Downy mildew of basil (caused by the oomycete Peronospora belbahrii) was observed on greenhouse-grown plants for sale at a large retail operation in New Jersey last week. As the spores (sporangia) of P. belbahrii may spread long distances on air currents, producers, retailers, and home gardeners in the Northeast should be careful to check basil plants regularly for symptoms of this disease now and in the coming months.
Yellowing of the upper leaf surface is often the first symptom of basil downy mildew. Yellowed areas are usually bordered by leaf veins. When spores are produced, a characteristic fuzzy, dark gray to purple growth on the underside of the leaves is evident. Sporulation on the upper surfaces of leaves may be seen in severe cases. Symptoms of downy mildew on basil can easily be mistaken for a nutritional deficiency. The fuzzy growth of spores on the underside of the leaf may look as if soil had been splashed onto the leaf under-surface. Five to 10 days may elapse between infection and the appearance of symptoms; the duration of the latency period depends upon temperature and light exposure.
Fortunately, Peronospora belbahrii is not known to survive the winter in the Northeast. The pathogen may overwinter in a greenhouse environment on susceptible plant material, but in the absence of live hosts it is unlikely to persist. It has a very limited host range, infecting only a handful of other Lamiaceae (mint family) plants including coleus, agastache, sage, and rosemary. Nepeta curviflora (Syrian catnip, or“za'tar chachla”) has been experimentally infected, so it is possible that other species of Nepeta may also be susceptible. There are several relatively new resistant cultivars of sweet basil available. These include ‘Prospera’ and ‘Amazel’ as well as the Rutgers DMR series (‘Devotion’, ‘Obsession’, ‘Passion’, and ‘Thunderstruck’); ‘Eleonora’ and ‘Everleaf’ have partial resistance.