Problem: Foliar Nematode (Hosta)
Aphelenchoides fragariae This disease is caused by a nematode that infests hosta leaves. Nematodes are microscopic, clear worms that usually infest plant roots, but this one infests the foliage of many plants. Nematodes enter leaves through the breathing pores. Inside, they feed on the tissues and may kill them. Leaf veins act as barriers until the leaf is wet; then the nematode swims through the water to infect other leaves. Foliar nematodes reproduce rapidly and are most damaging in wet-summer, warm regions of the country. They live in soil and dead plant tissue for years; some plants harbor them with no symptoms.
Remove and destroy infested leaves or plants by putting them in the trash. Do not compost them. Avoid wetting leaves when watering. Mulch the ground under plants; nematodes cannot move on dry surfaces. Remove dead foliage from the garden in the fall. Check new plants carefully for disease, and do not replant hostas in infested soil. Apply insecticidal soap if symptoms appear.