Pseudoperonospora cubensis is a plant pathogen causing downey mildew on cucurbit crops like cucumber, cantaloupe, pumpkin, watermelon and squash.
Life cycle and appearance of Downey mildew
Pathogens causing downy mildew are not fungi but Oomycetes and related to Pythium and Phytophthora. They survive as oospores. From these oospores or surviving mycelium sporangia are formed, structures in which zoospores are produced. However, in downy mildews, sporangia often germinate directly and the germ tubes infect the plants either through the stomata or directly by penetrating the cuticula. For germination free water is required. For most downy mildews, the optimum temperature is around 15 oC and the closer the temperature is to this, the shorter the leaf wetness period that is required.
Inside the plant, the pathogens produce haustoria, small organs with which the pathogen can take up nutrients from the living plant cells. The pathogens continue to grow inside the leaves and after a while new spore-bearing structures are protruding from the stomata. Because there are more stomata on the underside of the leaf, this is where most fluffy symptoms are found.
Downy mildew pathogens infect mostly the leaves but occasionally also stems and fruits. They cause lesions on the upper leaf surface, bordered by the veins, which are first yellow and then turn brown. On the underside of the leave fluffy growth appears which is first white and later turns grey-brown. This is actually the sporulation of the pathogens extruding from the stomata.