Agent: pseudofungus Pseudoperonospora cubensis Rostowz.
It is harmful first of all because of its being obligate parasite. It lives exclusively at the expense of its host - the infected plant. In addition, its spores hibernate well in soil, in remains of plants and can stay until they are completely rotted and after that they will be preserved even within 7 years! This pseudofungus affects only cucurbits: cucumber, zucchini, melon, squash, pumpkin, watermelon.
Disease spreads very quickly at optimal conditions (during a few days), affected plants will be in the area up to 5 m2 minimum! In particular, in greenhouses. Starting from a temperature of + 12 ° C and up to +20°C, as well as an increased level of humidity, spores of the false mildew enter an active stage of growth.
The first manifestation can be seen with the naked eye. Immediately after infection, large enough spots of angular form appear on the obverse of the leaves, painted in a green-yellow color. The next day, a plaque of gray shade becomes visible from the back of the leaf - this fungus has already begun its sporulation. Spots grow even more, darken to brown color and the leaves begin to dry up. As a result, cucumber bushes for further development lack the useful substances that they received from the leaves, and the plant dies.
Downy mildew is a common disease of cucumber bushes, which affects only the leaves, but causes a significant decrease in yield and premature death of the plant. Externally infected bushes can be identified by yellow and brown spots. They quickly expand, resulting in cucumber leaves dry up, then die and the bush itself. If not to take any measures for disinfection, the plant will die in 2-3 days. Another possible reason for the spread of mildew is the excessive density of rows of plants and watering with cold water.
In greenhouses are usually the most optimal conditions for mildew. High humidity creates optimal conditions for the rapid development of spores and their spread throughout the greenhouse, especially when condensation forms on its walls, as well as on the plants themselves. This disease can appear on cucumber bushes in any period of their development - from the appearance of the first leaflets to the period of fruit ripening. Also, the cause of the emergence of a transplant can be unbalanced soil (for example, an excessive amount of nitrogen in it or a lack of phosphorus).