By: Bonnie L. Grant, Certified Urban Agriculturist
Garden diseases are the bane of any gardener, especially when they threaten our food crops. Root knot nematodes in carrots are a major pathogen which also affects other food crops, such as onions and lettuce. Carrots affected by root knot nematodes display malformed, stubby, hairy roots. The carrots are still edible, but they are ugly and distorted with tough galls and thick skin. Additionally, root knot nematodes reduce yield. Root knot nematode control is possible through several corrective measures.
If you have discovered your carrot patch is yielding stumpy, rough looking, forked roots, you probably have a root knot nematode infestation. This pathogen is common in root crops but also in celery and lettuce. Symptoms vary slightly by plant variety, but in all cases crop production is reduced and the food looks unsightly. Root knot nematodes in carrots are particularly prevalent in northern regions. The good news is that you can control the pathogen to a certain extent.
Nematodes are tiny roundworms not visible with the naked eye. It takes a soil sample to positively identify the pests. They live in soil and feed on plant cells through several developing stages. The second stage juveniles are the only mobile stage and enter the roots. Later stages and adults become larger as the carrot root enlarges.
Any stage of nematode feeds on cells by piercing it with a mouthpart called a stylet. Females will break through the root and deposit eggs, which form galls. These become woody, hard and nearly unpalatable. There are almost 90 different nematode species that are directly associated with carrot development.
Recognizing parasitic nematodes in carrots is fairly obvious once you dig the roots up. On the surface of the soil, the foliage will be stubby and not well formed. Occasionally, it will also wilt. The roots will split and fork, looking like bizarre caricatures of possessed carrots. Some interesting forms appear but, overall, carrots affected by root knot nematodes will produce less edible roots, which are stunted and ugly.
In commercial growing, this represents less dollar yield and the roots collect more soil, requiring more extensive cleaning before the roots can be marketed. In the home garden, the less attractive roots can still be used, but some parts will be woody and preparation is more intense as opposed to the easily cleaned and peeled roots that are uninfected.
The most common treatments are crop rotation and allowing a field to lay fallow. Good sanitation practices such as cleaning machinery and tools is also useful. In some cases, solarization for 4 to 6 weeks can kill populations of some nematodes.
There are also several resistant crops that may be planted or a non-host plant can be installed. Such plants might be rye, fescue, corn, wheat, barley or sorghum. At this time, there are no resistant carrot varieties, but trials are underway and very soon these should be released.
There are a few soil fumigants that can be used up to 6 weeks before planting. They can be quite effective when used properly.
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Most gardeners are familiar with problems caused by diseases and insects because such problems are easily recognized. Few gardeners, however, are aware of the damage nematodes can cause.
Nematodes are microscopic (less than 1 mm long), wormlike animals too small to be seen with the unaided eye (figure 1). The majority of plant parasiticnematodes live in the soil and damage plants by feeding in large numbers on the roots, impairing the plant’s ability to take up water and nutrients. Severe root damage (figure 2) caused by nematodes typically results in aboveground symptoms that may include stunting (figure 3), yellowing of leaves (figure 4), loss of plant vigor and/or an overall general decline in plant performance. Damage is often more pronounced when plants are under other stresses such as lack of water or nutrients or when damaged by other diseases or insects. Although nematodes rarely kill plants, they can drastically reduce plant growth and yields. Nematodes are usually confined to localized areas in the garden spreading very slowly under their own power however, nematodes may be dispersed more rapidly by the movement of infested soil through cultivation, on soil clinging to garden tools and tillers, in water, or on roots of transplants.
Though there are at least 10 different genera of plant parasitic nematodes found in Alabama, the most important is the root- knot nematode. Root-knot nematodes have a wide host range, but the most serious problems occur on broadleaf crops. Root-knot nematodes attack the roots of plants causing distinct knots, swellings, or galls to form on the infected roots (figure 5). Galls may grow as large as one inch in diameter when they merge, but usually, they are not much larger than a pea. There are several different species of root-knot nematodes in Alabama and several may be present in any one location depending on which crops were grown previously (different species have different host ranges), sources of contamination, and geographical region in the state. Nematode species other than root-knot nematodes can cause damage to vegetables. These species include dagger, reniform, ring, stubby root, stunt, sting, root lesion, and cyst nematodes. With the exception of the cyst nematode, which produces distinctive egg-containing cysts on roots, the identification of these other nematode species requires laboratory analysis.
Nematode management requires long-term planning. No current control practice will permanently eradicate nematodes from the garden. Nematodes can be effectively managed in the home garden by the use of one or more of the following practices.
|Goosefoot||beets, Swiss chard, spinach|
|Composite||chicory, dandelion, endive, lettuce, marigolds, sunflower|
|Mustard (Crucifers)||alyssum, bok choy, broccoli, brussels sprouts, cabbage, cauliflower, collards, kale, mustard, radishes, turnip|
|Gourd (Cucurbits)||cucumber, gourds, cantaloupe, pumpkin, squash, watermelon|
|Grass||barley, corn, oats, rice, rye, wheat|
|Pea (Legumes)||alfalfa, beans, lupines, peanuts, English and southern peas, soybeans|
|Lily||asparagus, chives, garlic, leeks, onions, shallots|
|Buckwheat||buckwheat, rhubarb, sorrel|
|Rose||brambles, strawberries, apples, peaches|
|Nightshade||eggplant, pepper, petunias, Irish potato, tomato|
|Parsley||carrot, celery, chervil, dill, parsley, parsnip|
Cause Meloidogyne hapla , a nematode that also attacks other vegetable crops. The Columbia root-knot nematode, M. chitwoodi , is not an economic problem on carrots. Root-knot nematodes are sedentary endoparasites only second-stage juveniles (the infective state) and adult males (which may be rare) are in the soil. Northern root-knot nematodes are very damaging to carrot, and the tolerance is very low, so proper nematode sampling is essential.
Symptoms Localized areas of stunted plants. Infected carrots are typically forked and malformed. Numerous knots can be found on the tap root and secondary roots. When carrots are left in the soil for long periods in fall and winter, nematodes often enter lenticel areas and cause large galls.
Sampling Before planting, take soil samples with sufficient lead time to implement management procedures if necessary. For example, after soil fumigation, it may be several weeks before crops can be seeded. Fall sampling for planting in the following spring is an excellent strategy. When sending samples to diagnose a suspected root-knot problem, it is best to include carrots and roots as well as soil.
Chemical control Preplant soil fumigation.
Root-knot nematodes (Meloidogyne spp) are parasitic roundworms which can damage vegetable, fruit, and ornamental plants. These nematodes cause swellings in the roots which will result in stunted growth, wilting or death of the plant. The infected plants are more susceptible to fungal and bacterial attack as well. Treating nematode infections in plants is difficult and hence prevention of infection is the right option. The important methods to control the infection of root-knot nematodes in your garden are discussed here.
The natural way to protect your garden plants from nematode is to plant nematode cover plants before planting the garden plants. These cover plants release substances which are toxic to nematodes and hence repels nematodes. Growing plants which are resistant to nematodes will reduce the ability of nematodes to feed and reproduce in the soil. This will result in the lower number of nematodes in the soil while growing garden plants susceptible to nematodes. The painted daisy (Chrysanthemum coccineum) and French marigold (Tagetes patula) are highly effective as nematode repellent cover plants for carrots and other vegetables. Space the cover plants about seven inches apart and allow them to grow for about two months. After two months or before the flower heads form the seeds, mix the plant in the soil. This will improve the toxic substance content in the soil for nematodes.
There are vegetable and fruit plants which are resistant to nematodes causing root–knots. Planting resistant varieties in the garden can ensure good crop even when nematodes are present in the soil. Nematode-resistant varieties of beans, peas, red and green peppers, tomato, etc., are available at the garden centres or at the online stores. This method will be more effective if combined with crop rotation or nematode cover plants. Make sure that the same type of vegetable plants or fruits is not grown in the same site every season.
Crop rotation is the practice of growing different varieties of plants in a site each year. Planting nematode susceptible plants in a new location in your garden each year ensures that they are not attacked by the parasites. Succession planting or growing nematode susceptible and nematode resistant varieties in alternate seasons are also effective.
Root-knot nematodes need moist soil conditions for their growth, movement, and reproduction. Removing the roots of plants from the soil after each season and tilling the soil for two to three times before planting seedlings in the garden bed is very effective in controlling nematode growth in the soil. Tilling exposes any plant roots in the soil and will get dried by the sun. This will reduce the chance of nematode growth in the soil. Exposing the soil to sun or solarisation eliminate the pests and weeds in the soil. Gardeners can use thin and transparent plastic sheets to cover the soil and to increase the soil temperature in order to destroy any nematodes and other pests in the soil. It is ideal to raise the temperature to 120 F for better results.
Some plants which are used as green manures can kill nematodes present in the soil. Painted daisy, castor bean, ornamental crotalaria, and rapeseed can be used as green manure to kill nematodes in the soil.