Overview
Podosphaera xanthii is a fungal pathogen that causes powdery mildew disease primarily on cucurbit crops including cucumbers, melons, squash, pumpkins, and gourds. This obligate biotrophic fungus produces characteristic white to grayish powdery growth on leaf surfaces, consisting of conidiophores and chains of conidia (asexual spores). The fungal mycelium develops superficially on plant surfaces, sending specialized feeding structures called haustoria into epidermal cells to extract nutrients.
The disease typically begins as small, circular, white powdery spots on the upper leaf surfaces, which gradually expand and coalesce to cover entire leaves. Heavily infected leaves may turn yellow, brown, and eventually die. Podosphaera xanthii can also infect stems, petioles, and occasionally fruits, causing reduced plant vigor, decreased photosynthesis, and significant yield losses in susceptible cultivars.