MSU Extension

Growers on Michigan’s east side should consider preventive fungicide sprays for cucumbers and melons (watermelons, cantaloupe, muskmelon) now that downy mildew has been verified in Bay County over the weekend. A young downy mildew infection was detected on July 9, 2016, in a commercial cucumber field near harvest. It is likely that additional cucurbit fields in Bay County and nearby counties are also infected with the downy mildew pathogen and should be protected to prevent blighting. Michigan State University Extension suggests growers on the west side of the state remain vigilant and scout their fields for downy mildew.

Last minute rescue applications of fungicides fail to limit downy mildew and protect the crop. Several fungicides are no longer effective as a result of pathogen resistance. Field trials show that an infection that is not effectively managed causes yield loss and fruit distortion. Plants must be protected preventively.

An important reminder from last year is that when the weather conditions are especially favorable for downy mildew, timing of fungicide applications becomes crucial. In our MSU field plots, when the plants became infected almost immediately after emerging, it was challenging to limit disease development.

Each year my team evaluates fungicide active ingredients and programs, and the results are noted below. Fungicides that have just one downy mildew active ingredient should be tank-mixed with a general protectant fungicide such as Bravo WeatherStik (chlorothalonil) or mancozeb.

MSU Trial #1

This study included 17 fungicides and an untreated control.

Rating #1: At the first evaluation of disease on Aug. 19, 2015, downy mildew had become established on the untreated control, but was at a relatively low level. Several treatments were significantly better in protecting the plants from downy mildew than the untreated control and included Bravo Weatherstik SC, Ranman SC, Zampro SC, Gavel DF, Omega SC, Orondis and V-10208. Fungicides including Presidio SC, Previcur Flex and Tanos had the same disease level as the untreated control that did not receive any fungicides. Historically, Presidio, Previcur Flex and Tanos had been included in recommendations for downy mildew. While Presidio and Tanos had been dropped from the 2015 recommendations, Previcur Flex had remained based on 2014 field trial results.

Rating #2: Downy mildew progressed significantly from the first to the second rating that was taken Aug. 31; the untreated control received a disease rating of 7.3 (50 to 75 percent disease). Standout treatments included Orondis (1.0 equals 0 percent plant tissue diseased), and V-10208 (2.3 equals 0 to 3 percent disease). Other effective products included Zampro SC, Omega SC and Koverall (mancozeb) with ratings of 3.3 to 3.5 (3 to 6 percent disease). A moderate level of control was observed with Ranman SC and Gavel DF (4.0 to 4.3 equals 6 to 12 percent disease).

While several other fungicides were determined to be better than the untreated control, the amount of disease observed on the tissue was unacceptable with ratings of 5 or higher indicating at least 12 percent of the foliage was infected with a maximum of 50 percent disease assessed. Fungicides that did not protect the plant any better than the untreated control included Cueva, Cabrio DG, Previcur Flex SL, Revus SC, Forum SC and Priaxor SC.

Rating #3: The final rating was taken following a period of unseasonably warm weather that prompted plant growth resulting in a decrease in the overall appearance of downy mildew symptoms. As a result, the rating of the untreated control was 6.5 (25 to 50 percent disease) that indicated less disease than what was observed at the previous rating. The only fungicide treatment that scored a disease rating of 2 (0 to 3 percent disease) was Orondis. However, several treatments kept downy mildew limited to 3 to 6 percent disease and included Koverall DG, Ranman SC, Zampro SC, Gavel DF and V-10208. Presidio SC, Omega SC and Bravo WeatherStik SC were similar in their level of protection and resulted in disease ranging from 6 to 25 percent. Treatments that were not effective and similar to the untreated control included Priaxor SC, Forum SC, Revus SC, Tanos DF, Previcur Flex SL, Cabrio DG and Cueva.

¹Rated on the Horsfall-Barratt scale of 1 to 12, where 1=0% plant area diseased, 2=>0 to 3%, 3=>3 to 6%, 4=>6 to 12%, 5=>12 to 25%, 6=>25 to 50%, 7=>50 to 75%, 8=>75 to 87%, 9=>87 to 94%, 10=>94 to 97%, 11=>97 to <100%, 12=100% plant area diseased.

²Column means with a letter in common are not significantly different (LSD t Test; P=0.05).

MSU Trial #2

Nine treatments and an untreated control were included in this study.

Rating #1: At the first rating conducted Aug. 24, 2015, the untreated received a rating of 4 (6 to 12 percent) and nearly all treatments provided complete downy mildew control. Zing SC and Ranman SC were excellent fungicides when used along or in alternation with other materials.

Rating #2: On Sept. 14, a second disease rating was assessed and disease had progressed in both the untreated and all treatments. The trend was similar as seen in rating #1 where Zing SC and Ranman SC either alone or in alternation limited disease to commercially acceptable levels.

¹Rated on the Horsfall-Barratt scale of 1 to 12, where 1=0% plant area diseased, 2=>0 to 3%, 3=>3 to 6%, 4=>6 to 12%, 5=>12 to 25%, 6=>25 to 50%, 7=>50 to 75%, 8=>75 to 87%, 9=>87 to 94%, 10=>94 to 97%, 11=>97 to <100%, 12=100% plant area diseased.

²Column means with a letter in common or with no letter are not significantly different (LSD t Test; P=0.05).

MSU Trial #3

This trial included seven treatments and an untreated control.

Ratings #1 and #2: For the first rating conducted Aug. 25, 2015, disease in the untreated control had reached a rating of 4 (6 to 12 percent disease). All fungicide programs were effective in keeping downy mildew nearly completely controlled. The same trend was observed for the second rating Sept. 4. It was notable that the treatment with Bravo WeatherStik SC applied alone was comparable to other alternating fungicide programs that included Ranman SC, Gavel WG, Presidio, Zampro and V-10208.

¹Rated on the Horsfall-Barratt scale of 1 to 12, where 1=0% plant area diseased, 2=>0 to 3%, 3=>3 to 6%, 4=>6 to 12%, 5=>12 to 25%, 6=>25 to 50%, 7=>50 to 75%, 8=>75 to 87%, 9=>87 to 94%, 10=>94 to 97%, 11=>97 to <100%, 12=100% plant area diseased.

²Column means with a letter in common or with no letter are not significantly different (LSD t Test; P=0.05).

MSU Trial #4

This trial included eight treatments and an untreated control.

Rating #1: During the first rating, disease ranged from 25 to 50 percent on the untreated control. The following treatments provided exceptional control and included experimental fungicides (two) from Valent, Ranman SC, Zampro SC, Gavel DF and Orondis SC. Previcur Flex failed in limiting disease and was similar to the untreated control. While Presidio was better than the untreated control, the level of disease was not acceptable.

Rating #2: Disease was reduced in the untreated control compared to the first rating due to the unseasonably high temperatures that spurred new foliage. The pattern of treatment efficacy observed in Rating #1 was observed in Rating #2. It was interesting to note that Presidio SC and Previcur Flex SL were statistically similar to the untreated control and did not limit downy mildew disease.

¹Rated on the Horsfall-Barratt scale of 1 to 12, where 1=0% plant area diseased, 2=>0 to 3%, 3=>3 to 6%, 4=>6 to 12%, 5=>12 to 25%, 6=>25 to 50%, 7=>50 to 75%, 8=>75 to 87%, 9=>87 to 94%, 10=>94 to 97%, 11=>97 to <100%, 12=100% plant area diseased.

²Column means with a letter in common or with no letter are not significantly different (LSD t Test; P=0.05).

For current downy mildew news, go to MSU Downy Mildew News.

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