posted on June 17, 2009 14:03
Mary Hausbeck
Plant Pathology
Michigan pickle and cucurbit growers have battled downy mildew, incited by the water mold, Pseudoperonospora cubensis, for four consecutive years. Downy mildew causes symptoms on the leaves of vine crops (such as cucumber, squash, and melon) similar to a mosaic or angular leaf spot. The tell-tale symptom of downy mildew is the purplish/gray fuzz on the underside of the leaf that gives a somewhat “dirty” or “velvet” appearance.
On crops including pickle, slicing cucumbers, cantaloupe and watermelon, 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 2005-08 research identified a limited number of fungicides that are effective, but must be applied every five to seven days when the weather favors disease and the disease is found in the area.
While other cucurbits including squash, zucchini, pumpkins, and gourds may also be susceptible to downy mildew, it appears that the disease may be easier to control on these crops in comparison to cucumbers and the melons.
Downy mildew spore trapping
The downy mildew pathogen reproduces via tiny, microscopic spores that act as seeds of the pathogen. Six spore traps have been placed in Michigan counties spread across the state. 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 help to alert us to any influx of spores into those production regions. Since there is not a trap in each field, it is possible that we may miss an isolated spore mass coming into a particular region.
At this point, downy mildew sporangia are being picked up by all of our spore traps. The highest numbers of sporangia are being found in Monroe County. There may be an outbreak in or near Monroe County which would explain why these counts are so much higher than those for the other counties. Growers of pickling and slicing cucumbers and melon must consider themselves at high risk for developing downy mildew disease and apply their fungicide sprays immediately and use an interval of seven to 10 days between applications. Growers of other cucurbit crops including pumpkin, squash and zucchini should look for symptoms of the disease in the crop. Send any suspicious-looking plants to MSU Diagnostic Services for verification of the disease.
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
|
--
|
--
|
|
2
|
--
|
--
|
S0
|
2
|
8
|
S2
|
--
|
|
3
|
S15
|
S5
|
5
|
12
|
440
|
7
|
S15
|
|
4
|
3
|
2
|
2
|
2
|
465
|
0
|
7
|
|
5
|
5
|
2
|
3
|
3
|
100
|
2
|
10
|
|
6
|
2
|
20
|
5
|
10
|
60
|
0
|
18
|
|
7
|
7
|
12
|
0
|
3
|
28
|
2
|
3
|
|
8
|
2
|
0
|
0
|
5
|
0
|
0
|
3
|
|
9
|
3
|
2
|
7
|
12
|
5
|
7
|
8
|
|
10
|
|
|
2
|
0
|
7
|
8
|
|
|
11
|
|
|
3
|
0
|
8
|
0
|
|
|
12
|
|
|
|
8
|
7
|
|
|
|
13
|
|
|
|
8
|
8
|
|
|
|
14
|
|
|
|
|
27
|
|
|
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 the 2008 field season indicate that an effective spray program includes the following:
Cucumber
|
Applied before disease
(7-day intervals)
|
|
Applied after disease
(5-day intervals)
|
|
|
|
|
|
|
|
- Previcur Flex 6SC (2 day PHI)
|
- Previcur Flex 6SC (2 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: zucchini, squash, pumpkin, melon and gourds
|
Applied before disease
(7- to 10-day intervals)
|
|
Applied after disease
(7-day intervals)
|
|
|
|
|
|
|
|
- Previcur Flex 6SC (2 day PHI)
|
- Previcur Flex 6SC (2 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.]