Cucurbit downy mildew spore counts increase in Monroe County - fungicide sprays recommended

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

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! See fungicide recommendations accompanying this article.

Michigan pickle growers have battled downy mildew, incited by the water mold, Pseudoperonospora cubensis, for five consecutive years. Downy mildew is well-known for causing catastrophic losses in a brief period of time. Unprotected foliage can become completely infected and appear to be frosted within 10 days of initial infection. This downy mildew pathogen is resistant to commonly used fungicides including Ridomil Gold-based products and the strobilurin fungicides (i.e. Cabrio, Quadris, and Flint). Results from our 2005-09 research identified a limited number of fungicides that are effective, but must be applied every five to seven days when the weather favors disease.

Downy mildew spore trapping 2010

The downy mildew reproduces via tiny, microscopic spores that act as seeds of the pathogen. Eight spore traps were placed in six Michigan counties. These spore traps continuously sample the air and collect spores into the trap. These spores are imbedded on a film that can be removed and taken to the laboratory for identification and quantification. A compound microscope is needed to have enough magnification to identify any downy mildew spores that may be present on the tapes. The spore traps helped to alert us to any influx of spores into those production regions, but were not used to time fungicide sprays. 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.

Fungicide trials

Chemical control must be focused on using the most effective products, alternating the products, and applying fungicides at short intervals. Results from our downy mildew research in past research trials and those from field season 2009 (Fig. 1) indicate that an effective spray program includes the following: Previcur Flex (propamocarb hypochloride) plus Bravo (chlorothalonil) alternated with Tanos 50DF (cymoxanil + 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 (Table 1).

Cucurbit downy mildew updates can be found at

Figure 1. Evaluation of fungicides for control of downy mildew on pickling cucumber, 2009.Figure 1

Table 1. Recommended products for managing downy mildew on pickle.
Table 1

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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