Master Gardener℠ Volunteers Participate in Second Year of Basil Downy Mildew Trial

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Basil Downy Mildew (BDM) has had a devastating effect for both commercial and home growers on what once was considered to be an easily grown culinary herb. Here’s what causes this disease, and the results of recent research trials with Master Gardener℠ Volunteers in North Carolina to find good-tasting BDM-resistant varieties.

BDM is caused by a pathogen called Peronospora belbahrii, first confirmed in Florida in 2007. Sweet basil (Ocimum basiilicum) is its favorite host. BDM spores are carried by the wind, not soil-borne. The spores don’t survive North Carolina frosts, but spend their winters in frost-free locations or in warm greenhouses. Every summer they slowly come our way on warm breezes, traveling long distances until arriving in WNC in mid-to late summer. The spores land on plants and are most pervasive during wet, humid periods. BDM can also be transmitted via infected seeds or seedlings.

What to Look For

The first symptom of infection is a yellowing between the veins on the tops of older leaves; brown spots follow. Sporulation (fuzzy, powdery-looking grey areas which change to purple-brown) appear on the underside of leaves that match yellow areas on the top. The dark spores spread quickly and the leaves fall off. The entire plant generally dies within a couple of weeks.

BDM is a type of water mold, not an actual fungus, so fungicides are not effective. The best cultural methods to prevent BDM include: planting a resistant variety; planting in full sun; and avoiding wetting the leaves when watering.

Basil Downy Mildew Research Project

Master Gardener Volunteers from 22 counties in North Carolina participated in research trials in 2021 and 2022. Five BDM-resistant basil varieties (Prospera, Obsession, Devotion, Passion, and Thunderstruck were included, although other resistant varieties are available) were grown alongside the very popular Genovese (which is susceptible to BDM). Different Haywood County participants saw BDM in their trial plots in both years, and observed that all five resistant varieties were good at avoiding this disease.

In addition, blind taste tests were held in 2022 with all five resistant varieties, to find an equivalent flavor profile to Genovese. These tests were done using identical recipes for Caprese salad and Bruschetta. Prospera was judged to be the favorite followed closely by Obsession. New developments using Prospera include compact forms and a large leaf hybrid. Check seed catalogs for those that might grow best in your own garden.

Written by: Julie Taylor, Master Gardener Volunteer, Haywood County

Haywood MGVs seed basil for the start of the 2022 Trial Season. Photo credit: Inga Meadows

Wilson County MGVs planting basil. Photo credit: Cyndi Lauderdale

Chatham Co. MGVs participate in the basil taste test. Photo credit: Matt Jones