Sclerotinia crown and stem rot (SCSR) is one of the most destructive diseases of alfalfa (Medicago sativa L.) and other perennial forage legumes in the eastern USA. Sclerotinia is most severe in alfalfa seeded in late summer or early fall, especially when minimum tillage practices are used to establish the crop. This presents a production dilemma for producers, because both of these practices allow reduced pesticide and fuel use and promote soil conservation. In addition, late summer is the best time to establish alfalfa in the eastern USA. All commercially available alfalfa varieties are susceptible to this disease; however, progress is being made in selection of germplasm with disease resistance. The fungicide vinclozolin (Ronilan) is an effective control, but is not registered for control of Sclerotinia in alfalfa. Based on limited lab studies, late summer planting date may influence the relative severity of Sclerotinia crown and stem rot in alfalfa. The objectives of this project were to evaluate under field conditions the effect of planting date on the severity of Sclerotinia crown and stem rot in alfalfa, and to develop guidelines for an integrated approach to controlling Sclerotinia crown and stem rot in alfalfa.
The field experiment was successfully established in 1993 at the OSU Horticulture Farm, Columbus, in a grass-legume sod uniformly infested with sclerotia of Sclerotinia trifoliorum. Two cultivars differing in field resistance to SCSR were seeded with a no-till drill at four different dates (April and early, mid-, and late August). Plots were irrigated regularly to ensure uniform establishment. Each planting date-cultivar combination was subdivided into fungicide-treated (vinclozolin) and untreated plots so that data from each treatment combination can be expressed relative to a disease-free control. Treatments were replicated four times in a split-split-plot randomization of a randomized complete block design. Planting dates were whole plots, cultivars subplots, and fungicide treatments sub-subplots.
Weather conditions were favorable for sclerotia germination during the Fall of 1993. Apothecia emergence was substantial in all plots, so innoculum loads were heavy. Sclerotinia disease severity ratings will be taken in late winter through early spring of 1994 when the disease symptoms become apparent in the plots. Plant canopy height will be measured in early May 1994 as an indicator of degree of stunting and overall plant vigor, and yield data from four cuttings will be taken during the summer of 1994.
A second grass-legume field was inoculated in November 1993 with grain inoculum of Sclerotinia trifoliorum in preparation for a second seeding of this experiment in 1994.
Extension Program Implementation:
A field day will be offered to county Extension Agents and agribusiness representatives in early
spring 1994 and 1995 when SCSR injury is most apparent in the plots. The plots will also be part
of a tour organized by Landon Rhodes in September 1994 for representatives of the alfalfa
breeding industry. Preliminary results will be reported at the North American Alfalfa Improvement
Conference in July 1994. Upon completion of the project, the data will be reported in: 1) an Ohio
State University Extension Fact Sheet, 2) The Ohio Forage Report, 3) Ohio ICM Newsletter, 4)
Journal of Production Agriculture, and 6) Biological and Cultural Tests for Control of Plant
Disease. The results will be used to develop guidelines for an integrated approach to controlling
Sclerotinia crown and stem rot in summer-seeded alfalfa. This information will be shared with
Extension Agents and taught in Extension winter meetings for producers.
A report of the results from the 1993 experiment will be submitted in November 1994. A final report of the project will be submitted in November 1995.
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