Managing Nematodes in crops

Nematodes are among the most destructive and problematic pests for growers around the world, severely impacting crop development and yield. This nearly invisible killer affects a variety of crops around the world and is responsible for an estimated $80 to $125 billion of annual plant losses on a worldwide scale. Crops in serious danger include fruiting vegetables, cucurbit crops, carrots, strawberries, tobacco, potato, sugar cane, soybean, leafy vegetables and turf grass, among others.

Reducing nematode populations would result in more food production around the world, and thus represent a large step toward fighting crop losses, ensuring ample food supply, and stimulating farmers’ economic growth.

A variety of nematodes exist, both harmful and beneficial. The good nematodes include those that feed on bacteria harmful to plants. Another type of beneficial nematode includes those that fight fungus by puncturing the cell walls of the fungi and feeding on the internal contents, thus destroying it. There are also nematodes that attack various damaging insects. On the opposite end of the spectrum, predatory nematodes exist that feed on plants. 

The Impact of Nematodes:

Nematodes can be found just about anywhere. The amount of biological damage they cause to the plants upon which they feed, and the resulting economic damage, is cause for serious concern. Besides reducing crop yields, nematodes can affect the external appearance of the harvested crops. Nematode-infected potatoes appear lumpy and unattractive, carrots can appear forked and thus unsalable, and tomato plants will have withered leaves, leaving the tomato crop to get scalded by the sun. In the case of turf grass, nematode-affected areas simply can appear as withered and brown patches in an otherwise green, lush lawn. In addition to the direct damage they cause to crops, nematodes, even in low populations, enable easy penetration of other soil diseases to plants’ roots. 

Most plant parasitic nematodes, especially root-knot, get into the garden by either infested soil or infested transplants. Infested soil is easily moved from one garden to another on equipment — not only tractors and implements but also hand tools such as shovels, hoes and rakes. Shoes or boots can carry soil that contains nematodes. Wheelbarrow and vehicle tires can also move nematodes, especially when muddy. Thousands of root-knot juveniles can be present in a tablespoon of soil. Before moving equipment and tools from a known root-knot infested garden to a noninfested one, power-wash off all traces of soil or disinfect by washing with a 10 percent bleach solution.

Harmful nematodes such as root-knot can get into a garden through the generosity of neighbors or family members who share their favorite vegetable or flower transplants. Careful inspection of the roots of transplants for root-knot galls is helpful, but often on young roots they may be too small to be seen. Usually, transplants purchased from a reputable dealer will be free of root-knot nematodes.

Another source of nematode-infested soil is improperly or partially decomposed compost. Roots infested with root-knot galls should be destroyed and not composted. The compost pile needs to be properly managed to ensure that harmful nematodes do not survive. Heat generated from decomposition and winter temperatures can kill root-knot nematode, but not always. Care should be taken when adding compost back into the garden to ensure that nematodes and other soil-borne pathogens are not reintroduced and spread throughout the garden. For more information on composting

Plant Roots:

Root-knot nematode symptoms on plant roots are dramatic. As a result of nematode feeding, large galls or "knots" can form throughout the root system of infected plants. Severe infections result in reduced yields on numerous crops and can also affect consumer acceptance of many plants, including vegetables . The degree of root galling generally depends on three factors: nematode population density, meloidogyne species and "race," and host plant species and even cultivar. As the density of nematodes increases in a particular field, the number of galls per plant also will increase. Large numbers of nematodes penetrating roots in close proximity also will result in larger galls. Meloidogyne hapla (the northern root-knot nematode) produces galls less than half the size of those produced by M. incognita (the southern root-knot nematode) on the same plant hosts. Finally, each crop responds differently to root-knot nematode infection. Carrots typically undergo severe forking with galling predominantly found on lateral roots. Root-knot nematode galls on lettuce are beadlike . On grasses and onions, galls are usually small and barely noticeable, often no more than slight swellings . Depending upon the crop affected and the severity of infection, these symptoms can often result in significant economic losses to growers.

Management of nematode diseases:

If nematodes are causing damage, use one or more of the following management measures:

·         Crop Rotation

·         Early-season cropping

·         Organic matter

·         Soil analysis

·         Other Cultural Practices

·         Chemical Control

Crop Rotation:

Plant parasitic nematodes overwinter in the soil or in association with plant material. Crop rotation and weed control are very important in managing plant parasitic nematodes. Root-knot nematodes have a very wide vegetable, field crop, and weed host range. Soybean cyst nematodes have a much narrower host range, but when both nematode species are present, a rotation ideal for soybean cyst nematode reduction may favor buildup of root-knot nematodes.

Early-season cropping:

Lettuce, onions, radishes, leafy greens, green peas, early beans, and cabbage (and related plants) can be planted early and escape serious nematode damage. These spring-planted crops grow when temperatures are too cool for root-knot nematode reproduction and activity. The plants are harvested before nematode damage becomes serious. However, late-summer plantings of some of these crops for fall production can be severely damaged by nematodes. Nematode damage permanently retards growth and development of these plants.

Organic matter:

High soil organic matter helps retain moisture and adds to the available plant nutrients. Increased water and nutrients help plants fight nematode attack. Organic matter in the form of peat, manure or compost will increase the decay of organic matter, releasing nutrients through microbial action. The increased level of microbes in the soil favors the build-up of organisms that feed on all soil microbes, including nematodes. When adding compost, ensure that it does not include partially decomposed roots containing plant-parasitic nematodes or other soil-borne pathogens.

Organic matter can also be introduced into the soil through planting a green manure crop such as the legumes clover or vetch, or nonlegumes, such as rye. When used as green manure crops, these crops are planted in the fall or early spring and tilled into the soil before planting. Some evidence indicates the incorporation of this green manure crop produces compounds that are toxic to nematodes.

Soil analysis:

The most accurate way of diagnosing a nematode problem is a laboratory soil analysis. A good sample taken between late May and early September is essential for accurately assessing the presence of nematodes, the species involved and their relative abundance.

A good sample consists of several subsamples taken from different areas of the garden surrounding the roots of symptomatic plants. Garden soil containing roots should be randomly sampled if the area has a history of nematode infestation. Each subsample should represent the upper 7 to 8 inches of soil and an area of about 100 square feet. A hand trowel is an adequate sampling tool if the soil is soft or has been tilled recently. To collect a subsample, make a trowel-size hole 7 to 8 inches deep, and then remove a slice of soil from the side of the hole. This slice of soil is a subsample.

Collect the subsamples in a container, and mix thoroughly. From this mixture, remove about 1 pint as the sample. Promptly seal the sample in a plastic bag and submit for analysis. If shipment is delayed, keep the sample out of the sun and reasonably cool.

Other Cultural Practices:

Adequate water and fertilizer can minimize plant parasitic nematode damage. Plant parasitic nematodes reduce the plant root system’s ability to take up water and nutrients. Adequate water and fertilizer do not reduce nematode density but help plants to cope better with nematode damage, and might increase yield and reduce the symptoms of nematode damage.

Anything that moves soil can spread plant parasitic nematodes to other fields and within the same field. Thus, preventing infested soil and plant material from infesting fields will help with nematode management.

Chemical Control:

Seedling diseases, root diseases, and vascular wilts caused by soilborne fungi and nematodes can be destructive problems in the field and greenhouse. Soil-applied fumigants or nematicides may help prevent serious losses soilborne disease when combined with long term management practices. Soil fumigants are chemicals that are injected into the soil and emit toxic fumes that penetrate air spaces in the soil and kill microorganisms. Fumigants must be sealed into the soil with water or a plastic tarp to ensure that a lethal concentration and exposure time. Because fumigants are harmful to all living plants, a certain amount of time (from two weeks to two months) must pass between treatment and planting to avoid crop damage. Several nonfumigant nematicides are available for several vegetable crops. These generally are systemic compounds that also may provide good insect control. A number of factors affect the performance of these products, including soil temperature, soil moisture, soil tilth, organic matter, soil type, and time of application. Consult the product label for specific details on safe handling and application methods.