Association of level of resistance to Verticillium wilt with alfalfa forage yield and stand.

Although Verticillium wilt (VW), caused by Verticillium alboatrum Reinke & Berth., has been considered a serious disease on alfalfa (Medicago sativa L.) in North America since 1976, little research has been done to determine both the effects on forage yield due to resistant cultivars and the necessary levels of resistance required to adequately protect the crop. In 1985, 16 cultivars were evaluated both for resistance after inoculation by the root-soak method in a growth chamber, and for yield and plant stand after seed inoculation in sward plots in the field in Ithaca, NY. During three production years, 82 to 87% of the variation in yield among cultivars in the field was explained by level of VW resistance in the growth chamber test. When regressed with resistance level, yield in all 3 yr fit a quadratic curve, leveling at the cultivars classified as resistant (R). Compared with susceptible (S) and low-resistant (LR) cultivars, moderately resistant (MR) cultivars averaged 5% higher yield in the first production year, 19% in the second, and 20% in the third. Compared with MR cultivars, R and highly resistant (HR) cultivars yielded 7% higher in the first year and 10% higher in each of the next 2 yr. Plant stand was 125% higher for MR than S-LR cultivars and 33% higher for R-HR than MR cultivars. Verified by naturally infested trials, our recommendation, therefore, is to deploy cultivars R or HR to VW in New York and similar environments. Mean yield advantage due to VW resistance in the HR cultivar 'Oneida VR' was 0.52 tons/acre (valued at $44/acre at $85/ton hay) in the second production year and 1.28 tons/acre (valued at $109/acre) in the third year compared with the most susceptible cultivar 'Saranac AR'.