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NEMATODE CONTROL AND THE BEST ORGANIC MATTER

This December (2006) marks the 27th anniversary of the
introduction to Texas' gardeners of the best organic nematode control as well as a wonderful source of clean organic matter. On December 2, 1979, I wrote the first column about the use of cereal rye (Elbon) as a control for nematodes and as the fastest growing, cold-hardiest grass that can be used as a "green manure" crop for South Texas. Malcolm Beck introduced me to cereal rye as a cover crop. I was amazed by the growth rate and then was delighted to have Dr. Jerral Johnson, Texas A&M Extension Plant Pathologist, find quantities of scientific literature revealing it is one of the best trap crops for nematodes. All of these beneficial qualities of cereal rye were proclaimed to gardeners every year beginning in 1979 and ending in 1992 when I stopped writing for the newspaper. Columns were used such as: http://www.plantanswers.com/garden_column/110301/110101.htm
George Wechsler of Wolfe (now Calloway's) Nurseries and all privately-owned nurseries stocked and sold thousands of pounds of this beneficial product yearly.

Nematodes are unsegmented roundworms, different from earthworms and other familiar worms that are segmented (annelids) or in some cases flattened and slimy (flatworms ). Nematodes living in soil are very small and most can only be seen using a microscope. There are many kinds of nematodes found in the soil. Most of these are beneficial, feeding on bacteria, fungi, or other microscopic organisms. There are even nematodes that can be used as biological control organisms to help manage important turf insect. Unfortunately, the effectiveness of applications of beneficial nematodes is so extremely dependent on precise soil conditions and favorable environmental conditions that positive results which are actually caused by the beneficial nematodes are seldom achieved. There are also a group of nematodes that feed on plants -- these are called plant-parasitic nematodes. All plant-parasitic nematodes have a stylet or mouth-spear that is similar in structure and function to a hypodermic needle. The nematode uses the stylet to puncture plant cells, and then inject digestive juices and ingest plant fluids through it. The results can be seen at: http://www.plantanswers.com/tomato_root.htm

When the cereal rye was first introduced to the market, it was mainly recommended as a wonderful source of green manure organic matter. The nematode control aspect was just a favorable side effect which organic gardeners could enjoy. That was when home gardeners could purchase soil nematicides such as Vapam and Nemagon. Since then, a television segment on Sixty Minutes revealed that some factory workers were sterilized when manufacturing Nemagon so my wife purchased all available supplies. Neither of these products nor any effective nematode control products are now available to the home gardener.

After several years of gardening and growing susceptible plants, nematode populations increase to the point that damage becomes noticeable. Control practices include summer fallowing, rotation, adding organic matter, planting trap crops, removing diseased plants and using resistant varieties. There are few resistant varieties available and summer fallowing as well as rotation is impractical in a small home garden. The planting of cereal rye at this time of the year utilizes the control measures of planting a trap crop and adding organic matter. All these control measures are designed to reduce the soil's nematode population-none of these measures completely eliminate the entire population. Cereal rye can be planted by merely seeding directly on top of the garden soil and raking in. Apply seed at about the rate of 3/4 to one pound per hundred square feet of garden area to insure good coverage and adequate growth. Raking to give seed some coverage by soil helps. Be sure to water the rye regularly and fertilize with any type of fertilizer every three weeks for maximum growth. Remember that most of the organic material produced is in the root system rather than the top foliage. Shred rye and till the soil one month before planting is to occur so the massive rye root system will have adequate time to decompose.

High soil organic matter alone does not insure root knot nematode control. The higher the organic matter, however, the better the chance that antagonistic organisms will develop. Some soil fungi trap nematodes and use them as a food source. Some organic matter works better than others. Turning under a green manure crop such as small grains (cereal rye) or legumes (vetch) several weeks before planting is the best. Additional nitrogen may be necessary for adequate crop production because decay organisms in the soil use available nitrogen as they break down the green manure crop. Also, remember the information presented in "The Truth About Microbes" at: http://www.plantanswers.com/microbes.htm which stated: "The overall effect of fertilizer applications is to markedly increase microbial numbers and activity in soil through increased plant growth. The majority of soil microbes require organic carbon to grow and produce new cells. In grass systems, the vast majority of organic matter is produced from decomposing roots and leaves. Fertilization increases the amount of organic substrates available to soil microbes by increasing its source, the grass plants themselves. Thus, rather than producing "dead soil", judicious use of fertilizers invigorates soil microbes by allowing plants to produce more resources for them."

Sometimes people resort to home remedies to control nematodes such as planting marigolds or mixing sugar or lye into the soil. Of these three, only dense populations of marigolds are effective in controlling nematode populations. Marigolds secrete toxic compounds of an a-terthienyl type into the soil which kills nematodes but planting a few marigolds around annual plants in infested soil will NOT prevent infection. Marigolds also act as a trap crop. Nematodes enter their roots but are unable to complete their life cycle. Trapped nematodes die without reproducing. To be effective marigolds must be planted as a solid crop and grown for 90 days to begin secreting the three compounds of an a-terthienyl type to reduce the nematode population. See the write-up at:
http://aggie-horticulture.tamu.edu/PLANTanswers/earthkind/ekgarden27.html

It has been brought to my attention recently that nematodes can also damage turfgrass-especially in sandy soils. Plant-parasitic nematodes are probably the least understood and most difficult to manage of the turfgrass pests in Texas. As plant-parasitic nematodes feed they damage the root system and reduce the ability of the grass to obtain water and nutrients from the soil. Roots may be abnormally short and appear darkened or rotten when damaged by plant-parasitic nematodes. Root galls or knots associated with certain nematode damage to other crops are not always evident on grasses. When nematode population densities get high enough, or when environmental stresses such as high temperatures or drought occur, above-ground symptoms may become evident. Symptoms include yellowing, wilting, browning, thinning out, or death. Often weeds such as spurge and sedges become prominent as the grass thins out . Nematode damage usually occurs in irregularly shaped patches that may enlarge slowly over time. Be aware that similar conditions may be caused by other factors such as localized soil conditions, fungi, or insects.

Because plant-parasitic nematodes live in soil or roots, the pesticides used to kill them tend to be very toxic and are water soluble. Because of health risks and environmental concerns there are currently no toxic nematicides that are labeled for use on established home lawns. There are a number of products available for use on home lawns that are marketed as "organic", "biological" or "non-toxic" that claim to be suppressive to plant parasitic nematodes. Be aware that in order to be labeled for home use these types of products need to be safe, but do not need to be proven effective. Nernatologists at numerous universities have tested many of these products for efficacy, generally with disappointing results.

Adding soil amendments can help the grass tolerate nematode damage or possibly suppress nematode population densities. Remember that anything that can he done to improve root health is good. Colloidal phosphate incorporated into fine sand has been shown to help bermudagrass withstand attack by certain nematodes. Organic materials such as composted municipal sludge or manures can also promote grass health, and at the same time may stimulate fungi that attack nematodes.

Some of the best practices for managing nematode damage in home lawn include avoiding other stresses on the grass. Grass that is given proper watering and fertilization can often withstand higher levels of nematode infestation than grass suffering from drought or nutrient deficiencies. Over-fertilization should also be avoided, too much nitrogen can stimulate the production of succulent roots that are more susceptible to nematode damage. Grass that is mowed too low, or that is allowed to grow too tall between mowing, is also more prone to succumb to nematodes. Frequent mowing at moderate height is best. While minimizing these stresses can help, even the best managed lawns can suffer from nematode damage from time to time.

At present, the best management strategies for nematodes in the home lawn are aimed at increasing the grass' ability to tolerate nematode damage. These strategies include avoiding stress and promoting root vigor.