Foliar nematodes live and feed inside the leaves. When plants are wet, they move to the surface and travel in films of water to other plants or plant parts. Their movement inside the leaves is limited by the leaf veins, so spots often have an angular shape with veins at the edges. Defoliation can occur under high nematode pressure.
Fallen leaves contain many nematodes, which can survive in a dried condition for long periods of time. The nematodes become active again when moisture becomes available can move up the stems to new leaves. Early stage of disease can be noted as small circular lesions as seen in the picture.
The lesions enlarge and can cause heavy defoliation
Angular water soaked leaf spots are a key symptom for foliar nematodes.