Leaf spots of summer squash

Bacterial spot, caused by Xanthomonas cucurbitae, Pseudomonas syringae pv. lachrymans, and Erwinia persicina, manifests as water-soaked spots that transition into necrotic areas, often delimited by leaf veins. Fungal lesions include alternaria leaf blight (Alternaria cucumerina, Alternaria alternata) with characteristic concentric zonation and downy mildew (Pseudoperonospora cubensis), manifesting as angular yellow spots and sporulation on the underside of leaves.

Intraspecific differentiation of pathogens is most studied for downy mildew, which has numerous races with varying virulence. In summer squash breeding, resistance genes such as the Pm and MLO loci are used for protection against spot-causing diseases. It is important to distinguish pathogenic lesions from genetic silvering or virescence, which are varietal morphological traits.