Ohio

Planting Date Effects on Severity of Sclerotinia Crown and Stem Rot in Alfalfa

Principal Investigator:

 Mark Sulc, Dept. of Horticulture and Crop Science

Abstract:

 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. Late summer is an excellent time to establish alfalfa in the eastern USA. Adequate controls are currently unavailable to producers, and this disease is one of the most significant impediments to no-till summer seedings of alfalfa in this region. No commercially available alfalfa varieties are listed as resistant to this disease, although progress is being made in selection of germplasm with disease resistance. The fungicide vinclozolin (Ronilan) is an effective control, but is not registered for use in alfalfa. Based on studies in controlled environments, 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.

Field experiments were established in 1993 and 1994 at the OSU Horticulture Farm in Columbus. Two alfalfa cultivars differing in field resistance to SCSR were no-till seeded mid-May, August 1, August 16, and August 30 in a grass-legume sod uniformly infested with sclerotia of S. trifoliorum. Plots were irrigated immediately after seeding to ensure rapid and uniform establishment. Armor alfalfa is the susceptible check cultivar for Sclerotinia, and A9109 is an experimental line selected for improved resistance to Sclerotinia. Each planting date-cultivar combination was subdivided into fungicide-treated (vinclozolin) and untreated plots so that data from each treatment combination could be compared with a disease-free control. Treatments were replicated four times in a split-strip-plot randomization of a randomized complete block design. Planting dates were whole plots, cultivars subplots, and fungicide treatments were stripped across the cultivar sub-plots.

Weather conditions were favorable for sclerotia germination during the Fall of 1993 and 1994, and inoculum loads were sufficient to cause heavy damage by Sclerotinia both years. Sclerotinia disease severity ratings in the spring the year after seeding demonstrated that the four applications of vinclozolin effectively controlled the disease (Table 1). Without the fungicide, A9109 suffered less SCSR damage than Armor in the 1993 seeding, especially when seeded in late August. The cultivars did not differ in disease severity ratings in the 1994 seeding. Planting date significantly affected disease severity (% of stand affected) in the absence of the fungicide treatment (Table 1). Total season forage yield as a percent of the disease-free control was 98, 94, 85, and 73% for the mid-May, August 1, August 16, and August 30 planting dates, respectively. These data demonstrate that the risk of SCSR is minimal when seeding in the spring. When alfalfa is no-till seeded in late summer, the risk of SCSR damage can be dramatically reduced by seeding in early to mid-August, assuming timely rainfall to promote rapid emergence. These field data confirm results from controlled environments demonstrating that alfalfa seedlings should be at least 8 to 10 weeks of age at the time of apothecium emergence in order to reduce the risk of serious stand loss. In Ohio, this means that alfalfa should be planted in early August, because apothecium emergence normally occurs in mid-October. These data also demonstrate the potential for effectively reducing the impact of Sclerotinia in no-till establishment of alfalfa through the combined effect of earlier planting and improved cultivar resistance to SCSR.

Extension Program Implementation:

A field day was held for OSU Extension Agents in early spring 1995 when SCSR injury was very apparent in the plots. The plots were part of a tour organized by Dr. Landon Rhodes in September 1993 and 1994 for representatives of the alfalfa seed industry. Results were presented at the North American Alfalfa Improvement Conference in July 1994, and at the American Society of Agronomy Meetings in October 1995. The data will be presented at an OSU Extension Forage conference for producers at the South District office at Jackson, OH December 13,1995. The data will be reported in an Ohio State University Extension Fact Sheet, the Ohio ICM Newsletter, and at winter meetings and field days. A manuscript is in preparation for publication in the Plant Disease journal.

Table 1. Effects of planting date and fungicide treatment on severity of Sclerotinia crown and stem rot in no-till seeded alfalfa.

Planting Disease severity (%) * Total DM yield (T/A) Final stand density (%)
date No fungicide Fungicide No fungicide Fungicide No fungicide Fungicide
Mid-May 4 0 7.00 7.13 90 88
August 1 12 0 6.23 6.65 80 81
August 16 23 0 5.57 6.59 78 83
August 30 41 0 4.46 6.14 68 84
LSD (0.05) 6 NS 1.45 0.39 6 NS
* % of stand affected in May the year after seeding.
NS = treatment differences were not significant at the 0.05 probability level.
For further information contact Mark Sulc , Assistant Professor, Dept. of Horticulture & Crop Science, The Ohio State University or the Ohio IPM Office.

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