Herb growers and pesto lovers are feeling a bit stressed the past few years due to the increasing pervasiveness of basil downy mildew (BDM). Sweet basil plants, those most cherished by gardeners, are the most susceptible to the disease. BDM was first discovered in Uganda in 1993. The first reported outbreak of BDM in the United States was in Florida in 2007. By 2014 the Northeast was experiencing its worst outbreak. The disease has been reported in 30 states.
What causes BDM? Basil downy mildew is caused by the oomycete Peronospora belbahriian, an obligate parasite that can only survive within a living host, such as your basil plants and possibly basil seeds. (How the disease may be spread by seed is still not fully understood.)
How the disease spreads: The pathogen is transmitted by wind. The dark coloring of the pathogen protects it from the damaging UV rays of the sun–extending its lifespan and thus allowing it to travel great distances.
Identifying BDM: Many gardeners misdiagnose plants infected by BDM as a nutrient deficiency. What to look for:
• Bands of burned yellow on some leaves.
• Yellowing of the leaves. Plants may appear to be stressed by heat or nutrition deficits.
• Dark, sooty-looking spores can be seen on the underside of the leaf.
Plants at risk: No basil plant is completely resistant to BDM. Ocimum basilicum, or sweet basil, cultivars—‘Genovese,’ ‘Italian large leaf,’ ‘Poppy Joe’ and ‘Nufar’—are very susceptible to downy mildew.
The most resistant plants are O. x citriodorum and O. americanum cultivars like ‘Lemon Std’, ‘Lemon’, ‘Lime’, ‘Spice’, ‘Blue Spice’ and ‘Blue Spice Fil’.
Take action: Dispose of infected plants immediately. Do not compost, mulch or work into the soil. The disease is more prevalent in wet, humid environments. Therefore, space basil plants generously, allowing for maximum air circulation to help keep the plants dry.