Leaf mold manifests as chlorotic light-yellow spots on the upper side of the leaf blade, which eventually turn brown. On the underside of the leaf in the affected areas, a characteristic velvety coating develops, changing color from gray-white to olive or brown. The disease usually begins with the lower tier of leaves, leading to their abscission and a significant decrease in the photosynthetic activity of the plant.
The development of the pathogen depends directly on the microclimate in protected ground, where a high relative humidity of 75% and temperatures in the 20–27°C range create a favorable environment. A critical factor is the presence of condensation on the leaf surface, which promotes the germination of fungal spores. The incubation period of the infection ranges from 10 to 15 days.
Varietal resistance is provided by genetic Cf-genes that operate on the principle of recognizing parasite effectors and triggering a hypersensitive response. Since the fungus has high variability and the ability to form new races that overcome protection, gene pyramiding is used in breeding. Resistance assessment in variety testing is conducted using laboratory methods and in infectious backgrounds based on the absence of sporulation.