For the past seven years, a familiar scenario has been playing out on farms and in gardens across the U.S. A healthy, fragrant crop of sweet basil begins to display yellowing leaves. Upon closer inspection, the undersides of the leaves show signs of a menacing grayish sporulation. It is only a matter of time before the basil plant and others in proximity succumb to this new disease of basil, downy mildew.
Neither a fungus or a mold, downy mildew is the common name for a group of highly specialized plant pathogens called “oomycetes” that infect and feed off of living host plants. Each downy mildew is specific to its host plant. For instance, downy mildew of impatiens, another recent scourge, is specific to impatiens, while basil downy mildew affects only basil – with the most popular type, sweet basil, being the most susceptible.
Basil downy mildew favors heat and humidity, and by mid to late summer, when there is enough inoculum, the disease is widespread in our region. According to Extension Specialist in Vegetable Pathology Andy Wyenandt at Rutgers New Jersey Agricultural Experiment Station, basil downy mildew can’t overwinter in our region and can only survive the winter in southern Florida and Texas, where it is a year-round threat. The rapid spread to northern states during the growing season is through the planting of infested seed, by importing southern-grown plant material, or via weather patterns coming from southern states. [Read more…]