Diaporthe stem blight of cucurbits

The disease is caused by the fungus Diaporthe melonis, also known by its anamorph stage Phomopsis sclerotioides. This ascomycete fungus is a significant pathogen responsible for root and stem rot in various cucurbitaceae species, often leading to severe agricultural losses.

This pathogen primarily infects melons, cucumbers, pumpkins, and watermelons. It is particularly destructive in greenhouse conditions, where irrigation and temperature control can create ideal environments for the fungus to establish itself and spread rapidly between plants.

Symptoms typically begin as discolored, water-soaked lesions at the base of the stem near the soil line. As the infection progresses, these areas turn brown, dry out, and crack. Black, pimple-like structures known as pycnidia develop on the necrotic tissues, releasing spores that can further contaminate the surrounding soil and plant debris.

The development of Diaporthe melonis is heavily favored by high soil moisture levels and temperatures between 20°C and 25°C. The fungus persists in the soil through resilient sclerotia, which allow it to remain viable even in the absence of a host crop for extended periods, making rotation difficult.

Effective control strategies involve a combination of rigorous crop rotation—avoiding cucurbits for at least 4–5 years—and the implementation of strict greenhouse sanitation. Growers should ensure optimal soil drainage, remove and destroy infected plant parts immediately, and use preventive systemic fungicides to protect the root zone from early-season colonization.