The optimum time to seed alfalfa in many areas of the country is in the fall, when the ground is warm and the crop competes well with weeds. But a pocket of growers in California’s southern San Joaquin Valley don’t always dare to plant in September or October, when they’d get the best yields.

They’ve been hit hard – at times – by a fungus that can infect fields so badly some growers opt to disk those fields under.

The fungus, Sclerotinia, likes cool, wet weather and hits seedling alfalfa in particular, yet not on a consistent, yearly basis, says Carol Frate. She’s a University of California (UC) farm advisor studying the crown and stem rot disease.

“This disease has been the Achilles’ heel of that (fall planting) strategy because, if we have one of those wet, foggy winters, growers lose stands they have just planted,” she says.

So some decide it’s better to lose some optimum yield and plant in November or December. Others plant in late January or early February, which Frate advises.

Producers in areas of Illinois, Ohio, Delaware, and Kentucky, to name some other states, have also had to change practices in attempts to keep the pest at bay. But the southern San Joaquin Valley is surrounded by mountains and, when conditions turn humid and foggy, they can stay that way for weeks.

“If fog develops and we have some wet fields, the potential for disease is pretty bad, especially if we have apothecia,” she says.

Produced from black peppercorn-sized sclerotia, a hardy fungal structure that survives the summer conditions, apothecia are fruiting bodies that make and release fungi spores.

“Sclerotia can sometimes grow directly into the little threads (spiderweb-like mycelia) that fungi make and infect a plant. But when sclerotia produce these fruiting bodies, instead of maybe infecting one plant they happen to be next to, hundreds of thousands of spores are produced. So when your whole field is infected, or a huge part of it, it’s probably the result of a spore shower.”

A good stand of established alfalfa generally can hold up against Sclerotinia, says Dan Putnam, Extension forage specialist at UC Davis. An infected field will show the white spiderweb-like mycelia over the plants, which will also look wilted.

“Alfalfa will be set back quite a bit and may recover once conditions aren’t favorable for the pest. But it can kill most of the foliar growth and in bad cases go into and kill the crown,” he says.

The lush, dense crop canopy on seedling alfalfa encourages the disease as well. “If you have a canopy, even if you have sunlight on top of the canopy, conditions can be really nice for the disease underneath.”

Some recommend that growers who live in susceptible areas graze fall growth to reduce the canopy – and the disease’s opportunity to make use of it, he says.

At this point, there’s no fungicide on the market to control Sclerotinia in California. But Frate says there’s a promising product in the pipeline that she’s been testing.

“Looking at a trial conducted in the winter of 2004-2005, when we harvested our first cutting where we had no control – we just let it (Sclerotinia) go – we had the equivalent of 2.43 tons/acre of which 14.8% was weeds. With the fungicide I’m hoping gets registered in California by next fall, we had 2.78 tons/acre of which only 4.9% was weeds.

“That was one of the worst years. The following year, Sclerotinia was not as bad. In the 2006 trial, the fungicide treatment produced 2.62 tons/acre and the untreated control only 2.07 tons/acre.”