College teams up with state to battle pathogen infecting basil plants

University of Hawaiʻi at Mānoa
Posted: Feb 1, 2011

Basil leaf infected with Downy Mildew
Basil leaf infected with Downy Mildew
UH Mānoa’s College of Tropical Agriculture and Human Resources (CTAHR) is partnering with the state Department of Agriculture to combat Basil Downy Mildew (BDM). Last Friday, January 28, the pathogen (Peronospora belbahrii) was identified by basil growers at several farms in Waianae.
This is the first known incidence in Hawai‘i of the fast-spreading mildew, which has been found throughout Europe, Israel, New Zealand, Argentina, and some parts of Africa.  It reached the mainland U.S. and Canada in 2007.
Although Waianae is generally warm and dry enough that downy mildew is not a problem, the recent cool weather and unusual rains have created a receptive environment for the pathogen. 
Downy Mildew is characterized by black, gray, clear, or brownish spores on the undersides of the leaves, followed by yellowing, then black or brown leaf discoloration and die-off.  Affected basil is rendered unfit for sale or consumption, and some mainland basil growers have lost their entire crops due to BDM.
Basil is a $6.8 million (2009 farm gate value) crop in Hawai‘i, sold throughout the state and exported to the mainland U.S. and Canada. 
Says CTAHR plant pathologist Dr. Janice Uchida, “We are fortunate that our Cooperative Extension and farm safety field staff alerted us to this problem, allowing identification of  the pathogen and development of remediation strategies. Information on these strategies will be disseminated to basil farmers and the general public.” 
Because this downy mildew can be spread through infected seeds and leaves, as well as windborne spores, eradication must proceed in several steps.  Affected leaves are carefully gathered, burned or buried. New growth is treated with fungicide, and all tools and equipment are sanitized to reduce remaining spores.
Uchida stressed the importance of containing this new pathogen to ensure that it does not spread to wetter areas and the neighbor islands, where it can gain a stronger foothold. Both CTAHR and the Department of Agriculture are working to avert this.
Farmers who suspect the presence of this new Downy Mildew should contact CTAHR’s Agricultural Diagnostic Service Center at (808) 956-6706 for confirmation and detailed eradication techniques.
Additional information will be posted at CTAHR’s website at as it becomes available.