The first ever confirmed case of basil downy mildew in North Carolina showed up on a Chatham County organic farm this week (the disease was reported, but not confirmed, in NC last year). This disease is new to the U.S., first revealing its destructiveness on northeastern U.S. farms in 2008. It affects basil and possibly a few other plants in the Lamiaceae family.

Basil downy mildew can be overlooked by growers because the initial symptoms tend to be subtle: the plants shown below exhibited slightly yellowing leaves as their most noticeable symptom, which may resemble a nutritional deficiency. The fuzzy-looking sporulation occurs on the undersides of the leaves. The disease is serious because it renders the leaves unmarketable. The disease can be spread from infected seed or leaves or from wind-dispersed spores which can travel long distances.

North Carolina State University plant pathologist Dr. Frank Louws says that basil growers in the state are at high risk for seeing this disease on their basil. It will continue to sporulate and spread. Be on the look-out for this disease and try and catch it early. Remove infected plants to help slow the spread to other rows and/or fields. If you can catch it early enough and cull it and you might be able to slow it down. Unfortunately, according to Dr. Louws, odds are it has already spread billions of spores. The farther away you get from the hot spot the more diluted the pathogen will be. There are no conventional or organic fungicides that have shown to be effective against this disease.

Update: Ken Dawson of Maple Spring Gardens reports that they had the disease last year (unconfirmed by NCSU) and this is what he said: Last season we continued to harvest in portions of the row that were unaffected. A few weeks later, as the weather dried out, the abandoned and infected portions of the row put out new growth that was not affected and we began to harvest again.

The sudden importance of basil downy mildew will hopefully lead to more control recommendations in the future.

Learn more about this new disease from Cornell University.