Author
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RHODES, L - OHIO STATE UNIVERSITY |
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SULC, R - OHIO STATE UNIVERSITY |
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Ariss, Jennifer |
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PIERSON, PAULETTE - MONSANTO COMPANY |
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MC CORMICK, JOHN - OHIO STATE UNIVERSITY |
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Submitted to: North American Alfalfa Improvement Conference
Publication Type: Proceedings Publication Acceptance Date: 7/16/2006 Publication Date: 7/16/2006 Citation: Rhodes, L.H., Sulc, R.M., Ariss, J., Pierson, P.E., Mc Cormick, J.S. 2006. Field Testing of Alfalfa Cultivars for resistance to Sclerotinia Crown and Stem Rot: Problems and Progress. In Proceedings of the joint meeting of the 40th North American Alfalfa Improvement Conference and 19th Trifolium conference. July 16-19, 2006, Bloomington, MN. Interpretive Summary: Technical Abstract: Sclerotinia crown and stem rot (SCSR), caused by Sclerotinia trifoliorum, often causes severe losses in late-summer seeded alfalfa. The disease may be especially destructive when no-till methods are used. Most alfalfa cultivars presently available may be severely damaged when inoculcum concentration is high and environmental conditions are favorable for disease development. Identification of cultivars or experimental lines with commercially acceptable levels of resistance to SCSR would be a major advance in controlling this disease. A program of field testing of alfalfa cultivars and experimental lines was begun at the Ohio State University, Columbus, in 1987. Entries submitted by alfalfa seed companies and public institutions were evaluated under field conditions. Although some variation in methods has occurred since the beginning of the testing program, the basic procedures have involved: 1) Establishment of alfalfa plots within an area consisting of a killed forage legume sod which has been uniformly pre-inoculated with S. trifoliorum; 2) Late-summer seeding of alfalfa plots using no-till methods; 3) Natural inoculation of alfalfa plants by ascospores disseminated from apothecia emerging within the plot area; and 4) Comparison of entries in the trial to a fungicide-treated, disease-free check cultivar. To obtain a suitable plot area, a red-clover sod was seeded at least one year prior to the establishment of the alfalfa trial and inoculated with a mycelial inoculum of S. trifoliorum. Red clover plants were infected by S. trifoliorum, and sclerotia were produced on stems and crowns of infected plants. During the spring and summer, sclerotia falling to the soil surface became buried in the upper 1 cm of soil. During the summer prior to alfalfa plot establishment, the red clover sod was treated with glyphosate. Alfalfa entries were seeded in late August or early September using no-till methods. Typically, apothecia emerged in October or November. When apothecia were abundant, the primary infection period usually continued until mid-December. Check cultivars included for year-to-year comparisons were Armor, Cimarron VR, Hi-phy, Trident II, and Vernal. To achieve a disease-free control treatment, Vinclozolin (Ronilan 50 DF) was applied to plots of Armor alfalfa at 2 lb. formulated product per acre in mid-September, mid-October, mid-November, and mid-March. Unsprayed Armor served as the designated susceptible check cultivar. Data on disease development were recorded in late April or early May. Beginning in late May, plots were harvested 4 times at approximately 35-day intervals, and dry matter yields were determined for each entry. Of the 17 trials conducted since 1991, 10 had sufficient disease development to evaluate differences between cultivars. In these 10 trials, cultivars which had undergone selection for resistance to SCSR were among those with lowest disease severity. In general, both first-harvest yield and total yield (4 harvests) were inversely correlated with disease severity. In years with moderate to high levels of disease severity, cultivar resistance was easily observed. However, in Ohio, the inability to achieve a consistently high level of disease pressure from year to year has been the most serious obstacle to evaluating resistance to SCSR in the field. |
