Threat of downy mildew on cucumbers remains high for southeast Michigan growers

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 threat of downy mildew continues to drive the spray recommendations. Last Friday, the North Carolina State University Cucurbit Downy Mildew Forecast issued the following prediction for southeast Michigan:

Serious Threat of downy mildew developing for Thursday (June 22), Moderate Threat for Friday (June 23), and Low Threat for the weekend (June 24, 25).

If this week’s weather forecast holds (cloudy and rainy), it is likely the downy mildew threat will increase again.

What about the threat of downy mildew to Michigan growers outside of the southeast region? According to the Cucurbit Downy Mildew Forecasts that have been generated, the southeastern portion of Michigan has the highest risk. However, that does not mean that the other regions of the state have a zero chance of disease.

Downy mildew also threatens other growing regions outside of Michigan. Last week, the downy mildew risk was high for northern Ohio and southern Ontario and moderate for southwest and south-central New York and northern Pennsylvania.

These forecasts are based on the downy mildew biology, weather predictions and the trajectory of the air currents carrying the spore loads. The mat of velvet that forms underneath the leaves are actually masses of spores that are commonly referred to as sporangia. These spores are formed during the night when the leaves become wet with dew, rain or fog. The spores are produced on spore stalks. The spores mature (or ripen) during the early morning hours of (1:00 to 5:00 AM). As the morning progresses, the leaves begin to dry, the relative humidity falls and a breeze may begin to blow. As the morning dew dries, the spore stalk begins to dry and twist, dislodging the mature (ripened) spore from its stalk and placing it into air currents. The air currents can then move these spores long distances. Rain showers are thought to scrub these spores from the atmosphere and deposit them onto fields. Once the spores are dropped from the air onto the foliage, wetness is needed for the spore to germinate and cause a new infection.

Fungicides are most effective when they are applied prior to the spores landing on the leaf’s surface. Once the spores have germinated and penetrated the leaf, it becomes much more difficult to limit downy mildew. A preventive approach will always be more effective than trying to stop the disease once it has appeared.

Spore traps have been placed in six Michigan counties and include: St. Joseph, Allegan, Monroe, Bay, Van Buren and Saginaw counties. The spore load has been relatively high from Monroe County during certain days when the weather was favorable for spore production. The downy mildew spores detected in the other spore trapping regions in the state have been negligible. Please remember that the spore traps may not be in the correct site to pick up all influxes of downy mildew spores into a region. At this point, we are using these traps as a research tool. Any information that we learn from these traps will be shared over the course of the growing season.

Are fungicide sprays still needed?

Growers of cucumbers or cantaloupes in the southeast Michigan region should be on a 5-day alternation program.         

Growers of pumpkins, squash, or watermelons in the southeast Michigan region should be on a 7-10 day alternation program.

Alternate these sprays:

  • Previcur Flex (1.2 pint) + Bravo (or Mancozeb)
  • Tanos 50DF (8 oz.) + Mancozeb (or Bravo)

Please note: Mancozeb is not registered for use on pumpkins, but Maneb or Manex could be used.

If Previcur Flex is not readily available, two Tanos 50DF applications may be made in a row prior to changing up the spray program with Previcur Flex. The fungicides will be more effective when applied prior to the appearance of downy mildew. Tanos 50DF has a 3-day pre-harvest interval (PHI) and Previcur Flex has a 2-day PHI. The addition of Mancozeb increases the PHI to 5 days. Bravo has a 0-day PHI.

At this point, downy mildew appears to be isolated to Monroe County. No new fields have been reported to have the disease. It is possible that the two fields that are currently diseased will be out of production sometime next week and will be burned down. Growers of cucumber and/or cantaloupes in the rest of the state have a lower risk of downy mildew developing than the growers in southeast Michigan who are in the immediate vicinity of the infected fields. How low is the risk to the Michigan areas outside of Monroe County and what should be done to protect cucumbers and melons in the northern and western parts of the state?

The fungicide program that offers the least risk of downy mildew developing also carries the highest cost: a 7-10 day alternation program with the fungicides Previcur Flex (1.2 pint) + Bravo (or Mancozeb) alternated with Tanos 50DF (8 oz.) + Mancozeb (or Bravo).

A fungicide program that offers a moderate level of risk with a more moderate price tag includes a mancozeb only program applied every 7 to 10 days. Mancozeb is available under the trade names of Dithane, Penncozeb, Manzate and others. Please note: Mancozeb is not registered for use on pumpkins, but Maneb or Manex could be used. The thought behind using this program is that if a downy mildew outbreak develops in the growing region, Previcur Flex and/or Tanos would be added to the mancozeb immediately.

While some growers have not applied any fungicides, the relatively high use of fungicides early on likely prevented the spread of this disease to nearby young cucumber and melon fields. Although the disease was initially detected on June 9, the downy mildew was well established and was likely present in the two fields as early as mid-May. We will continue to handle any suspect samples of downy mildew and will visit any fields with unusual symptoms.

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