The causative agent of the disease is the oomycete Pseudoperonospora cubensis, which exhibits high variability, leading to the emergence of new aggressive pathotypes and the overcoming of breeding resistance. Symptoms manifest as angular yellowish-green spots on the upper side of the leaf, which eventually turn brown and coalesce. A diagnostic feature under high humidity is the appearance of a grayish-violet or whitish layer of sporulation on the underside of the leaf blade.
In southern regions, the disease can occur in a latent form, manifesting as small chlorotic spots, which complicates visual diagnosis. The consequences of the infection include premature drying and shedding of leaves, deterioration of fruit marketability, and plant death. When cucumber plantings are affected, the fruits lose their taste qualities and characteristic coloration.
For breeding resistant varieties, Dm genetic markers are used, including the specific genes Dm1.1, Dm5.1, Dm5.3 and the non-specific STAYGREEN (CsSGR) gene. The historically significant dm-1 gene has ceased to provide protection due to the evolution of the pathogen. In the international classification of varieties, resistance to the pathogen is labeled with the Pcu code, divided into HR (high) and IR (intermediate) levels.
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Downy mildew.