West Michigan tree fruit regional report – April 21, 2015

We are off to the races for the 2015 season.

Crop stage and degree days

Apple are mostly at the half inch green stage with some early varieties at tight cluster and later varieties just at quarter-inch green. Degree day totals have caught up to about normal accumulations after a few days last week in the 70’s. We are likely to fall behind again with the forecast being for much below normal temperatures.

Tree fruit diseases

Apple scab. Most areas have had two wetting events for the start of scab season. Rain in the morning on April 16 did not result in a scab infection and most area weather stations recorded 5 hours or less of wetting. I only caught 2 spores per rod at both my Sparta and Standale monitoring sites.

For the rain that began on Sunday, April 19, all area weather stations are now up to at least a moderate infection period and with the forecast, this will likely be a heavy infection if it continues raining for several days as forecasted. I checked spore rods at the Sparta location at 10 a.m. on April 20, after a few hours of daylight rain, and there were only 3 spores per rod. When I checked them again at 7 p.m. on April 20, there were 342 spores per rod. Apparently, they needed a bit of moisture to get them to release. I suspect that the 342 were released with the heavier rainfall in the mid- to late- afternoon of April 20. This was a good 16 to 18 hours into the wetting period, so it makes the latter portion of this rain event more important than the beginning, and should help us as we try to get back in to get recovered. The entire wetting event is significant, but this delay of spore discharge is different than the usual heavy release at the beginning of a wetting period.

This is the first apple scab infection period of the 2015 season and hopefully you had a good fungicide cover spray on ahead of it. I feel that tissue did move with the two 70 degree Fahrenheit days ahead of this rain, but not so much growth that a full cover spray within 48 hours of the start of the rain the evening of April 19 would have not been enough to keep you covered. Once this rain is ended, you will want to get back out there right away and get covered up again and perhaps get a little back action out of the fungicide you choose.

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