Integrating nemade-resistant crops into sugar beet rotations

Sugar beets (Beta vulgaris) are grown on 180,000 acres in the Eastern Rocky Mountain region and represent more than $120 million in annual income. Total variable costs to grow the crop are $500/acre or more, nearly half of which is spent on pest control (Hewlett and Bastian, 1992). Sugar beets have been Wyoming's most profitable crop, in spite of the high input costs.

The sugar beet (cyst) nematode (SBN), Heterodera schachtii, is the most damaging pest of sugar beet in Wyoming and many other areas worldwide. As much as half of the crop can be lost if this pest is not controlled. In a recent survey (Gray, in press), 96% of Washakie County (Wyoming) fields were infested, and 72% were above the estimated damage threshold of 2.8 eggs and/or juveniles cm-3 of soil (Griffin, 1981; Robb et al., 1992). In a more recent survey (Gray and Gill, 1995, unpub.), 54% of fields had only one year and 41% had two years between sugar beet crops.

Temik 15G® (aldicarb), a nematicide/insecticide, is applied to 30% of Wyoming's sugar beet acreage (Legg, et al., 1992). There is an increasing use of Telone II® (1,3-dichloropropene), a soil fumigant (biocide). Cost of aldicarb and 1,3-dichloro-propene at full-label rate is about $90 and $120-$130/acre, respectively. These nematicides are restricted-use chemicals and represent a significant environmental risk to air and water, as well as a health risk to applicators. They have been found in ground and surface water in several states (Roberts and Thomason, 1981). Future availability of these materials is uncertain.

Sustainability of the sugar beet industry is threatened by the continued use of nematicides. In spite of increased use of nematicides, soil populations of the SBN are still relatively high. In some areas there has been a decreased acreage of sugar beets due to buildup of pests. This has resulted in poor yields and quality of beets, increased inputs and the need to grow sugar beets more remotely from the factory, with an increased cost due to transportation.

Sugar beets are intensively grown in Wyoming. For the most part, legume, green manure or cover crops have not been utilized. Extensive tillage, use of heavy equipment and many trips over the field have led to such problems as soil compaction, reduction in tilth, depletion of organic matter and greater likelihood of leaching of soluble nutrients and pesticide residues. The results of the proposed research would contribute information to these areas of concern.

The increase in "flex" acres proposed in the 1995 Farm Bill provides an opportunity to try new and alternate crops without penalty. There will likely be encouragement for the concepts of sustainability, integrated pest management and conservation of natural resources.

In Germany and the Netherlands the nematicides that control SBN have been removed from the market due to ground water contamination. SBN-resistant radish and mustard, which act to "trap" and prevent SBN reproduction and effectively reduce soil populations have been developed and used successfully.

At grower meetings, there has been considerable interest in the recent findings of work in Wyoming, which show the potential for reduced nematicide use and fall grazing of trap crop radish following malt barley. Growers, however, have suggested that more work be done on modifying other rotations to accommodate the use of trap crops. They further request that alternatives methods for establishing trap crops, which will facilitate their planting at a very busy time, be evaluated.

The SBN occurs throughout the sugar beet-growing areas in the western U.S.(Christie, 1959). Continuous feeding by large numbers of nematodes and the release of a toxin into sugar beet roots results in stunted plants. High soil populations of SBN may result in poor emergence and poor stands (Stelle, 1989).

Relative to control, crops are categorized as either susceptible (soil population of SBN is increased), neutral (population unaffected) or resistant (population of reduced) (Roberts and Thomason, 1981). Crops susceptible to SBN include sugar beet and table beet, radish (Raphanus sativa), most Brassica crops and weeds, including mustards. Neutral plants include small grains, pea, dry bean, corn, grasses and alfalfa (Medicago sativa). Resistant plants include the recently developed radish and mustard trap crop cultivars (Spittel, 1986).

The resistance mechanism of these cultivars is as follows: (1) root exudates stimulate eggs within cysts (dead females of H. schachtii) to hatch, releasing infective juveniles, as occurs with susceptible plants; (2) infective juveniles are attracted by exudates and infect roots, as with susceptible plants; (3) development of juveniles into adult females is inhibited and nematodes starve, while reproductive cycles are completed in roots of susceptible plants (Christie, 1959 and Spittel, 1986). Gardner and Caswell-Chen, 1993, in the U.S., reported the effectiveness of nematode trapping.

The SBN has been controlled by crop rotations, nematicides and, more recently, by trap crops. Resistant sugar beet cultivars would be the ideal control, however, agronomically- desirable SBN-resistant cultivars are not available (Lange and De Bock, 1994). The time required to reduce the soil population below the damage threshold level with non-host crops depends on the initial soil population of SBN (Gray, et al., 1992). In Utah, a 6-year sugar beet rotation resulted in holding SBN populations below the damage threshold (G.D. Griffin, pers. comm.), avoiding the need for nematicides. The common rotations in Wyoming, sugar beet-corn-dry bean and sugar beet-malt barley, are too short for adequate control of even moderate soil populations of SBN. Lengthening rotations may not be economically practical due to the high value and potential profitability of the sugar beet crop and the lack of adapted and profitable cash crops for Wyoming's climate (Koch, et al., 1995). However, modifying the rotation with alternative crops has merit, as it would allow the advantageous use of trap crops.

A recent analysis, based on yields the past five years shows that with average prices, the gross margin for wheat and corn are about the same. The current high price of wheat provides an opportunity to substitute wheat for corn. In addition to providing more growth of trap crops following main crop harvest, other advantages of wheat are: (1) providing a winter cover, particularly important on coarse-textured soils; (2) better control of weeds and other pests affecting the summer annual crops in the rotation; (3) reduced cost of production, compared to corn, dry beans and sugar beets; (4) spreading of the seasonal farm workload; and (5) greater diversification of cash cropping.

Although there is little information on price and cost of production for pea/oat (Pisum sativum/Avena sativa) forage mix, these species are well adapted to Wyoming growing conditions. Three to 3 1/2 tons/acre of high-quality hay are regularly obtained with this mixture. The pea in the mixture increases dry matter and protein content over straight oats (Green et al., 1995). This crop offers similar advantages as winter wheat in a sugar beet rotation, except that it is spring-planted. It may be more appealing than winter wheat to crop-livestock producers.

Varieties of fodder radish (Raphanus sativa) and yellow mustard (Sinapsis alba), selected for resistance to SBN (trap crops) and their ability to reduce SBN populations, are a promising alternative to nematicides. Technology for substituting trap crop use for nematicides was first developed in Europe (Peterson, 1992). Trap crops are an integral part of European sugar beet production. However, there is a need to determine the best way to utilize trap crops in the U.S. where climate, soils, irrigation, rotations and management of sugar beets differ. Planting full-season trap crops, an area of current research in Germany and Holland, would maximize their ability to cleanse the soil of SBN and contribute forage and/or green manure, however, the economics may favor second-crop use. Knowing the effectiveness of full-season, compared with second-cropping, would be helpful in shaping future farm programs emphasizing integrated pest management. SBN-resistant trap crop cultivars, bred in Europe, are now available in the U.S. (Hilleshog Mono-hy, Inc.).

In Wyoming, trap crops, used as a second crop following malt barley, have reduced soil SBN populations 50-75% over a 10-week period (August to mid-October) (Gray, et al., 1994). Grazing of trap crops in the fall has produced 220 to 250 lb/A of lamb without significant effect on nematode control or sugar beet yields (Koch, et al., 1995). Sugar beet yield the following year was increased 4 to 5 tons per acre with trap crop radish and without nematicide. Cost of growing trap crops is less than the cost of the full-label rate of aldicarb or 1,3-dichloropropene.

Following dry beans and corn, the effect of trap crops on SBN control and subsequent sugar beet yield was not adequate to justify expense of growing trap crops. This is related to the much shorter time of favorable conditions following dry beans and corn harvest, compared to malt barley. These unpublished results are summarized in a final report (September 1995) to the granting agency (USDA-SARE program) and are in agreement with those of Wilson, et al. (1993) in Nebraska.

Initial tests in Wyoming show that trap crops utilize large amounts of residual soil nitrogen (N). Mid-summer plantings have produced more than 1 ton/A of low fiber dry matter. Plow down of this high-quality organic matter would be expected to alleviate some of the soil problems mentioned above. In a grazing study (unpub. data), trap crop mustard planted in late July reduced residual soil nitrates 40% in the top three feet of soil (unpub. data). Additionally, organic matter additions to the soil have been reported to stimulate activity of naturally-occurring antagonists and parasites of nematodes and other soil-borne pathogens (Stirling, 1991).