Basil downy mildew was first observed in the USA in fall 2007 in Florida. Seed contaminated with the pathogen was likely the initial source of the pathogen. Wind-dispersed spores are considered a more important source of the pathogen for field-grown plantings and gardens. There have been occurrences in greenhouses in winter for which contaminated seed seemed the likely pathogen source.
The causal pathogen is Peronospora belbahrii.
Sweet basil is more susceptible than spice types.
Other types of plants are not susceptible.
The pathogen cannot survive in soil overwinter.
Sources of the pathogen:
1. Wind-dispersed spores (sporangia) from other affected basil plants. The pathogen produces an abundance of spores easily moved long distances by wind; this explains the widespread occurrence of this disease every year. Sporangia are short-lived.
2. Seed contaminated with Peronospora belbahrii.
3. Plants and cuttings infected where produced before purchase.
Favorable conditions for development of downy mildew:
At least 85% relative humidity. The pathogen does not require leaves to be wet from rain or dew in order to infect or produce spores, but abundant sporulation has been observed following an evening with heavy dew.