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Flowers, Fruits, and Frass

Local and statewide information on a variety of current topics for home gardeners and market growers.
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Downy Mildew on Basil

According to Dr. Mohammad Babadoost, University of Illinois plant pathologist who specializes in vegetable crops, there are currently over 550-600 acres of commercially grown basil grown in Illinois. The Illinois basil industry is valued between $ 10,000-30,000 per acre. Horticulture educator, Kelly Allsup, states this may be a concern for gardeners as well because herbs are popularly grown for culinary and ornamental purposes and have been relatively pest and disease free till now.

In 2009, Downy mildew (Peronospora belbahrii) was detected in Illinois and has become a major concern for small growers. The leaf damage caused by the fungus makes the basil unmarketable. One challenge for growers is that the fungus is quite difficult to distinguish from a nutrient deficiency because the leaves turn yellow first. However, once the leaf is turned over you will see brown sporulation. Yellow leaves advance to necrotic spotting covering the plant. Other crops affected by downy mildew are lettuce, crucifers and cucurbits. The disease thrives in wet, cooler temperatures, but is still very viable once the temperature rises.

University of Illinois suggests planting less susceptible varieties. For instance, sweet basil (Ocimum basilicum) is more prone to downy mildew than some of the purple varieties. Good plant practices to minimize disease include planting in full sunlight; ensure good air movement and the use of drip irrigation to prevent water from sitting on leaves. It is suggested to destroy plants on a sunny day to minimize length of time the fungal pathogen remains viable. Few fungicides have been labeled to control this disease on basil. The active ingredient Phosphorus acid has been shown to give control. To date, Dr. Babadoost's fungicide trials at the University of Illinois, have demonstrated ineffectiveness in organic fungal treatments.

When spraying fungicides, be sure to read the pesticide label in order to find out what crops can be sprayed, diseases controlled, pre-harvest interval and personal protective equipment. It is against the law to spray crops not on the pesticide label. When in doubt, visit your local extension office for information on control and identification of Downy Mildew.

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