Foliar nematodes (Aphelenchoides spp.)
Foliar nematodes are extremely small - approximately 0.8 mm long - colorless roundworms. They need a film of moisture to move around.
Foliar nematodes feed within the plant's spongy mesophyll and spend most of the growing season in leaves. The nematodes cannot move across main leaf vains while inside the leaf, so infected areas of leaves are bounded by the veins. Foliar nematodes may overwinter on debris and in the crown of plant buds. They may infect a number of herbaceous perennials, including hosta, astilbe, lilium and delphinium.
Management: Dispose of all infected plants and plant material. Avoid splashing water on susceptible plants. Selectively remove leaves from plants just expressing symptoms of foliar nematod damage. Insecticides with translaminar properties may be useful in controlling foliar nematodes.