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Divergent Selection in Alfalfa for Resistance to Sclerotinia Crown and Stem Rot

Dep. of Agronomy, Mississippi State Univ., Mississippi State, MS 39762
USDA-ARS, Crop Science Res. Lab., Forage Res. Unit, P.O. Box 5367, Mississippi State, MS 39762
Appl. Res. Exp. Div., Gyogone, Isein, Rangoon, Myanmar

^* Corresponding author.

Sclerotinia crown and stem rot (SCSR) caused by Sclerotinia trifoliorum Eriks. is often a serious disease on alfalfa (Medicago sativa L.) during cool, wet periods of the year. No cultivars have been developed with increased resistance to this disease. This research determines the selection responses to one cycle of divergent selection for resistance to SCSR. Plants of four genetically different populations were evaluated for resistance to SCSR using a stem-tip inoculation technique. Selectious were made at 4 and 8% selection intensities for both resistance and susceptibility. Selections were polyerossed to produce four experimental synthetics (two selected for resistance and two selected for susceptibility). Plants grown from seed of each experimental synthetic and from remnant seed from each source population were evaluated for resistance along with ramets of the parental plants of the synthetics. Experimental synthetics selected for resistance at selection intensities of 4 and 8% had necrotic regions 4.0 and 4.5 cm long, respectively. The experimental synthetics selected for susceptibility at intensities of 4 and 8% had necrotic regions 7.4 and 8,2 cm long, respectively. Means of each of the experimental synthetics were significantly different (P 0.05) from the mean (6.1 cm) of the unselected population. Ileritability was estimated at 0.49 (SE = 0.14) using a regression of offspring parent. Resistance to Sclerotiula is heritable and can be increased or decreased using the stem-tip inoculation technique.

Received for publication October 4, 1993.