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Fruit and Vegetables |
Downy mildew has been detected from several fields of slicer cucumber in Sampson County. In some cases the disease is causing as much as 20 to 25 percent defoliation (Figure 1; Figure 2). This has caught growers by surprise, since downy mildew usually occurs much later in the season and is generally not of great concern to cucumber production.
We have been tracking cucurbit downy mildew outbreaks in the U.S. since 1998. The typical yearly pattern begins with disease outbreaks in southern Florida in February. As the season progresses, outbreaks are reported from progressively more northern locations, arriving in North Carolina in July or August. Go to the following web site to see the forecasting system and for additional information on downy mildew: http://www.ces.ncsu.edu/depts/pp/cucurbit/. The disease is usually detected first on the most susceptible cucurbits - squash and pumpkin. While it is not difficult to find downy mildew on cucumber on the fall crop, the disease rarely becomes a big enough problem to warrant fungicide applications. This is largely due to the excellent disease resistance in most commercial cultivars. So it is surprising to not only find downy mildew this early, but to find it on cucumber.
The first line of defense against cucumber downy mildew is the use of resistant cultivars and these are widely available. The slicer cultivar Speedway does not have resistance to downy mildew, but Dasher II and Conquistador do have resistance. The disease resistance profile of the cultivar grown should be determined before deciding on a fungicide spray program. Several commercially available fungicides have good activity against cucurbit downy mildew. These include chlorothalonil, Tanos, Gavel, Pristine, Cabrio and Ridomil Gold Bravo. All of these products work best when applied preventatively and all but chlorothalonil require alternation with fungicides that have a different mode of action. Resistance has been reported to Ridomil Gold and the effectiveness of this product may be greatly reduced if the fungus is resistant. For a list of fungicides and their activity on cucurbit diseases, go to the following web site: http://ipm.ncsu.edu/agchem/chptr6/615.pdf.
If you encounter an outbreak of cucurbit downy mildew, please report it to the forecasting system at: http://www.ces.ncsu.edu/depts/pp/cucurbit/form/form.htm.
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Last modified on June 7, 2004 by Stephen J. Toth, Jr.
