No new cases of downy mildew reported but fungicide sprays still needed

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.

It has been a relatively quiet week with no new cases of downy mildew found. The two cucumber fields with downy mildew have not been destroyed and will likely be in production for the next four weeks. We are working to determine the threat that these fields pose to the cucumber and cantaloupe producers in the state. The diseased fields have been under an intensive fungicide program to keep the downy mildew limited.

The downy mildew pathogen moves from field to field primarily via air currents. It reproduces via tiny, microscopic spores that act as seeds of the pathogen. Spore traps have been placed in one of the diseased cucumber fields and also in four other Michigan counties: St. Joseph, Allegan, Bay, Van Buren and Monroe.

These traps are research tools that are manufactured in England and cost approximately $5,000 to $6,000 each. Inside of the trap there is a drum that has a cellophane tape with a sticky surface. This tape rotates past an intake orifice at the rate of approximately 2 mm per hour. Anything in the air that flows past the cellophane tape becomes embedded in the sticky material on the tape and includes spores, pollen, insect parts, debris, etc. At the end of 7 days, the cellophane tape is removed from the drum, cut into 24mm segments and is mounted onto a microscope slide. 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 will help to alert us to any influx of spores into those production regions. These traps cannot be used as a substitute for the fungicide sprays. Since we do not have a trap in each field, it is possible that we could miss an isolated spore mass coming into a particular region.

Are fungicide sprays still needed? Yes. Many cucumber and cantaloupe growers have now applied two fungicide sprays and have protection through this week. At this juncture, I recommend that growers continue with the same fungicide program. The recent series of rains will be favorable for downy mildew development.

Alternate these sprays:

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

Growers of cucumbers or cantaloupes in southeast Michigan region should be on a 5-day alternation program. Growers in the rest of the state should be on a 7-day alternation program. 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 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.

Some later planted fields may be covered with a banded application of fungicide. As further sprays are needed, cost becomes difficult to bear. However, with downy mildew present in the state and the weather providing the moisture needed for the disease, I do not think that we have a choice. Fungicides must be applied preventively and frequently.

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