Basil plants that originated from a nursery on the central coast of California nursery were recently found infected with the downy mildew pathogen, Peronospora belbahri, in a home and garden store in the state.
This is a new pathogen that can cause serious damage to both culinary and ornamental basil plants, says Surendra Dara, strawberry and vegetable crops advisor for University of California (UC) Cooperative Extension in Santa Barbara and San Luis Obispo counties. Symptoms include yellowing of the area between the veins on the upper leaf surface and growth of fungal mycelia and purplish grey spores on the lower surface. Yellowing on the upper leaf surface can be confused with nutritional deﬁciency, so careful examination of the lower side is important.
Basil downy mildew was ﬁrst reported in the United States in 2007 in Florida and in several eastern states by 2008, Dara says. It was also found in California by 2009. Infected seeds and plants are the main sources of infection. Growers should try to obtain clean seeds, he advises.
Spores can be dispersed by wind and overhead irrigation can also aid in the dispersal of the inoculum. Regular monitoring of ﬁelds for early detection of the infection and timely application of fungicides is important for managing this disease. Proper ventilation and temperature control in the greenhouses is also critical, as prolonged leaf wetness, high humidity and cool weather promote the disease development.