Basil plants in a nursery in Ohio were confirmed to have downy mildew by Nancy Taylor of OSU’s Plant and Pest Diagnostic Clinic (Fig. 1). The pathogen that causes basil downy mildew, Peronospora belbahrii, is seed borne and favored by cool, humid/rainy conditions. Overhead irrigation in the greenhouse/nursery promotes the development and spread of the pathogen, as does rainfall outdoors. Symptoms on leaves start with diffuse yellowing and browning that can look like sunburn. Leaf lesions later become black (Fig. 2). A diagnosis of downy mildew is made when spores of the pathogen are observed on the underside of lesions (Fig. 3); the best time to look for these dark purple-black structures clustered together is early in the morning.
There are differences in susceptibility of basil varieties and types to downy mildew, with sweet basil generally the most susceptible. Fungicides are available for downy mildew management in basil – see Dr. Meg McGrath’s article for recommendations for both conventional and organic systems. However, as is the case for the closely related cucurbit downy mildew, fungicides work best when applied preventatively – before symptoms are observed. Many, but not all of the fungicides are allowed for use in greenhouses or other protected culture systems such as high tunnels. See page 45 of the 2017 Midwest Vegetable Production Guide for Commercial Growers for a listing of allowed uses of fungicides for protected culture.
If you suspect downy mildew in your basil crop, you may send or drop off samples to the OSU Plant and Pest Diagnostic Clinic in Reynoldsburg or the OSU Vegetable Pathology Lab in Wooster (call, text or email Sally Miller (firstname.lastname@example.org)/330-466-5249 or Francesca Rotondo (email@example.com/865-640-5621).