by Paul Vincelli

A late-March seeding of alfalfa in Adair County, Kentucky was diagnosed last week in the University of Kentucky Diagnostic Labs as Aphanomyces root rot. Throughout the field, a large proportion of the seedlings exhibited stunting, purpling, and even death. The producer noticed the symptoms during the last week of April. Although this could be an isolated case, it is more likely that other cases have occurred, at least sporadically.

Aphanomyces root rot typically is associated with sustained periods of wet soils. Soil moisture in Adair County during much of April was on the moderately dry side. Dry soils don't completely arrest the activity of the fungus, however, and the 3½ day period of rainy weather that was recorded in the Campbellsville Weather Station from 11-14 April was apparently enough to trigger activity of the organism in the soil. More recent periods of extended rainfall, such as this past weekend, could have also been favorable for infections in other locations.

The variety seeded in the case we diagnosed was one with a "R" (=Resistance) rating to Aphanomyces. This means that 31-50% of the seedlings of that variety show resistance to Aphanomyces in standardized tests. This also means that anywhere from 50-69% of the seedlings are susceptible, and that is very consistent with what was reported from the field: widespread damage to approximately half to two-thirds of the seedlings, with the other seedlings appearing healthy.

University of Kentucky research in the 1990's clearly implicated Aphanomyces as a major risk to spring-seeded alfalfa in many Kentucky soils. Based on that research, we have recommended that spring seedings be made with alfalfa varieties having an R or HR ("High Resistance" rating, meaning that over 50% of the seedlings exhibit resistance) rating to Aphanomyces. In our previous research, these two levels of resistance have performed equally well, even in sites with severe pressure. However, under certain circumstances, the experience in Adair County suggests that varieties with an HR level of resistance are needed. In any cases, a springtime seeding of a variety with a lower rating than "R" to Aphanomyces is unwise.

As a reminder, the levels of susceptibility/resistance to diseases in alfalfa are:

0-5% resistance
Susceptible (S)

6-14%
Low resistance (LR)

14-30%
Moderate resistance (MR)

31-50%
Resistance (R)

51+%
High resistance (HR)