Mix it up

03/01/2005 02:00:00 AM

Downy mildew does not overwinter in areas where hard frosts occur, but migrates north from the southern vegetable growing regions of Florida and Texas. Every year is a whole new disease scenario. Last season, heavy dews in spring and a tropical storm in July created the cool, wet conditions favoring the development of downy mildew in cucumbers, and Holmes says pressures were so strong the disease became an economic problem in spite of inbred resistance.

Holmes says many growers were caught completely off guard with losses averaging 40 percent on cucumbers in the mid-Atlantic. Watermelons were also hit hard in certain regions.

"We were really devastated by the disease on cucumber. It was definitely the worst epidemic anybody can remember," he says. "Downy mildew is a serious problem all over the world, but our resistance has held up over the last several decades."

As a result of last year's epidemic, growers this season will be much more vigilant and have fungicides in the wing when the disease risk is high.

In addition, a University of North Carolina Web site provides a downy mildew tracking service, so growers throughout the country can see how the disease is progressing and at what time it might appear in their growing region.

The information will allow you to make preventative sprays in advance of disease symptoms but close to the period they are likely to occur. Controlling downy mildew before symptoms appear is a much better strategy--both for managing the disease and its potential resistance to fungicides--than waiting for an outbreak and attempting to control it curatively.

Pathogens evolve
Holmes says last year's epidemic was either due to favorable conditions or an evolving organism that may be overcoming some level of varietal resistance when the pathogen is in high enough numbers.

"We don't really know, but perhaps it's the two combined that contributed to this problem," he says.

The scenario at any rate provides a perfect case study for the importance of managing disease with resistance in mind to keep both fungicides and resistant varieties available for as long as possible.

Holmes says no one knows whether downy mildew will appear again in Northeastern cucumbers and other cucurbits as severely as it did in 2004. But an integrated program of monitoring the path of the disease, weather conditions and symptoms, and using resistant varieties and effective preventive fungicides as conditions warrant should help growers stay ahead of downy mildew.

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