Downy mildew on cucumbers: Michigan growers need to be on high alert

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.

Downy mildew has been verified on field cucumbers in Ontario and Ohio this week. Michigan will be next, and it is likely that downy mildew has already infected cucumbers in our state but hasn’t yet been detected. Remember, that we’ve had very high airborne sporangia counts from southeast Michigan and low numbers of sporangia have been detected in all spore traps in our six locations. The cool, wet and overcast weather of the recent days are absolutely perfect for downy mildew development. Because of the airborne sporangia at our trapping sites, the findings of downy mildew in the field in regions adjacent to Michigan, and the weather, growers must consider the risk of downy mildew to be very high. Cucumbers and melons must be protected with fungicide prior to downy mildew becoming established in the field. Waiting until the disease is detected on cucumbers or melons before applying fungicides is likely to be a disaster. Growers cannot play catch-up with this disease on crops that are especially susceptible including cucumbers and melons.

Growers of cucumbers and melons in Michigan should apply proven fungicides immediately. Fungicides should be reapplied at weekly intervals.

Growers of winter squash, summer squash, zucchini and gourds could consider waiting until downy mildew is detected within their immediate growing region before beginning a downy mildew fungicide program.

Table 1. Downy mildew spore trap daily totals (counts/m3/day) by location

Date Michigan counties
E. Allegan W. Allegan Bay Dundee Monroe Saginaw VanBuren
June 1 S0 S8
June 2 S0 2 8 S2
June 3 S15 S5 5 12 440 7 S15
June 4 3 2 2 2 465 0 7
June 5 5 2 3 3 100 2 10
June 6 2 20 5 10 60 0 18
June 7 7 12 0 3 28 2 3
June 8 2 0 0 5 0 0 3
June 9 3 2 7 12 5 7 8
June 10 40 3 2 0 7 8 12
June 11 5 0 3 0 8 0 5
June 12 7 2 18 8 7 8 2
June 13 2 0 3 8 8 2 2
June 14 2 5 2 18 27 13 10
June 15 7 5 7 2 8 5 13
June 16 8 3 2 7 15 0 13
June 17 0 0 0 0 2 0 3
June 18 3 3 7 7 2 12 3
June 19 3 0 23 2 3 5 2
June 20 2 5 2 7 7 0 5
June 21 8 10 7 3 3 15 12
June 22 17 10 0 13 10 5 2
June 23 2 3 23 2 12 3 7
June 24     10 3 5 2  
June 25     5 3 7 0  
June 26       7 8    
June 27       8 3    
June 28       2 0    


Table 2. Spray recommendations.

CUCUMBER
APPLIED BEFORE DISEASE (7-day intervals) APPLIED AFTER DISEASE (5-day intervals)
  • Gavel 75WG (5 day PHI)
  • Presidio 4FL (2 day PHI)
  • Presidio 4FL (2 day PHI)
  • Previcur Flex 6SC (2 day PHI)
  • Previcur Flex 6SC (2 day PHI)
  • Ranman 3.6SC (0 day PHI)
  • Ranman 3.6SC (0 day PHI)
  • Tanos 50WG (3 day PHI)
  • Tanos 50WG (3 day PHI)
 

Alternate products and mix each with either:

  • Dithane (mancozeb) 3 lb or
  • Bravo (chlorothalonil) 1.5 pt

Alternate products and mix each with either:

  • Dithane (mancozeb) 3 lb or
  • Bravo (chlorothalonil) 2 pt
VINE CROPS, such as zucchini, squash, pumpkin, melon and gourds
APPLIED BEFORE DISEASE (7- to 10-day intervals) APPLIED AFTER DISEASE (7-day intervals)
  • Gavel 75WG (5 day PHI)
  • Presidio 4FL (2 day PHI)
  • Presidio 4FL (2 day PHI)
  • Previcur Flex 6SC (2 day PHI)
  • Previcur Flex 6SC (2 day PHI)
  • Ranman 3.6SC (0 day PHI)
  • Ranman 3.6SC (0 day PHI)
  • Tanos 50WG (3 day PHI)
  • Tanos 50WG (3 day PHI)
 

Alternate products and mix each with either:

  • Dithane (mancozeb) 3 lb or
  • Bravo (chlorothalonil) 1.5 pt

Alternate products and mix each with either:

  • Dithane (mancozeb) 3 lb or
  • Bravo (chlorothalonil) 2 pt
[Please note: Gavel 75WG and Dithane are not registered on some vine crops.]

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

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