The Homeowners Column

The Homeowners Column

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Alert for New Disease of Basil

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Sandra Mason
State Master Gardener Coordinator

Which states lead in the production of basil? Illinois is likely not one of your first guesses. However basil is big in Illinois. Approximately 600 acres of basil are planted in Illinois with a value between $10,000 and $20,000 per acre. Unfortunately a serious disease, basil downy mildew, is threatening basil production.

Diane Plewa, University of Illinois Extension Educator and diagnostician, recently wrote in UI Extension's Home, Yard and Garden Pest Newsletter, that a basil sample from Wisconsin was recently diagnosed with downy mildew at the University of Illinois Plant Clinic. Basil downy mildew was a serious problem last year and, depending on the weather, we may be seeing more of it in 2014. This pathogen affects both home growers with a few basil plants and producers who cultivate commercial basil in Illinois.

According to Dr. Mohammad Babadoost, UI plant pathologist in the Department of Crop Sciences who specializes in diseases of vegetable crops, this disease is very serious for Illinois growers. While there are a number of other important downy mildew diseases, including the infamous impatiens downy mildew, basil downy mildew is host specific and will not infect other plant species.

Basil downy mildew was first reported in the United States in 2007. By 2009 it had reached Illinois late in the growing season. It is caused by a fungal-like water mold that flourishes in cool wet weather, so the disease is generally worst at the beginning and end of the growing season. The pathogen goes dormant in hot, dry weather.

Symptoms first appear as diffuse yellow areas on the upper surface of leaves. The pathogen produces spores and translucent, thread-like structures on the underside of leaves, giving them a dirty fuzzy appearance best seen with a hand lens. The disease progresses quickly, with affected leaves turning brown and falling from the plant; within a few days an entire plant can be defoliated. Typically nearby basil plants are already infected even if they are not yet showing symptoms.

It is unknown if the pathogen can survive the winter in Illinois; however, it may overwinter in greenhouses, or travel in on cuttings. The spores can travel long distances by wind. A small initial number of spores can quickly lead to a huge infestation.

Dr. Babadoost's laboratory has been conducting experiments for the past 5 years to identify management options for basil downy mildew. Presently management including the use of fungicides requires a pesticide applicator license.

To avoid downy mildew of basil:

As with all plants, inspect leaves for any signs such as fuzzy growth on the undersides of leaves before purchase.

Downy mildew tends to be less virulent with red or purple basil cultivars so these may be good alternatives.

Don't crowd basil plants. Give them plenty of room to allow good air circulation. Because the pathogen needs moisture to thrive, reducing humidity and leaf wetness is critical.

Plant basil in full sun.

Water at base of plants. Do not water basil via overhead sprinklers (especially at night) and avoid any other conditions that may promote leaf wetness. Fungi love wet leaves.

Be on the lookout for this disease, especially during cool temperatures (spring or fall). Early detection is key.

Bring suspected plants to UI Plant Clinic: S-417 Turner Hall, 1102 S. Goodwin Ave.

Urbana, IL 61801; Phone: 217-333-0519;

Sanitation (removing and destroying diseased plants as well as nearby basil plants) is an important management technique.

Diseased plants can be allowed to dry thoroughly or buried in an unused part of the garden. However do not place suspected downy mildew plants in the compost pile.

University of Illinois Extension Home, Yard and Garden Pest Newsletter, published online free of charge, is a fabulous resource for home gardeners and commercial growers. Sign up for email alerts so you never miss an issue.

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