Basil downy mildew was first reported in the United States in south Florida in 2007. By 2008 and 2009 downy mildew had become a widespread problem on basil on the east coast of the U.S. both in greenhouses and in fields. The pathogen can be seedborne; however, the major means of new outbreaks is thought to be through windborne spores of this pathogen. Spores are easily wind-disseminated and can travel long distances. Symptoms on the upper leaf surface of irregular leaf yellowing and browning are often mistaken for nutrient deficiency or other abiotic problems. When the pathogen is producing spores, a grayish fuzz appears on the underside of leaves. High humidity and leaf wetness favor the development of this pathogen. Cultural controls that reduce leaf wetness, such as ensuring adequate plant spacing, good ventilation in the greenhouse and irrigating early in the morning, can help slow the development of downy mildew. In greenhouse situations, removed diseased plants and crop debris out of the greenhouse as soon as possible. There are currently no resistant varieties of basil; however, some trials have shown Ocimum basilicum cultivars to be more susceptible than Ocimum x citriodorum and Ocimum americanum cultivars. Fungicides labeled for use on herbs to control downy mildew may be used, but there are few of these. Copper and phosphite products with appropriate labels may be used preventatively. Biological fungicides may also be used, although there is little information currently on their effectiveness. Research on fungicides effective for this disease on basil is ongoing and more information may be available in the near future.
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