Alternaria leaf spot

Alternaria leaf blight (A. alternata f.sp. cucurbitae) is a fungal pathogen of Cucurbits.

Symptoms
Symptoms appear in late autumn, mainly on the leaves of the middle and upper part of the plants. Necrotic flecks, surrounded by a chlorotic halo, appear on the leaf, and these enlarge to spots which may coalesce to form lesions up to 5cm (2in) or more in diameter. The lesions appear circular in shape and bear black-brown fructifications of the pathogen. Severely-infected leaves become yellow, senescent, and die. No other part of the plant is affected. During the winter, when relative humidity in the plastic houses is high and plant vigor is reduced due to fruit bearing and unfavorable climatic conditions (reduced illumination and average air temperature lower than 15°C (59°F)), infection progresses rapidly throughout the crop resulting in severe damage within a few days.