Downy mildew (Peronospora sp.) on basil was a major problem on both field- and greenhouse-grown basil during 2008 and 2009. High humidity and extended leaf wetness favor pathogen development.
Univ. of Mass. plant pathologists Rob Wick and Bess Dicklow said the production practices of increasing spacing between plants and watering earlier in the day can help to reduce the amount of time leaves remain wet. Increasing air movement in the greenhouse with horizontal air flow fans can help to reduce condensation. Infected plants should be removed and destroyed to hinder further spread of the disease.
Field trials conducted in New Jersey in 2009 determined that commonly produced sweet basil (Ocimum basilicum) cultivars including ‘Poppy Joe’ and ‘Nufar’ were the most susceptible to downy mildew. Least susceptible basils include the lemon and spice types such as O. x citriodorum and O. americanum cultivars, including ‘Lemon’, ‘Lime’, ‘Spice’, ‘Blue Spice’ and ‘Blue Spice Fil’.

Pictured: The discolored foliage on the upper leaf surface of a basil plant caused by downy mildew could be confused with a nutritional deficiency.
Photo by Rob Wick, Univ. of Mass.