Current Procedures for Field Testing Alfalfa Cultivars for Resistance to Sclerotinia Crown and Stem Rot

L. H. Rhodes and R. M. Sulc
The Ohio State University
Columbus, OH 43210

Sclerotinia crown and stem rot (SCSR), caused by Sclerotinia trifoliorum, is a serious disease of late-summer seeded alfalfa. Although differences in resistance to SCSR among alfalfa cultivars have been reported, all cultivars presently available may be severely damaged when inoculum 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. In 1991, a program of field testing of alfalfa cultivars and experimental lines was begun at the Ohio State University Waterman Farm, Columbus, Ohio. Entries are submitted by alfalfa seed companies and evaluated under field conditions. Although some modification of procedures has occurred since the beginning of the testing program, the following procedures are currently being employed.

A red-clover/orchardgrass sod is seeded at least one year prior to the establishment of the alfalfa trial. Grain inoculum, consisting of 1 part wheat to 1 part oats colonized by S. trifoliorum, is spread uniformly throughout the red clover/orchardgrass sod during the fall or spring. Red clover plants become infected from mycelium growing from the colonized grain and sclerotia are produced on stems and crowns of infected plants. During the spring and summer, sclerotia fall to the soil surface and gradually become buried in the upper 1 cm of soil.

Alfalfa entries are seeded in late August or early September using no-till methods. Irrigation is supplied as necessary to insure stand establishment and sclerotial germination. Typically, apothecia emerge in October or November and release ascospores which infect alfalfa plants. The primary infection period usually continues until mid-December. To achieve a disease-free control treatment, Vinclozolin (Ronilan 50 DF) is 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 serves as a susceptible check cultivar.

Plots are monitored from February through May for symptoms of SCSR. Data on disease development (percent of plot area affected by Sclerotinia) are recorded from late winter through spring as needed. Beginning in late May, plots are harvested 4 times at approximately 35-day intervals, and dry matter yields are determined for each entry. In October, plots are undercut and industry representatives are invited to select plants for breeding programs.

1995 Central Alfalfa Improvement Conference Proceedings