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Forge Nematode Story

Contents

 METHODS
 RESULTS & DISCUSSION
 LITERATURE CITED

Winter Cover Crops for Managing Root-Lesion Nematodes Affecting Small-fruit Crops in the Pacific Northwest by Thomas A. Forge,1, Diane Kaufman,2 and Russell E. Ingham1 1Dept. of Botany and Plant Pathology, Oregon State University, Corvallis, OR. 2North Willamette Area Research and Extension Center, Oregon State University, Aurora, OR. Winter cover crops are recognized to be an important component of sustainable agricultural practices in the Pacific Northwest. Winter cover crops reduce erosion and weed growth, and their sustained use may ultimately improve soil fertility and structure. Some researchers have recently begun to recognize that certain winter cover crops can also be beneficial for the management of plant parasitic nematodes and soil-borne diseases. To date, most research on the use of cover crops for management of nematodes and soil- borne diseases has focused on plants in the family Brassicaceae, which includes cabbages, mustards, and rapeseed. In the 1960's Papavizas (1966) demonstrated that the green manures of certain brassica plants reduced root-rot of pea plants caused by the fungus Aphanomyces euteiches . Research since then has expanded the list of plant pathogenic fungi that may be managed with brassica green manures to include Rhizoctonia solani (Manning & Crossan 1969), Thielaviopis basicola (Adams 1971; Papavizas 1968) and Fusarium oxysporum (Ramirez-Villapudua & Munnecke 1988). Some cultivars of brassicas contain relatively high concentrations of compounds known as glucosinolates (Carlson et al . 1987). When brassica plants are incorporated into soil as green manures, the glucosinolates are broken down by enzymes from the plant and soil bacteria to form several potentially biocidal compounds such as isothiocyanates (Lewis & Papavizas 1971, 1970). Most researchers believe that the compounds released by glucosinolate degradation are responsible for the reduction in pathogen populations caused by brassica green manures. Nematode management: Hassan Mojtahedi and coworkers (1993, 1991) at Washington State University recently demonstrated that the green manures of rapeseed and sudangrass could be used to reduce populations of the Columbia root-knot nematode, Meloidogyne chitwoodi , and the northern root- knot nematode, M. hapla , both of which are important pests of potato in the Columbia basin. Sorghum and sudangrass contain a compound known as dhurrin (Harrington 1966). Like the glucosinolates of certain brassicas, dhurrin is broken down by microbial enzymes in soil to yield compounds toxic to nematodes. Some potential cover crops may also be antagonistic to nematodes while the plants are growing, although the mechanisms of this type of antagonism are not as well known as for green manure effects. Nematologists have known since the 1960's that marigolds are antagonistic to many species of plant parasitic nematodes. Sudangrass has also recently been reported to be antagonistic to nematodes prior to incorporation as green manure (Ingham & Kaufman 1992; Olson 1984).

use and may ultimately be detrimental to soil fertility and structure. We have begun trying to find winter cover crops that, in addition to improving soil quality and reducing erosion and weed growth, may be used to reduce populations of P. penetrans . Currently, we are trying to identify nematode-antagonistic crops that may be grown during the fall and winter before replanting caneberries, strawberries or fruit-trees. Eventually, we hope to explore the possibilities for growing these crops during fall and winter between the rows of established vines or trees. In 1992 we tested several potential cover crops in a greenhouse experiment to determine their effects on P. penetrans (Ingham & Kaufman 1992). Here we describe the results of a field trial conducted in the fall and winter of 1993-1994 to evaluate the effectiveness of rye cv Wheeler, oats cv Saia, rapeseed cv Humus, sudangrass cv Trudan 8, and sudangrass x sorghum hybrid cv SS 222. In this trial, the effects of these crops on populations of P. penetrans was compared to a winter fallow which was allowed to become colonized by indigenous weeds. METHODS The experimental plots were established in a cooperating grower's field near Gresham, Oregon. The field had been planted with strawberries from May, 1989 to August, 1993, and was to be replanted with strawberries in the spring of 1994. On August 31 of 1993 each cover crop was planted in five replicate 10 x 20 foot plots arranged in a randomized complete block experimental design. Nematode populations in soil and roots were sampled at the time of planting, on November 30, and on April 11. Nemacur was applied to all plots in January. The cover crops were plowed down in mid-May and strawberries were planted at the end of May. Nematodes populations in soil and roots of the strawberry plants were sampled on July 15, 1994. At each sample date, eight 1- inch diameter x 12 inch deep cores were taken from each plot, pooled and mixed thoroughly. Roots were collected by digging five randomly selected plants of the appropriate cover crop with a trowel. The nematodes were extracted from 250 g soil and from approximately 25 g of roots and counted. RESULTS & DISCUSSION Weeds and Crop performance: In November the dominant weed species in fallow plots at this site was cornspurry (Spergula arvensis ). In April the dominant weed species was little bittercress (Cardamine oligosperma ). Common groundsel (Senecio vulgaris ) was the second most common weed in November and April. Both varieties of sudangrass grew very poorly as a result of low autumn temperatures and late planting date. By February the sudangrass plots were indistinguishable from weed plots and were dropped from analyses for the April and July sample dates. Nematode populations: The preplant population density of P. penetrans in the study area, averaged over all plots, was 875 nematodes per 250 g of dry soil. By November 30 (Figure 1), the nematode population densities in roots and soil under 'Wheeler' rye were significantly less (P < 0.05) than population densities under weeds and all other cover crops except the 'Saia' oat. On April 11 (Figure 2) the nematode population densities in soil under 'Wheeler' rye were significantly less (P <0.05) than under 'Humus' rapeseed and weeds but not 'Saia' oat. Nematode population densities in roots of the dominant weed (little bittercress) were significantly

supported by our previous greenhouse research (Ingham and Kaufman 1992), in which 'Saia' oat suppressed root-lesion nematode populations relative to strawberry and 'Humus' rapeseed ('Wheeler' rye was not previously tested in the greenhouse). The large numbers of P. penetrans in soil and roots of 'Humus' rapeseed at all pre-incorporation sample dates indicate that it is a good host for the nematode. If the incorporation of rapeseed residues reduced nematode populations, it was not sufficient to offset nematode reproduction which occurred on the rapeseed while it was growing. It is important to emphasize that the response of P. penetrans to 'Wheeler' rye and 'Saia' oats appears to be cultivar specific. Previous research (Townshend 1989) also demonstrated that 'Saia' oat was a poor host for root-lesion nematodes, but in the same research the cultivar OAC Woodstock was found to be a good host. Previous research also indicates that rye is generally a good host for root-lesion nematodes (Olthoff 1980; Dunn & Mai 1973), but the cultivars used in those studies were not specified. The mechanisms of nematode suppression by 'Wheeler' rye and 'Saia' oat are not known. The nematode populations were suppressed under 'Wheeler' rye and 'Saia' oat, relative to the other crops, before incorporation of green manures in the spring. Consequently, we have hypothesized that these cultivars are directly antagonistic to P. penetrans and green manure decomposition is not necessary for nematode control. Two attractive possibilities are that these cultivars function as trap crops or produce root exudates that are actively antagonistic to the nematodes. Alternatively, these crops may simply be poorer hosts for the nematodes than 'Humus' rapeseed and weeds that occurred in our fallow plots. Currently, 'Wheeler' rye and 'Saia' oat are not readily available in the Pacific Northwest and we hope that they will become more available as interest in their use as cover crops increases. 'Wheeler' rye is also very effective at reducing symphylans (Peachy & William 1994) and weed growth (Peachy & William 1994, Kaufman et al. 1993). Although 'Wheeler' rye is known to be allelopathic to many weeds, it does not appear to have any negative effects on yield of raspberry when grown between rows of established vines (Kaufman et al. 1993). Our results are encouraging and suggest that it may be possible to reduce the need for pre- plant fumigation by planting winter cover crops that are antagonistic to P. penetrans . However, it is very difficult to draw firm conclusions from one year of field data and we are continuing to evaluate potential winter cover crops, in the greenhouse and field, for their effects on P. penetrans . LITERATURE CITED Adams, P.B. 1971. Effect of soil temperature and soil amendments on Thielaviopsis root rot of sesame. Phytopathology 61: 93-97. Carlson, D.G., M.E. Daxenbichler, C.H. VanEtten, W.F. Kwolek & P.H. Williams. 1987. Glucosinolates in crucifer vegetables: Broccoli, Brussels sprouts, cauliflower, collards, kale, mustard greens, and kohlrabi. Journal of the American Society of Horticultural Science 112: 173-178. Chan, M.K.Y. & R.C. Close. 1987. Aphanomyces root rot of peas. 3. Control by the use of cruciferous amendments. New Zealand Journal of Agricultural Research 30: 225-233. Dunn, R.A. & W.R. Mai. 1973. Reproduction of Pratylenchus penetrans in roots of seven cover crop species in greenhouse experiments. Plant Disease Reporter 57: 728-730. Ingham, R.E. & D. Kaufman. 1992. The potential of cover crops and soil amendments for suppression of root-lesion nematodes (Pratylenchus penetrans ) in small fruits. Proceedings of

Manning, W.J. & D.F. Crossan. 1969. Field and greenhouse studies on the effects of plant amendments on Rhizoctonia hypocotyl rot of snapbean. Plant Disease reporter 53: 227-231. Mojtahedi, H., G.S. Santo & R.E. Ingham. 1993. Suppression of Meloidogyne chitwoodi with sudangrass cultivars as green manure. Journal of Nematology 25: 303-311. Mojtahedi, H., G.S. Santo, A.N. Hang & J.H. Wilson. 1991. Suppression of root-knot nematode populations with selected rapeseed cultivars as green manures. Journal of Nematology 23: 170- 174. Olsen, H.C. 1984. Influences of rotation crops and management systems on Pratylenchus penetrans associated with Solanum tuberosum production. M.S. Thesis, Michigan State University, East Lansing, MI. Olthof, Th.H.A. 1980. Screening rye cultivars and breeding lines for resistance to the root-lesion nematode Pratylenchus penetrans . Canadian Journal of Plant Science 60: 281-282. Papavizas, G.C. 1968. Survival of root-infecting fungi in soil. IV. Effect of amendments on bean root rot caused by Thielaviopsis basicola and on inoculum density of the causal organism. Phytopathology 58: 421-428. Papavizas, G.C. 1966. Suppression of Aphanomyces root rot of peas by cruciferous soil amendments. Phytopathology 56: 1071-1075. Peachey, E. & R. William. 1994. Influence of grain cover crops on rowcrop pests. Pacific Northwest Sustainable Agriculture 6:4-5. Ramirez-Villapudua, J. & D.E. Munnecke. 1988. Effect of solar heating and soil amendments of cruciferous residues on Fusarium oxysporum f.sp. conglutinans and other organisms. Phytopathology 78: 289-295. Townshend, J.L. 1989. Population densities of four species of root lesion nematodes (Pratylenchus ) in the oat cultivars, Saia and OAC Woodstock. Canadian Journal of Plant Science 69:903-905.