Control of Nematode Populations
Fallow may be used as a preplant nematode control when nematode assay reports indicate a potential problem in the intended planting site. Fallow is the practice of keeping land free of all vegetation for varying periods of time by frequent disking, plowing, or harrowing of the soil, or by applying herbicides to prevent plant growth. At least two principles of nematode control are represented by fallow. The first principle, and perhaps the most important, is starvation of the nematode. Plant-parasitic nematodes are obligate parasites, which depend on living hosts for the food necessary to develop, mature, and reproduce. Therefore, in the absence of a host plant, the nematode will die after the stored food in its body has been depleted. Most plant-parasitic nematodes probably do not survive for more than 12 to 18 months, and many do not survive the first 6 months.
The second principle involved in fallow is death through desiccation and heat. With some exceptions, most species of nematodes will die if exposed to the drying action of the sun and wind. Sometimes, the term "fallow" is used to mean letting the plot go without a crop, but allowing a weed cover to develop to minimize soil erosion. This practice is not suitable for reducing nematode populations since the weeds may serve as hosts for the nematodes.
Although no single crop will act as a "magic bullet" against all nematodes, specific problems may be controlled successfully in preplant situations by planting certain cover crops. Cover crops work for different reasons, depending upon the specific nematode-crop combination. In some cases, the cover crop may be a nonhost for the nematode. Since nematodes do not feed on plants outside of their host range, such plants will starve out the population with the same effect as fallow cultivation. Other plants are known to be susceptible to nematode invasion but prevent the development of larvae into adults. Such plants are referred to as "trap crops."
Crotalaria, for example, has been used successfully in this way to reduce populations of root-knot nematodes. Still other "antagonistic" plant species have been found to naturally exude chemicals toxic to nematodes. French marigolds and asparagus are examples of such crops. While antagonistic and trap crops have been used successfully to control nematodes, little is understood about the principles involved. Clearly, this is an area in which more research is needed.
The use of crop rotation to reduce nematode populations is a very effective and widely used land-management practice. This practice was used by growers long before its significance as a means of nematode control was recognized. To be an effective control practice, crops that are unfavorable, or nonhosts, for nematodes must be included in the rotation sequence. Although many people think of crop rotation as a short-term strategy against nematodes, some long-term crop rotations can be very effective for fruit production. For example, rotations of asparagus, raspberries, and Christmas trees might be an option.
Organic Manuring and Soil Amendments
In some instances, adding large amounts of organic materials to soil results in reduced populations of plant-parasitic nematodes and higher crop yields. The reduction in nematodes is thought to be caused, at least in part, by an increase in natural enemies of nematodes. In addition, the presence of decomposing organic materials in the soil apparently provides host plants with some tolerance to nematode attack. Decomposition products of organic matter and plant residues may also be detrimental, directly or indirectly, to plant-parasitic nematodes, as demonstrated by the butyric acid released by the decomposition of cover crops such as rye and timothy.
Other examples of green manure crops and soil amendments reportedly effective for reducing plant-parasitic nematodes include ground sesame stalks and crabmeal. Some research suggests that ammonia released by decomposition of these soil amendments might be the active killing agent. This theory has been supported by reduced nematode populations after incorporation of agricultural-grade urea in the soil.
Nutrition and General Care of the Host
The deleterious effects of nematode damage to certain crops can be offset to some degree by proper nutrition, moisture, and protection from adverse conditions such as cold that place plants under stress. Practices that tend to offset the damage caused by nematode attack include irrigation, conservation of moisture by mulching, fertilization, protection of plants on cold nights, and control of root and foliar diseases caused by other pathogens. It should be pointed out, however, that these are only delaying tactics, and if susceptible crops are grown continuously, the nematode population will reach proportions that will cause serious damage. The rapidity of disease development and the magnitude of the damage will depend on the host and nematode species involved, the resistance or tolerance of the host, and various factors in the environment that favor or deter development of the disease.
Some research has shown that soil population levels of several nematode species may be changed by host nutrition and, similarly, that disease development and severity are more pronounced in infected plants that are deficient in one or more essential nutrients. Nematode infection also has been found to cause an increase or decrease in the concentration of one or more minerals in leaf or root tissue. The interactions among host, parasite, and nutrition are complex, and the application of such information to fertilization programs designed to minimize crop plant nematode damage is just beginning.
Sanitation and Nematode-Free Planting Stock
The land-management and cultural practices discussed above reduce nematode populations in plots to varying degrees. Most of these measures have limitations; the degree of control is erratic; and sometimes those factors actually responsible for the reduction in nematode populations are not fully understood. Sanitation and the use of nematode free planting stock, however, are sure and effective means of nematode control.
The cost of these practices is small, yet many growers continue to use nematode-infected transplants. Although pathogenic nematodes are already widespread, indiscriminate use of nematode-infected plants and plant parts introduces new species into many fields and consequently complicates control measures. Furthermore, nematodes introduced in this manner are in a favorable position for survival since they are already in or close to host plant tissues.
TitleControl of Nematode Populations
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