Symptoms
The initial symptoms include stunting of the plant and wilting and yellowing of the foliage. These symptoms are subtle and may be overlooked unless compared to adjacent healthy plants. On severely infected plants, the symptoms are more obvious. Symptoms are often restricted to scattered, uneven patches in the field. Although these symptoms are indicative of many diseases, root knot nematode can be distinguished by observing the roots for swellings or galls. These galls interfere with water and nutrient uptake and result in the above-ground symptoms observed on infected plants.
Yield losses are determined by population levels of the nematode in the soil and the extent of other stresses. Because of the patchy distribution of the nematode in the field, yield losses across a field will be highly variable. Losses in highly infested areas may exceed 75%, while other areas may not suffer any losses.
Causal Agent and Disease Development
Root knot nematodes (Meloidogyne incognita) infect seedlings at any stage of growth. Above-ground symptoms are usually not visible until mid to late season during flower initiation and boll set. Galls form within a few weeks of planting but usually are not readily visible until mid-season.
Distribution of the root-knot nematode in a field is usually in uneven clumps. The nematode occurs in every soil type but favors sandy, coarse soils. Soil sampling is the best method to determine if root-knot is present in a field.
Control
Management of root-knot nematode is accomplished by crop rotation with non-host crops, planting resistant cultivars or applying nematicides. Peanuts are not a host for the root-knot nematode and make an excellent rotation crop. Grain sorghum and root-knot resistant soybean cultivars are also effective in lowering root-knot levels. Susceptible soybeans, corn and most vegetables, along with most of the winter legume cover crops, are susceptible to the root-knot nematode. The winter grains like wheat oats and rye are also hosts, but nematode infection and reproduction are reduced because of low soil temperatures. Fallowing is not effective because many weeds are hosts.
There are no cultivars with high levels of resistance to the root-knot nematode, but some varieties have moderate levels of resistance. Consult recent variety publications to identify varieties with the highest level of resistance.
In highly infested fields with the potential for severe losses, the application of nematicides will reduce soil populations and improve yields. These materials provide a zone of protection around the developing roots of plants the first four to six weeks after planting and permit the establishment of a healthy root system. Temik and Nemacur are effective non-fumigant nematicides, and Telone II is an effective fumigant nematicide. Non-fumigant nematicides are applied at planting in the furrow or may be side-dressed, and fumigant nematicides are applied seven to 14 days preplant.