West central Michigan vegetable regional report – July 27, 2016

Don’t let your guard down and let foliar diseases develop. While weather has been mostly dry, high dew points have still created leaf wetness, which can help foliar diseases develop.

Even though there was no rain, I had to wear a rain jacket when checking weather sensors in this asparagus field last week. Dew like this in combination with warm temperatures is all foliar diseases need to develop. Photo: Ben Werling, MSU Extension.

Even though there was no rain, I had to wear a rain jacket when checking weather sensors in this asparagus field last week. Dew like this in combination with warm temperatures is all foliar diseases need to develop. Photo: Ben Werling, MSU Extension.

Michigan State University Extension plant pathologist Mary Hausbeck recently cautioned that although weather has been dry, high dew points can still create the moisture needed for disease to develop (see photo). Instead of a normal crop report, I wanted to illustrate this with data from our Hart Enviro-weather station.

Up until this past weekend, it has been very dry. However, even with the lack of rain and high temperatures, high dew points can create dew on leaves. Extended dew periods are all many foliar diseases need to develop. For example, there was no rainfall at the Hart Enviro-weather station from Monday afternoon to Tuesday, July 25-26. However, dew points were high enough that dew formed when temperatures cooled around 11 p.m. Monday (Figure 1). Dew persisted until 9 a.m. the following morning.

Looking at the month of July (Figure 2), the Hart Enviro-weather station recorded multiple days with wetting periods of at least six hours. Just as notable is average temperatures during these wet periods have been in the mid- to high 60s in recent days.

Warm air temperatures during periods when dew is on leaves help promote foliar disease. The main message is don’t let your guard down! Though it’s been dry, dew is forming and foliar fungicide programs will still be necessary for many foliar diseases.

Hart weather data graph 1.

Figure 1. Weather data from the Hart Enviro-weather station starting from 6 p.m. on July 25 and ending at 6 p.m. on July 26. The blue line (starting in bottom left corner of graph) shows the percentage of each hour that dew was present. The orange line (starting near the 80) shows air temperatures.

Hart weather data graph 2.

Figure 2. Data from the Hart Enviro-weather station showing the number of hours a leaf wetness sensor recorded wetness (black line) plus average air temperatures during those wet periods (gray bars).

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