Nematodes are microscopic, unsegmented, wormlike animals that live in water, soil, plants, and even in animals. A few nematodes are plant parasitic. Still fewer species feed on the foliage or in the stems of plants. One nematode that we have discussed in detail is pinewood nematode, the pathogen causing pine wilt (refer to issue no. 13). The foliar nematode is a less common plant parasitic nematode in Illinois. We see this occasionally on hostas and some other perennial garden plants. In recent years, the foliar nematode has been a problem in hosta production, so you will want to become familiar with its symptoms.
The foliar nematodes that we see in Illinois are Aphelenchoides ritzemabosi and A. fragariae. They only attack plant parts that are above ground, but reports indicate that they can overwinter in the crown of perennials. It is likely that our cold winters and droughty summers are our best management for this nematode, but it is still a serious pathogen in mild, wet years or indoors. The nematodes can be spread in cuttings (stems of chrysanthemums, leaves of African violets) and can move from plant to plant in splashing water or through leaf contact. Although the nematode is inside the foliage, it easily moves out to the leaf surface if water is present. It can move around the leaf and back in leaf pores. Moisture is essential to movement of the nematode. You can guess that the recent mild winter and wet spring and summer have been conducive to the growth of this nematode.
The most common symptom caused by foliar nematodes is a wedge-shaped necrotic area that is bordered by the leaf veins. The area may first appear yellow or reddish but becomes brown. Hostas, ferns, begonias, African violets, and chrysanthemums are a few hosts that may show this symptom. Foliar nematodes may also feed in the buds and growing points of some plants, including chysanthemums, lilies, African violets, and others. Plants may be stunted and bushy from apical death and regrowth of side buds, and leaves may be crinkled and distorted. There are many hosts of foliar nematodes. Consult Report on Plant Diseases No. 1102 for details, but common garden plants often infected include chrysanthemums, columbines, coral bells, and hostas.
This nematode can be seen with a dissecting scope or even a good hand lens. Cut up the dead area with some scissors and soak in a shallow glass dish. Use a light source under the dish and observe the nematodes swimming around in the water. It takes many nematodes to cause injury, so there should be many clear, wormlike animals moving around in the extract. Because nematodes can also help decay plant material, you will want to have the identity of your nematodes confirmed by a nematologist. Our nematologist is Dale Edwards, who can be reached at (217)244-2011.
In a home garden, this nematode might be controlled by removing affected leaves, but more than likely affected plants will need to be rogued. Often winter temperatures take care of the rest. Indoor plantings that are affected also need to be removed and care taken to avoid splashing between plants when watering. Of course, inspect any new purchases carefully for symptoms of foliar nematodes. Buy only vigorous-looking plants with good foliar color.