Basil plants with severe downy mildew symptoms in a big box store garden center in Wooster, OH, 2021. Photo by F. Rotondo.
We received a report on August 8 from an Ohio Master Gardener of sweet basil downy mildew in a garden near the OSU main campus in Columbus, Franklin County. We often see this disease in Ohio for the first time in late July or early August, although our reporting, aside from sentinel plots in northwest and central Ohio, depends on the growing and gardening communities giving us a heads up when observed. So, downy mildew may have been present in Ohio earlier than this first report. The pathogen, Peronospora belbahrii, does not overwinter in Ohio and arrives most years on air currents from the south. The pathogen is also seedborne and has been introduced earlier on basil seedlings and transplants in nurseries and retail stores as we observed in 2017 and 2021. For future plantings, there are sweet basil varieties now available with good resistance to downy mildew. These include Prospera Compact DMR, Prospera DMR, Prospera Red DMR, Rutgers Devotion DMR, Rutgers Obsession DMR, Rutgers Passion DMR, and Rutgers Thunderstruck DMR, available from a number of seed companies as organic or non-organic seeds. Resistance in the varieties may break down under severe disease pressure from favorable weather conditions (cool, overcast, high humidity, rainy) and high inoculum levels, so crop protectants may also need to be applied. A detailed listing of fungicides and biologicals registered for downy mildew management was published recently by Dr. Andy Wyenandt, Rutgers University.
Downy mildew was also reported on cucumbers in Huron County, Ohio this week.