adhere to a single plant family and can infect multiple plant families, including cucurbits, tomatoes, and peppers. This pathogen can target various plant parts, including leaves, stems, and fruits.

Different Colletotrichum species have preferences for specific crops. For instance, Colletotrichum capsici mainly affects peppers and eggplants, while Colletotrichum coccodes targets tomatoes and potatoes. Colletotrichum orbiculare primarily affects cucurbits, including cucumbers, melons, and watermelons.

Optimal Conditions for Infection: Colletotrichum thrives at temperatures between 18-28°C (64-82°F) with free moisture on leaf surfaces for over 10 hours. The pathogen survives on infected plant debris in the soil and can persist for over two years in the absence of host plants.

Disease Cycle: Anthracnose does not involve a sexual stage in its life cycle. It spreads through seeds, seedlings, or plant debris in the soil. The pathogen’s conidia (spores) spread via rain splash, wind, or irrigation water. Therefore, frequent and prolonged rainfall conditions favor the development and spread of anthracnose.

Chemical Control of Anthracnose

Protective Fungicides: Effective management of anthracnose often involves the use of protective fungicides. Commonly used fungicides include:

• Copper-based fungicides
• Mancozeb
• Carbendazim
• Chlorothalonil

In Worldwide, imidazole (e.g., myclobutanil) and triazole (e.g., propiconazole, difenoconazole) fungicides are frequently used. Internationally, newer fungicides like methyl benzimidazole carbamate (e.g., pyraclostrobin) and azoxystrobin are popular. However, reports indicate increasing resistance to these fungicides in some regions. Consequently, it is often necessary to apply fungicides with different modes of action in combination to manage resistance effectively.

Conclusion

Anthracnose, caused by Colletotrichum species, poses a significant threat to cucurbit crops, including cucumbers, melons, and watermelons. Recognizing the symptoms and understanding the disease cycle are crucial for effective management. By applying appropriate fungicides and monitoring environmental conditions, growers can reduce the impact of this damaging disease on their crops. Regular rotation of fungicides with different action mechanisms can help mitigate resistance and enhance disease control.