Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
The University of Delaware is currently conducting research on root knot nematode risk assessment and management in lima beans. As this project progresses, we aim to bring new tools and information on the best ways to manage this pest.
While we have had a cold winter and considerable soil freezing, it is still unclear how root knot nematode populations will be affected in 2014
There is good news for vegetable growers on the nematode front. There are several new nematicides being trialed for vegetable crops. For example, NIMITZ™ (fluensulfone) nematicide, manufactured by MANA, is expected to receive federal approval later in 2014. NIMITZ is a broad-spectrum, non-fumigant nematicide, which is highly effective in controlling plant-parasitic nematodes, providing particularly good control of root-knot nematodes. It has been effective ahead of crops such as tomatoes. Initial registration will be for fruiting vegetables and cucurbits. Registration for carrots, strawberries and other crops is expected to follow.
While this is good news on chemical front, rotations also should be considered in managing root knot nematode. One issue is that several cover crops such as crimson clover and hairy vetch can maintain and increase populations over the winter and early spring. Some winter annual weeds are also hosts.
Some growers in the region have started using nematicide seed treatments (Avicta, Votivo) on field corn. Some research studies have shown increased corn yields with nematicide seed treatment products in root knot infested fields while others have not; however, root knot nematode populations later in the season have not been different from the controls. Therefore it is unclear if seed treatments in corn will have any benefit in reducing root knot populations for following vegetable crops. See the following reports for more information: http://www.aces.edu/timelyinfo/PlantPathology/2011/March/pp693.pdf
https://sites.aces.edu/group/timelyinfo/Documents/2014%20Corn%20Nematicide%20TI.pdf
Concerning rotations, I thought that it would be a good time to reprint information that Bob Mulrooney, retired Extension Plant Pathologist, wrote in 2011 on managing root knot nematodes with rotations:
“Root knot nematodes are microscopic roundworms that are widely distributed in Delaware and Eastern Shore Maryland 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, snap beans, and lima beans due to compromised root systems evidenced by galling on the roots. Root knot populations are favored by the sandy, low organic matter soils. There are several species of root knot nematodes that could occur here but Southern root knot nematode, Meloidogyne incognita, is the one most commonly found. Most of the field crops grown in Delaware; corn, small grains, sorghum, alfalfa, sorghums, and grass hay are not good hosts of root knot and can reduce populations of root knot, but not eliminate them.”
“Small grains (wheat, barley, oats, rye, triticale) are a good rotation crop because if planting is delayed until soil temperatures at planting depth are below 65°F, when 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.”
“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 resistant varieties still are limited in group 4 soybeans. 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. 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.”
“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. As a caution, 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. 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.”
If you are sampling in the spring, a bioassay would be a better way to detect root knot nematodes. See http://extension.udel.edu/weeklycropupdate/?p=6586 for information on how to conduct a bioassay.