Foliar nematodes may infect hosta, causing brown areas on the leaf that are delineated by veins. The result is often a brown striping of the leaves. Large, wedge-shaped areas may appear in the leaves. Injury may occur on leaves, buds, or flowers. You won't find any fruiting bodies of fungal spores in this area; the tissue death is the result of a microscopic roundworm, a nematode. This nematode lives in the leaves and crown of the plant. It is microscopic and can be de-tected by teasing apart leaf tissue with forceps while observing with a dissecting microscope. Leaf tissue soaked in water overnight will yield the nematodes as well.
These nematodes reproduce within the leaves. They move in and out of the stomata and may be on the leaf surface if there is a film of water. They can be splashed or moved from plant to plant on anything wet, even on people. They may also move into the soil and cause problems for some time to come.
We discussed foliar nematodes in issue no. 8 of this newsletter with other hosta problems. In that issue, I stated that it was questionable whether the nematode could overwinter in Illinois. A hosta breeder from New Jersey wrote to tell me that the nematode can survive temperatures as low as 40 degrees below zero. He said that foliar nematodes are a huge problem to nurseries and home gardeners alike in the Northeast.
Another grower in Minnesota wrote to tell me that the nematode can survive Minnesota winters. It seems clear that the nematode can overwinter in Illinois and that we should be inspecting plants very carefully for nematode symptoms. Keep an eye out for this nematode problem, and I will keep you posted as confirmed cases are found in Illinois.