At the Eden Shale trial (seeded Apr 96; severe pressure from ARR), such variables as seedling health, alfalfa yield, and stand vigor during the seeding year were all significantly (P<0.05) improved by using ARR-resistant varieties. In 1997, alfalfa yield and stand vigor were also substantially better in varieties resistant to both ARR and Phytophthora root rot (PRR) than in varieties resistant to PRR alone, demonstrating value to ARR resistance past the seeding year. At the UKREC trial (seeded Apr 97, moderate pressure from ARR), while seedling health was improved significantly during the seeding year, yields in 1997 were not correlated to percent ARR resistance. Likewise, stand survival through the winter was substantially poorer only in varieties susceptible to PRR. We concluded that, under conditions highly conducive for the disease, Kentucky producers can expect a substantial improvement in seedling health by using ARR-resistant alfalfa varieties. Varieties with ARR resistance ratings of R or HR are expected to consistently outperform susceptible varieties under severe disease pressure, and under some environmental conditions, may do so under moderate disease pressure, as well.
Although our data regarding the agronomic value of ARR-resistant varieties were convincing, we documented sources of variation in plant health which also merit discussion. For example, varieties with a similar ARR resistance rating did not perform equally with respect to various measures of plant health following a severe outbreak of ARR; in simple terms, alfalfa varieties with the same ARR resistance rating are not created equal. While this result is not unexpected, it is significant and warrants affirmation. Another, less intuitive, result was that, under severe ARR pressure, varieties exhibiting improved plant health and performance also exhibited greater variation in plant health and performance. In the Eden Shale trial, variances for seedling health, alfalfa yield, stand vigor, and stand density increased greatly as the means for these variables increased. This pattern was not observed in the UKREC trial, where disease pressure was moderate. This result is important, because it suggests that, under high disease pressure, performance of the currently available ARR-resistant varieties may vary considerably within or among fields. This result is consistent with our own observations in commercial fields, in that we have documented fields planted to alfalfa varieties highly resistant to ARR with rather serious cases of the disease. The reason for this variation is being investigated.