Black rot of cucurbits

The fungus Diaporthe cucurbitae (anamorph: Phomopsis cucurbitae) is the causal agent of black rot, a significant disease affecting cucurbit crops. It is known for causing localized lesions on leaves and stems, as well as rapid decay of the fruit.

Typical host plants include pumpkin, squash, cucumber, melon, and watermelon. The pathogen is particularly damaging in greenhouse environments but can also lead to significant losses in open-field cultivation during humid growing seasons.

The primary signs of infection include dark-colored, irregular spots on foliage. On stems, the disease manifests as elliptical lesions or sunken cankers that can cause wilting of the parts above the infection site. On the fruit, the disease starts as small, watery spots that expand into large, dark-brown or black necrotic zones covered in fungal pycnidia.

Environmental conditions such as high humidity and frequent rainfall are critical for the spread of Diaporthe cucurbitae. The fungus overwinters as mycelium and fruiting bodies in decaying crop debris, which acts as the primary inoculum source for subsequent seasons. Spores are dispersed through splashing rain or irrigation water.

Effective management strategies require an integrated approach. Important measures include the removal and destruction of crop debris post-harvest to reduce inoculum levels, the implementation of a 3-year crop rotation, and the application of protective fungicides. Ensuring good air circulation within the canopy helps to minimize favorable conditions for fungal growth.