By SHELLY STRAUTZ-SPRINGBORN
Michigan Correspondent
 
EAST LANSING, Mich. — Downy mildew has been confirmed in cucumber fields in seven Michigan counties.

According to Michigan State University Department of Plant Pathology professor Mary Hausbeck, the disease has been found in cucumbers in counties on the eastern side of the state, including Arenac, Bay, Ingham, Macomb, Midland, Saginaw and Tuscola.

On June 23, Hausbeck reported on her website that “the spore trap tapes from last week are being counted and show an increase in Monroe County. This increase in spore counts and the finding of downy mildew in a Canadian field require immediate action by all
Michigan cucumber growers. Spray downy mildew fungicides now.”

Downy mildew reproduces via tiny, microscopic spores that act as seeds of the pathogen. They are carried through the air by wind.
Hausbeck’s research team has eight downy mildew traps in six Michigan counties. These spore traps continuously sample the air and collect spores into the trap, which are embedded on a film that can be removed and taken to a laboratory for identification.

“The spore traps helped to alert us to any influx of spores into those production regions, but were not used to time fungicide sprays,” Hausbeck reported. “Since we do not have a trap in each field in a county, it is possible that we may miss an isolated spore mass coming into a particular region.”

On July 7, Hausbeck reported in her MSU Extension Crop Advisory Team (CAT) Alert that downy mildew found July 6 on cucumber leaves in fields in Bay, Saginaw and Arenac counties “showed lesions that appeared to be about seven days old.

“The current weather conditions that are hot and dry do not favor rapid spread and disease development. However, once the temperatures moderate and more moisture become available, I expect the downy mildew to increase in severity and spread.”

During the following week, much of Michigan experienced widespread rain, warmer temperatures and higher humidity – conditions that promote spread of the disease. On July 14,
Hausbeck’s MSU CAT Alert urged growers to “be on the lookout for downy mildew.”

Michigan pickle growers have battled downy mildew for five consecutive years. It causes symptoms on leaves similar to mosaic or angular leaf spot. A distinct symptom of downy mildew is dark, purplish-gray fuzz on the underside of the leaf that gives it a “dirty” or velvety appearance. The fuzz  may be most evident in the  morning and can infect leaves of all ages.

Downy mildew is known for causing catastrophic losses in a short amount of time. Unprotected foliage can become completely infected and appear frosted within 10 days of initial infection.
According to Hausbeck, chemical control must be focused on using the most effective products, alternating those products and applying the fungicides every five to seven days. Results from Hausbeck’s 2009 downy mildew research indicate that an effective spray program includes Previcur Flex (propamocarb hypochloride) plus Bravo (chlorothalonil), alternated with Tanos 50DF (cymoxanil and famoxadone) plus mancozeb.

Presidio and Ranman are newly registered products that are highly effective against downy mildew and against Phytophthora crown, fruit and root rot. All fungicides should be used in a tank mix with another fungicide effective against downy mildew.

Visit www.veggies.msu.edu for more information about downy mildew and chemical control.