Basil belongs in everyone’s herb garden. It is easy to grow, and it will save you from having to purchase overpriced fresh basil at the grocery store. However, the ‘Genovese’ basil cultivars that are most commonly grown for culinary purposes are susceptible to a disease called basil downy mildew (BDM).
Basil downy mildew is caused by an oomycete (water mould) pathogen called Peronospora belbahrii, making it related to the causal agent of cucurbit downy mildew (Pseudoperonospora cuebensis) and, more distantly, to pathogens responsible for many root rot diseases (Phytophthora spp.). The pathogen thrives in warm, humid conditions, and requires warm conditions throughout its life cycle, limiting its year-round presence to southern Florida, or in infested greenhouses further north. This means that, like its cucurbit downy mildew cousin, most infections in North Carolina don’t occur until the spores have arrived on wind currents from further south in the mid to late summer.
Dr. Margaret McGrath of Cornell University maintains the Basil Ag Pest Monitor website (basil.agpestmonitor.org) which tracks the movement of the disease each year. The severity of the disease can vary in different areas from year to year depending on pathogen dispersal patterns, local environmental conditions, and host availability. Last year, basil downy mildew infections in North Carolina were relatively mild.
Symptoms begin as yellowing (chlorosis) of the leaves. Early symptoms are often mistaken for nutritional deficiencies. Both nitrogen and magnesium deficiency can occur in basil (the latter especially in container grown plants), but the yellowing would appear relatively uniformly on the lower leaves if the chlorosis was nutrient related. If the yellowing is caused by early basil downy mildew infection, the distribution pattern usually appears more randomly. Within a few days, more definitive signs emerge as the infection advances. On the underside (abaxial) leaf surfaces, the spore-releasing structures (sporangiophores) develop, resulting in the black, downy appearance of the disease’s namesake. Infected leaves eventually turn brown and die.
Remove infected leaves and plants, as they can serve as a further source of inoculum. Professional growers have access to several fungicides. Some products are labeled for use in organic production, but the efficacy of these products is low. While some fungicides available to home gardeners can help mitigate disease spread, the best management strategy is to plant resistant basil cultivars. Some basil cultivars that are more associated with southeast Asian cooking show less severe basil downy mildew symptoms, including ‘Red Leaf’ ‘Queenette’ (Thai basil), ‘Lemon’, ‘Lime’, and ‘Blue Spice’.
The ‘Genovese’ cultivars many associate with Italian culinary traditions (think insalata caprese, pizza Margherita) are highly susceptible to basil downy mildew. Recently, researchers at Rutgers University and Bar-Ilan University in Israel have bred several new ‘Genovese’ type cultivars that are highly resistant to basil downy mildew. The Rutgers University team developed ‘Rutgers Obsession DMR’, ‘Rutgers Devotion DMR’, ‘Rutgers Passion DMR’, and ‘Rutgers Thunderstruck DMR’ which are produced and marketed by VDF Seeds and Johnny’s Selected Seeds. The Bar-Ilan University developed ‘Prospera’ cultivar seeds are distributed by Genesis Seeds Ltd.
North Carolina Extension Master Gardener Volunteers from 18 counties across the state are now testing all five of these cultivars at NC Cooperative Extension County Centers or in their own home gardens. We piloted this study last year to determine how Master Gardener Volunteer research projects could work. We found volunteers were enthusiastic about doing research trials and thought they would be beneficial to outreach and education efforts. Master Gardener Volunteers will assess not only how these resistant cultivars perform in the garden, but also how they appear and taste compared to the susceptible traditional ‘Genovese’ cultivar. The trial is funded by the NC State Extension Youth and Consumer Horticulture Working Group.
Stay tuned for updates on the project late this winter. In the meantime, consider trying some of the resistant cultivars out yourself! For more about basil downy mildew, visit: https://content.ces.ncsu.edu/basil-downy-mildew.
Matt Jones is the Horticulture Extension Agent at the NC Cooperative Extension Chatham County Center.