Charles Overstreet, Xavier, Deborah
Tomato root that is heavily galled by the Southern root-knot nematode.
Many of the vegetables that are planted in Louisiana are subject to attack from plant-parasitic nematodes. The Southern root-knot nematode has been our most problematic nematode pest. Root-knot nematodes are particularly destructive and found in about 25% of home gardens in our state. The reniform nematode is another pest found on vegetables but has not caused nearly as much of a problem. Since vegetables are grown year-round in Louisiana, it is important to know what nematode injury looks like and what producers can do about them.
The southern root-knot nematode is a sub-tropical or tropical nematode found throughout the warmer areas of the world. Root-knot nematode likes warm conditions so it is not active in the soil until soil temperatures warm up to 65°F. Likewise in the fall these nematodes are no longer very active once temperatures drop back down to this level. This leaves a 5-6 month period that these pests are active in our state. Since each female lays hundreds of eggs and a generation may be only 25-30 days-long, large populations can develop quickly in the soil. Root-knot nematode seems to favor soils that are coarse-textured such as loamy sands or some silt loams. However, they can be introduced on contaminated equipment or infected transplants and survive fairly well on all but very heavy soils.
The reniform nematode is also considered to be a sub-tropical or tropical nematode and is also found in many countries around the world. This pest doesn’t like cold weather either and is not active when soil temperatures are below 60°F. It has a very fast life cycle that may be only 18-20 days and often builds up to extremely high levels in the soil. Reniform seems to do slightly better in soils that have more loamy but can very well in sandy to fairly heavy clay soils. This nematode is very tough and is much more difficult to reduce populations by some of the normal practices that are used on root-knot such as tilling to dry out the soil or fallowing.
Both of these two nematodes live in the soil and impact the roots of plants. Most of the damage that is visible above ground is the result of damage that was previously caused to the root system. Root-knot nematode produces a very distinctive symptom on the roots (galling) that makes it easy to recognize. Galls can range in size from being just barely visible on crops such as corn to extremely large as found on butterbeans, okra, or tomatoes. The size of the gall may also be an indication of the number of nematodes present. Large galls have a lot more nematodes present in them than small galls which may have only one or two. The amount of galling that is present may also give you an indication of just how serious a problem exists. Plants with only a few galls may function almost normally and little damage may be visible. Large numbers of galls on the roots of plants can be a good indication that a major problem is present. Ideally, the best thing to do is observe the roots of plants after they are finished producing and look for the presence of galls. A few galls may mean that a problem is building and corrective action should be taken. Severe galling means that future plantings in this location are likely to be seriously damaged and corrective action is necessary. Galling disrupts the flow of water and nutrients in a plant and really expresses damage during dry periods. Increasing available water and nutrients has been one of the management strategies for many years. Plants above-ground may be stunted, wilt early, yield poorly, or die prematurely. When you discover plants that fit any of these foliar symptoms, you need to be sure to check out the root system to look for visible galling.
Reniform nematode is much more subtle than the root-knot nematode and can easily be misidentified. It doesn’t produce any distinctive symptoms on the roots. The large numbers that attack the roots weaken them and slow down growth and fruiting potential of many vegetables. Fortunately, fewer vegetables are attacked by reniform than the Southern root-knot. This makes it a lot easier to develop rotational plans for management. Above-ground symptoms are usually stunting and reduced yield.
Damage by nematodes usually only occurs when high numbers of the nematode are present at the time of planting. Nematode populations have ups and downs each year depending on past crops, growing conditions, winter survival, predators, and parasites. In years where large numbers of these pests survive, damage can be severe on many crops. Once established in a garden, these pests are likely to appear on spring, summer, or fall vegetables.
Just about every type of vegetable that we plant in our gardens can be attacked by root-knot nematode. Those planted and growing during the hot summer months such as tomatoes, okra, butterbeans, squash, cucumbers, and melons are generally some of the most severely damaged. Since the damage is done to the roots of these plants, all most people ever see is that the plants are stunted, wilt easily during dry weather, yield poorly, and die prematurely.
Identifying the problem
Since plant nematodes inhabit the soil, having a soil sample checked for the presence of nematodes is one of the best methods of to determine if there is a problem. Sampling should be done similar to that of soil collected for nutrient analysis or may be done at the same time. About 15-20 soil cores or trowel slices to a depth of about 6-8 inches should be collected from the area that you are checking. Large fields should be broken up into 20-25 acre units to pinpoint specific areas with problems in a field. Gardens, flower beds, ornamentals, or lawns may need to be sampled separately. Nematode samples should be enclosed in a plastic bag to prevent the soil from drying out. Your local county agent can provide the necessary forms to be included with the sample and mail it in for you.
Although nematodes may be present and potentially causing damage, several management options will reduce the levels of nematodes to allow success when growing susceptible crops. Try using a combination of several of these practices if you start having serious problems with nematodes on vegetables.
Resistance There are only a few crops that have varieties resistant against the southern root-knot nematode. Tomato varieties include Better Boy, Big Beef, Celebrity, Champion, Crista, Mountain Fresh Plus, Roma, and Terrific or any other variety listed with a “VFN”. The “N” stands for root-knot resistance. Southern peas are the other major vegetable with resistance. Several Southern pea varieties to use against the root-knot nematode include Mississippi Silver and Mississippi Purple. Even if you are not interested in growing peas for sale or personal consumption, allowing this crop to grow for 6-8 weeks can drastically reduce the numbers of nematodes present.
One of the best methods of limiting damage to vegetables is through rotation. Don’t keep planting the same susceptible vegetables in the same area each year. Although most vegetables are susceptible to root-knot nematode, some are less susceptible. Several plants such as broccoli, beet, Brussels sprouts, cabbage, collards, corn, English peas, garlic, mustard, onion, and potato are less susceptible to injury from root-knot nematode primarily because they are grown during cool weather when this nematode is not as active. Look at the figure below to get some ideas of how to rotate crops.
Figure 1. Example of crop rotation within a growing season to reduce root-knot nematode damage to sensitive vegetables.
Soil solarization is a method of using the natural energy of the sun to sufficiently heat the soil to temperatures high enough to kill many soil pests. A clear plastic tarp is used to cover the soil, trapping the heat beneath. Pests that may be killed by solarization include nematodes, plant disease organisms, and weeds. Most nematodes are killed when the soil temperature goes above 118º F. However, several weeks may be required to sufficiently heat the soil to a depth of 6-8 inches where most nematodes are found. The hottest months are the best time for solarization and include June, July, and August. Because many spring vegetables are finished by May or June, the soil can be treated before it is time to plant fall crops. These procedures should be followed to ensure success with soil solarization.
The high temperatures that develop beneath the plastic tarp will kill many of the common pests found in the soil. Since the temperature of the soil will decrease with depth, several weeks of exposure to the sun are required to reduce pests that are deep within the soil. Solarization can never eliminate nematodes, weeds, or soil diseases but can certainly reduce their incidence. This treatment should allow a successful planting of a susceptible crop for at least one growing period. Currently, there are no nematicides available for use in the home garden. Solarization can be a useful management tool to protect vegetables in the garden.
Included under this category are such things as weed control, crop residue destruction, and disinfection of equipment. Since many weeds are hosts of nematodes such as root-knot, it is important that management practices include a good weed control program. Plants should be tilled up or removed as soon as they are through producing to prevent any further nematode development. Root-knot nematodes are sensitive to desiccation so tilling and drying the soil during hot weather may help.