Downy Mildew Close-up

Downy Mildew Spore Growth on Basil Leaves

Downy mildew of basil is a new destructive disease that appears to be here to stay. In the first years of its appearance in the U.S., complete crop losses occurred for some growers because basil leaves with any mildew are unmarketable. Applying fungicides frequently and starting before first symptoms are considered necessary to control basil downy mildew effectively.

“Sweet basil (Ocimum basilicum L., Fam. Lamaiaceae) is the most commercially important annual culinary herb crop grown in the United States. Sweet basil is grown for culinary use for both fresh and dry consumption and as a source of essential oil and oleoresin for manufacturing perfumes, food flavors, and aromatherapy products.

Basil downy mildew… is a new disease of basil in the United States. …In the United States, the pathogen was first discovered in Florida in the fall of 2007. Since that time, basil downy mildew has been found throughout the eastern United States and in regions of commercial basil production in the Midwest and California.

Once basil plants become infected and develop symptoms, they are no longer marketable as a fresh product. …Currently, there is no known resistance or tolerance to basil downy mildew leaving 100% of the sweet basil acreage in the eastern United States vulnerable to the pathogen. Without adequate chemical control options and genetic resistance, basil downy mildew has the potential to destroy basil production in the eastern United States and in all other areas where basil is being produced.

Selection criteria such as foliar morphology, plant architecture as well as the presence of secondary metabolites and other factors that provide a less favorable microenvironment to the pathogen need to be examined as potential avenues for developing downy mildew-resistant sweet basil cultivars. Until this can be achieved, basil growers will have to rely on multiple applications of the few commercial fungicides currently registered to produce a marketable crop. Additionally, for organic basil growers, control of basil downy mildew will be even more challenging because there are fewer approved products labeled for organic use.”

Authors: Wyenandt, C. A., et al.
Affiliation: Department of Plant Biology and Pathology, Rutgers University
Title: Susceptibility of basil cultivars and breeding lines to downy mildew (Peronospora belbahrii)
Source: HortScience. 2010. 45(9):1416-1419.