Basil downy mildew has become more common over the past few years with 2014 being one of the worst years in a long time. It is relatively new to North America, and something all gardeners who grow basil should start to be on the lookout for.
Basil downy mildew is a fungal disease that was first reported down in North America in Florida in 2007. It has since spread across the continent, and especially along the Eastern seaboard. One of the primary symptoms is yellowing of the leaves, which is easy to overlook and often associated with a nutrient deficiency. If not treated, entire crops can be lost as the plant can spread easily between varieties and affect young and mature plants.
As the disease progresses, dark and fuzzy spore spots can be seen on the underside of the leaves. Basil downy mildew is humid conditions with average to cooler conditions. Very hot weather can minimize disease transmission.
Basil downy mildew can be spread via infected seeds or transmitted through the air, making it easy for it to travel long distances. No conventional or organic fungicides have been shown to be effective in treating basil downy mildew. Remove infected leaves and/or infected plants, minimize wetness on moisture on plant leaves during humid periods, and increase air flow and plant spacing to minimize the likelihood of the mildew taking hold.
There are some down mildew resistant basil varieties as well that can be purchased.
Cover Image by Forest and Kim Starr, used under its Creative Commons license.