Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Root knot nematodes are microscopic roundworms that are widely distributed in Delaware agricultural soils and can cause varying degrees of damage to susceptible crops. Most of the damage caused by root knot nematodes is evident as stunting on vegetables such as pickling cucumbers, cantaloupe, watermelon, squash, and lima beans. Root knot populations are favored by the sandy, low organic matter soils in Kent and Sussex counties. There are several species of root knot nematodes that could occur here but Southern root knot nematode, Meloidogyne incognita is the most commonly found root knot species here. Most of the field crops grown in Delaware; corn, small grains, sorghum, alfalfa, Sudan grass, and Sudex are not good hosts of root knot and can reduce populations of root knot, but not eliminate them.
Small grains are a good rotation crop because if planting is delayed until soil temperatures at planting depth are below 65°F, root penetration does not occur. Soil temperatures are generally too low during most of the small grain growing season for root knot to increase. Small grains are poor hosts to begin with and the crop basically avoids infection or penetration by root knot. In my long career here I have never seen root knot nematode affect small grains (wheat, barley, oats, triticale, etc.).
Unfortunately soybeans, unless they are a resistant cultivar, are susceptible to root knot nematode. Soybeans can tolerate low populations of root knot without producing visible symptoms but yield loss can occur depending on growing conditions, especially low rainfall. High populations and adverse growing conditions can cause stunting as severe as that produced by soybean cyst nematode. Root knot resistance has been available in soybean for a long time but it has been in group 5 or later maturity groups. Recently, advances have been made to incorporate resistance into group 4 varieties. There are a few resistant group 4s available that were posted on the VIPS website (http://www.vipsoybeans.org): Schillinger 479.RC, Southern States RT 4470N, HS HiSoy FS 41T80, FS HiSoy HS 4426, FS HiSoy HS 46T80. There should be an updated list on the VIPS site soon of soybean varieties rated in 2010. Another source of information is a soybean variety selector from North Carolina. There are no group 4, only group 5 resistant soybeans on this site http://www.soybean.ncsu.edu/soyvar/. Root knot resistant soybeans would be an excellent rotation crop for vegetable growers who plant susceptible fresh market or processing vegetables.
Field corn varies in its susceptibility to southern root knot nematodes. There is no current data on corn hybrid screening for resistance to root knot nematodes. Earlier studies indicated that there was wide variation in susceptibility to root knot in corn. A test conducted at Auburn University in 2009 of hybrids grown in the South showed that none were resistant. Irrigated corn is not likely to be damaged by low to medium root knot populations, but can support damaging population levels if followed by a susceptible vegetable crop. It might be very difficult to even see symptoms of high root knot populations in field corn especially if it is irrigated. Nematode numbers in corn seem to be increasing but it is difficult to know for sure. It has been thought that the increase in Bt corn and the shift to pyrethroid insecticides has had an impact. Growers are not using the granular and liquid carbamate and organophosphate insecticides at planting which would suppress nematode populations as well as control the target insects.
Managing Root Knot Nematodes
Rotation is often a limited control strategy for root knot because it has such a wide host range. Alfalfa and oats are thought to be the safest crops to use in a rotation to reduce root knot nematodes. Increasing organic matter in fields with low organic matter and high levels of root knot or other plant parasitic nematodes can have a suppressing effect on root knot populations. Fall planted rape and other mustards may also be useful to suppress root knot populations as a biofumigant when they are plowed under before they go to seed in the spring prior to planting the crop. Rape can be infected with root knot if populations are high and soil temperatures are above 65°F at planting or the fall is warm. (See under small grains above). It is the decomposition of the plant parts when tilled into the soil that releases the chemicals that kill the nematodes, not root exudates from living plants. Soil sampling in the fall right after harvest is the best way to know if you have high root knot populations in your soil. Unfortunately spring is not the best time to sample because the nematode overwinters primarily as eggs which are not detected in the methods used for processing soil samples for nematode analysis. Soil sampling this time of year can underestimate the number present or not detect low populations.
Symptoms
Typical stunting from root knot nematode in an irregular area of a lima bean field
Heavily galled and stunted lima bean roots dug from an infected area. Soybeans can be this badly infected as well. Corn and other field crops produce much smaller and less evident galling