Basil downy mildew, first identified in the United States two years ago, is spreading rapidly on the East Coast and has also been found in California and the Midwest. Plant pathologists warn that basil crops everywhere are at risk, and they urge growers to inspect their plants for signs of the disease.
Basil downy mildew first appears as yellowed leaves, leading many growers to assume there is a nutrient deficiency, sunburn, or water stress. However, gray, fuzzy sporulation on the underside of the leaves reveals the presence of downy mildew.  Because the foliage is the edible part of the plant, basil downy mildew makes crops unmarketable.
Margaret Tuttle McGrath of Cornell University’s Long Island Horticultural Research Center, says there are few fungicides labeled for downy mildew on herbs at this time, but more are expected to labeled for it in the future as the disease becomes more established. Actinovate AG is an OMRI-listed fungicide available to organic growers for use on herbs.
The disease is relatively new worldwide as well as in North America, McGrath said. It appeared in greenhouse basil crops in Switzerland in 2001, and has since moved across Europe, the Mideast, and Africa. It was first reported in south Florida in October 2007. In 2008, it was reported on the East Coast, Kansas and Missouri and in Canada. This year, it has also been reported in California. Before these recent outbreaks, it was known only to have occurred in Uganda in 1933.
McGrath says the disease may be spread in contaminated seed and in wind-dispersed spores. She asks growers to report it on a special website she has created (http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html#Report), to try to determine if a monitoring program can be used to help growers be prepared for its arrival, similar to the monitoring and alerting system used for cucurbit downy mildew.
In North Carolina, where the disease was confirmed for the first time this year, Extension is advising growers to remove infected plants to help slow the spread. But a grower who said he had it last year (though it was not confirmed by North Carolina State University) said that he left infected plants in the field and after the weather dried out, they put on new growth that was not infected and was marketable.