Nematodes are microscopic worms that are free-living or parasites of plants or animals. Most plant parasitic nematodes live in the soil and attack plant roots. Foliar nematodes, in contrast, feed on the aboveground parts of plants. Recently, a chrysanthemum infected with the foliar nematode, Aphelenchoides ritzemabosi, was submitted to the Plant Disease Clinic.
The chrysanthemum foliar nematode measures about 1 mm long, invisible to the naked eye, but easily visible under a microscope. Plant parasitic nematodes have a structure in their mouth end, called a stylet, which is used to puncture plant cells and obtain nutrients.
Symptoms. The symptoms produced on chrysanthemum leaves are fairly characteristic. Initially, yellow to brown spots develop that are contained between the larger leaf veins, giving the appearance of fan-shaped or V-shaped lesions. Eventually, entire leaves turn brown and become brittle. The disease tends to progress from the lower to the upper leaves.
Disease cycle. Nematodes overwinter in dead, infested leaves on the ground or between the scales of infected buds. In the spring, the nematodes become active and swim up the plant stems. The stems must be covered with a film of water, from rain, irrigation, or humidity, in order for the nematode to move upward. The infected leaves then develop spots that coalesce to form the large V-shaped lesions. The leaves from infected buds may be small, distorted, and fail to develop. Eventually infected plants parts collapse, turn brown, and die. The dead and infected leaves that fall to the ground can then serve as an overwintering source of inoculum to repeat the disease cycle.
Control. There are several measures that can be taken to control this disease. Sanitary practices are especially important. Remove and destroy plant tops and debris in the fall. Space plants to allow for good air flow so leaves dry quickly after rains. Avoid overhead irrigation. Mulch the surface around the chrysanthemums in the spring to cover any infested debris that may have been left on the soil surface. If symptoms are observed during the season, remove and destroy diseased leaves and several healthy leaves above the infected area.
Research is currently being conducted to determine whether the use of systemic insecticides will aid in control of the nematode.
This article originally appeared in the November 11, 1994 issue, p. 155.
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