Kentucky alfalfa producers should watch for signs of Sclerotinia crown and root rot in stands seeded late last summer, says Paul Vincelli, University of Kentucky (UK) Extension plant pathologist.

The disease, caused by the Sclerotinia trifoliorum fungus, typically becomes active in mid- to late autumn. Young seedlings in late-summer-seeded stands are especially vulnerable to it. They haven’t had time to develop adequate resistance before the pathogen produces infectious spores.

In contrast, spring-seeded stands are able to develop larger, more resistant crowns prior to the infectious period.

White threads or filaments of fungal growth, coming from crowns of dying or dead plants during humid weather, are signs the disease is present and active.

If you can’t find the filamentous growth but still notice wilting and dying plants, Vincelli says, reseeding a forage legume is still risky. That’s because the fungus will resume activity and attack seedlings when weather permits.

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Usually, the best option is to seed with non-legume forage crops – cool-season grasses seeded before April 15-20 or summer annuals for seedings made after May 1.

While no commercial varieties offer complete resistance to Sclerotinia, some provide partial resistance, Vincelli notes. He recommends Phoenix and Cimarron SR varieties, which offer partial resistance under Kentucky field conditions. “Be aware that these varieties can still suffer stand loss from the disease, but they will suffer considerably less stand loss than the many susceptible varieties on the market,” he says.

For more information, check out this fact sheet from UK Extension.

Here’s more on forage pests:

Scary Sclerotinia Hurts Alfalfa Yields

August Alfalfa Seedings Fool Weeds, Diseases