Elizabeth Beggins alerted me to this problem which she is seeing in a number of beds in the community garden. The following information is from Johnny’s Seeds. Diseased plants should be pulled and put in the trash. Do not put diseased plants in the compost bins.
Basil Downy Mildew is a new disease to North America and Europe. It was first reported in Europe in 2001 and was identified in the U.S. in Oct. 2007. In 2008, downy mildew was confirmed in both field-grown and greenhouse-grown basil in many states including FL, NC, PA, NJ, NY, MA, KS, and MO. Prior to these outbreaks it was only known to occur in Uganda back in the 1930’s. To date the pathogen has been reported in 38 states throughout the U.S., including Hawaii, with devastating results.
DESCRIPTION: The pathogen (Peronospora belbahrii) is a water-mold (oomycete) that can be spread by contaminated seed, by infected basil leaves, and as wind-dispersed spores. Spores of the pathogen are capable of being dispersed long distances. Infected basil leaves produce an abundance of spores. Thus the pathogen can spread widely once introduced to an area. The optimum environmental conditions for disease development occur at high humidity levels with extended periods of leaf wetness.
SYMPTOMS: Basil Downy Mildew can easily be mistaken for a nutritional deficiency. The infected leaves develop a diffuse yellowing on the top of the leaf separated by veins. On the underside of the leaves, distinctly vein-bounded patches appear. When spores are produced, a characteristic purplish gray, fuzzy growth on the underside of the leaves is evident.
For a gallery of photos see link below. http://www.longislandhort.cornell.edu/vegpath/photos/downymildew_basil.htm
PREVENTION: At this time no commercially viable tests are available for the detection of infested seed. Prevention is the best course of action in all cases. Foliage should be kept dry. Use drip irrigation or bottom-watering wherever possible. Set up plantings to ensure good airflow within and between rows. Harvest the crop early if the disease is present or the risk of infection is high due to disease presence in the area. Carefully remove and destroy any infected plants. Fungicide applications need to begin before the disease is present to obtain effective control. Few products are labeled for this use. Additionally the post-harvest interval and label restrictions limit the use of many fungicide materials. Actinovate AG®, MilStop®, and Oxidate® are all OMRI-approved materials labeled for use on herbs to suppress foliar diseases including Downy Mildew.