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Publication #PP271

Downy Mildew of Basil in South Florida1

Shouan Zhang, Zelalem Mersha, Pamela D. Roberts, and Richard Raid2

Introduction

Downy mildew of basil, caused by Peronospora belbahrii, is a new destructive disease that has spread rapidly in the U.S. since its first detection in South Florida in 2007. In 2008, basil downy mildew was detected in Kansas, Massachusetts, Missouri, New Jersey, New York, North Carolina, and Pennsylvania. The disease was subsequently identified in Alabama, California, Connecticut, Delaware, Georgia, Illinois, Indiana, South Carolina, Vermont, and Virginia in 2009, in Kentucky, Ohio, Texas, and Wisconsin in 2010, and in the Hawaiian Islands in 2011. Sporulation on the abaxial leaf surface and chlorosis of the adaxial leaf surface make basil leaves unacceptable for the marketplace. Downy mildew has also been reported in many other countries, such as Switzerland, Italy, France, Belgium, Israel, New Zealand, and South Africa during 2001-2005. Damage from the downy mildew disease has also been reported in Cameroon, Canada, Cuba, and in greenhouse-grown basil in Argentina. Farmers in Europe have experienced the impact of the downy mildew disease earlier (2001), but reports of the disease have continued as recently as 2010 from Hungary. This implies the possibility that the disease may already exist in other countries but has neither been detected nor reported. Prior to these recent outbreaks, downy mildew had only been reported on basil in Uganda in 1933.

Recently, downy mildew was also observed on ornamental plants related to basil, particularly coleus (Solenostemon spp.) and salvia (Salvia spp.), which belong to the Lamiaceae family and includes basils (Ocimum spp.), mints (Menta spp.), sages (Salvia spp.) and other aromatics. The oomycete pathogens (Peronospora spp.) that cause downy mildew on coleus and basil were demonstrated to be genetically different. However, there are many basil-related ornamentals that are also hosts to the downy mildew pathogen, affecting basil grown for use as an herb. Although sporangiophores and sporangia of the basil downy mildew pathogen (Peronospora belbahrii) are capable of dispersing long distances, it is believed that contaminated seed is most likely the way that the basil downy mildew pathogen has been able to move between geographically separated areas. The pathogen found in Florida has been shown to be genetically the same as that in Switzerland.

Symptoms

The downy mildew pathogen that infects plants related to basil, such as the ornamental plants coleus and salvia, is present in Florida. However, it is not known whether this is the same downy mildew pathogen and which particular hosts are infected by this strain, although the isolate from coleus is reported to infect basil in greenhouses in New York.

Symptoms of downy mildew initially appear as yellowing of basil leaves (Fig. 1). Upon close observation of individual leaves, an irregular-shaped chlorosis or typical vein-bounded symptoms are clearly discernible when contrasted to the green foliage of healthy basil leaves (Fig. 2). Growers generally do not realize their basil plants are infected with downy mildew disease since the most noticeable symptom on affected basil is leaf yellowing (Fig. 1), which is phenotypically similar as the result of a nutritional problem. The discolored area may cover most of the leaf surface. On the underside of leaves, a gray, fuzzy growth may be apparent upon visual inspection (Fig. 3). Under high humidity, the chlorotic areas on the leaf turn from dark brown to black very quickly. Sporangia, the reproductive structures of the pathogen, are easily detected under magnification and are a diagnostic for this disease (Fig.5).

Figure 1. 

Symptoms of downy mildew on field-grown basil (left) compared with healthy basil plants (right)


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Figure 2. 

Top view of basil leaf a) without any disease and b-d) exhibiting chlorosis and vein-bounded yellowing of downy mildew


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Figure 3. 

Sporulation evident on underside of the basil leaf


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Figure 4. 

Microscopic mount of a) down-hanging aseptate sporangiophore of Peronospora belbahrii and b) germinating sporangium of the basil downy mildew pathogen


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Control

Few fungicides are currently labeled for downy mildew control on basil. Some phosphorous acid fungicides are effective against downy mildew under herbs on the current label. These fungicides were effective in fungicide efficacy experiments with applications started before or after initial symptoms were found. Actinovate AG is an OMRI-listed fungicide that is labeled for use on herbs and for suppressing foliar diseases, including downy mildew. Other fungicides are expected to be labeled for this use in the future.

Although few fungicides are specifically labeled for this disease, some fungicides that are labeled for basil may be useful in disease management. Reducing the period of leaf wetness by avoiding overhead watering may also be helpful. Heavily infected plants should be discarded. If possible, isolate new plantings to reduce inoculum spread from older plantings. The pathogen is believed to be seed transmitted. Consult the University of Florida IFAS Plant disease Management Guide and current labels for specific and current fungicide recommendations.

Footnotes

1.

This document is PP271, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date September 2009. Revised August 2012. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Shouan Zhang, assistant professor, and Zelalem Mersha, postdoctoral research associate, Department of Plant Pathology, Tropical Research and Education Center (REC)--Homestead FL; Pamela D. Roberts, professor, Department of Plant Pathology, Southwest Florida REC--Immokalee FL; Richard Raid, professor, Department of Plant Pathology, Everglades REC--Belle Glade FL; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.