Study of Morphobiological Characteristics of the Pathogen Causing True Powdery Mildew in the Family Cucurbitaceae L. Crops

The studies assess the current species composition of cucurbit powdery mildew pathogens to identify the sources of cucumber resistance in Moscow region. The object of the study was a causative agent of powdery mildew in cucurbits. The research material included 17 cucumber breeding lines and a collection of differential plants (Lagenaria siceraria, Cucumis sativus, Cucumis melo, Cucurbita pepo, Citrullus lanatus, and Luffa cylindrica). Powdery mildew development was monitored in protected ground conditions in 2021–2023. Isolation of Podosphaera xanthii from affected plants and study of the morphobiological characteristics of the fungus were performed using classical methods of phytopathology and light microscopy. The racial composition was studied based on the specific reaction of differential plants of the Cucurbitaceae family in response to infection. Immunological screening of cucumber lines for resistance to the causative agent of powdery mildew was carried out under introduced infection by ranking into resistance groups on a five-point scale. Based on the presence of fibrosin bodies in the conidia of the studied isolate, lateral germination of the germ tube after 12 h, and linear dimensions (length = 30.5 ± 3.1 μm, width = 17.7 ± 2.7 μm, shape index = 1.8), the pathogen was assigned to the species P. xanthii. The degree of disease development on differential plants was as follows: L. siceraria, C. sativus, C. pepo, and C. melo, R ≥ 60%; C. lanatus and L. cylindrica, R = 0%. The susceptibility response of differential plants indicates the presence of races one and two of P. xanthii. Phenotypic evaluation of breeding lines for resistance to P. xanthii allowed us to isolate new source material (L-143, L-168-2, L-168, L-130, and L-208-2, R = 0%; L-211, R = 0.3%; L-234, R = 8.3%; and L-210, R = 9.6%) for inclusion into the breeding process for creating cucumber varieties and hybrids resistant to powdery mildew.