Foliar nematodes are not very common in the Iowa landscape, but the ISU Plant & Insect Diagnostic Clinic does receive samples occasionally. Nematodes are worm-like organisms that have a wide and varied diet. Many species are predaceous and feed on other nematodes and some species are used for biological control of insect pests. Many other species feed on fungi or bacteria. Nematodes that feed on plants are considered plant parasitic nematodes and some of these species are considered pests, including foliar nematodes.
If you grow soybean you may be familiar with the soybean cyst nematode, which is also a plant parasitic nematode, but it has a very different life than foliar nematodes. Pine wilt is another case of a plant problem caused by a plant parasitic nematode.
Signs and Symptoms
Foliar nematodes, like other plant parasitic nematodes, have a piercing-sucking mouth (stylet) that allows them to pierce into plant cells and consume the contents. Foliar nematodes kill the plant cells they consume and enough nematodes feeding on a leaf can cause visible dead spots. Foliar nematodes will feed between the leaf veins so the pattern of dead spots will match the leaf vein pattern. A recent sample of foliar nematodes on a coralbell shows the unique pattern left by the nematodes feeding between the leaf veins. A microscope is needed to confirm a diagnosis of foliar nematode. Cut leaf tissue is placed in a drop let of water and the nematodes are easily seen wriggling out of the leaf tissue into the water.
Foliar nematodes can feed on a wide variety of plants in the landscape and so it is important to avoid bringing them into your yard. It is always good to closely inspect any new plants carefully for insect or disease problems before purchasing them.
If you have foliar nematodes there are some strategies that can help minimize the damage. First of all consider removing and destroying infested plants completely from the landscape or moving them away from plants not showing symptoms. Remove and destroy any leaves with symptoms. Remove and destroy dead leaves in the fall as the nematodes overwinter in the leaf debris. Foliar nematodes move in water and move up onto leaves by splashing and the film of water on leaves, so do not overhead water.