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Foliar Nematodes

June 13, 2001

The words “plant nematode” generally bring to mind a root-related problem in Illinois. We also have some nematodes that cause problems to aboveground plant parts in this state. Pinewood nematode is a very common example. The foliar nematode of hosta has been identified occasionally in Illinois, and it is an up-and-coming problem in the landscape business. Foliar nematodes can overwinter here and may infest many herbaceous hosts.

Nematodes are microscopic unsegmented worms that live in plant stems, leaves, and roots. Foliar nematodes include several species from the genus Aphelenchoides that commonly feed inside leaves of hostas, as well as chrysanthemums and begonias. Many other perennials may also be infested, including anemone, creeping phlox, ground ivy, windflower, heuchera, and others. Plant infection occurs when the nematodes migrate from the soil surface up to the plant stems and onto leaf surfaces in a film of moisture. They enter leaves via stomates and begin feeding on parenchyma cells. The leaves eventually die and fall to the soil surface, where they are the source of reinfestation.

The most distinctive field symptom of foliar nematodes is lesion formation limited by leaf veins. Leaves with parallel veins display stripes of necrotic tissue; and, in plants with netted veins, the lesions have the appearance of an angular leaf spot. As damage increases, the tissue becomes necrotic and dries out, leaving a large dead patch of tissue. Leaf blotches progress down and outward in the plant from upper leaves. In plants with angular veins, these dead areas have a fanshaped pattern that is yellow-brown to gray in color. Often there is a Vshaped, damaged area bordered by leaf veins. These symptoms should alert you to the possibility of foliar nematodes, but similar symptoms can also be caused by bacterial pathogens.

Although foliar damage is seldom lethal, plant aesthetics are ruined, and vigor is reduced. Plants under nematode stress are more susceptible to damage from other diseases and unfavorable weather conditions. Confirming foliar nematodes is relatively easy. Cut symptomatic leaf tissue into 1-inch squares and soak the leaf tissue in a shallow bowl of water. After 24 hours, the nematodes should be visible with a 10X hand lens. They will appear as whitish, moving strands. Rotting tissue can be infested with sapro-phytic nematodes so, when you are in doubt, consult a specialist.

Nematodes are disseminated by water. This includes rain, dew, overhead sprinklers, wet gardeners, wet gardening equipment, and possibly wet animals. It is always best to work with plants when they are dry. Nematodes can also survive in a dormant state for up to a year in leaf debris in warm climates. In Illi-nois, the nematode probably survives in the protection of the crown of plants protected by mulches. The best pest management practices we can recommend are:

    1. Buy disease-free (symptomless) plants and propa- gating stock.
    2. Do not water foliage. Water the soil.
    3. Remove and destroy infected plants.
    4. Disinfect tools if you are working with suspect plants.

Foliar nematodes are extremely difficult to eradicate once they are established. Follow the above suggestions to avoid a problem. Nematicide recommendations are not available. Consult the disease report RPD no. 1102, Foliar Nematode Diseases of Ornamentals, for more details. The RPD is available in Extension offices or on the web at http://www.ag.uiuc.edu/~vista/ horticul.htm.

Author: Nancy Pataky Molly Pate


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