Northwest Michigan fruit report – June 18, 2019
Cool and wet conditions continue in northwest Michigan. We are assessing crop size in cherries, and growers are thinning apples.
Not much has changed with the weather during the past week. We continue to have cool and wet weather across the region. The forecast is predicting conditions will dry out this week, with rain moving back into the region for the weekend. This dry period is an opportunity for growers to get into the orchard to cover up for the next rain event. This weather has been a huge challenge for growers to cover up the fast-growing tissue to protect for diseases. Diseases have a foothold here at the Northwest Michigan Horticulture Research Center. We were late with our cherry leaf spot applications, and we have a pretty good infection across many of our tart cherry blocks. Hopefully growers were able to start their leaf spot programs earlier as this season has been unforgiving for disease development.
We continue to be about two weeks behind our normal growing degree day averages. We have accumulated 707 growing degree days (GDD) base 42 and 340 GDD base 50. Our 30-plus year averages are 1,003 GDD base 42 and 559 GDD base 50. We have had over 3 inches of rain since the beginning of June, and we had rain Wednesday, June 12, through Saturday, June 15, last week. Jeff Andresen’s forecasts suggest there will be a change in the weather next week, and we should see warmer temperatures and less moisture.
The sweet cherry crop seems to be lighter than we anticipated last week. We have observed drop in the small fruits that likely were not pollinated. The crop still looks quite good, and fruit is sizing; Ulster and Emperor Francis are at 13 millimeters at the Northwest Michigan Horticulture Research Center.
The tart cherry crop is quite variable across the region. Areas to the south seem to have a bigger crop load than some of the orchards to the north. We are still assessing the size of the crop for 2019.
Apple thinning is underway across the region. We are at the optimal thinning timing at the Northwest Michigan Horticulture Research Center. Honeycrisp are 10 millimeters and Gala are 8 millimeters. The next few days are likely going to be the best opportunity to thin apples. Temperatures are predicted to be in the mid-60s to low 70s for the next four days. The carbohydrate model is showing that we have an excess of carbohydrates, which means the trees will be more resistant to thinning (Fig. 1). The model is recommending growers increase their chemical thinner rates by 15% for the next four days. Take advantage of this thinning window as rain is expected to move back into the region for the weekend. Temperatures will still be on the cooler side, but warmer than in recent days, and warmer conditions will improve thinning activity.
Primary apple scab is still ongoing at this time. Our colleague Amy Irish-Brown located on the Ridge north of Grand Rapids, Michigan, is still catching spores on her spore rods and has yet to call the end of primary. Spores are mature, but rain is needed for the remaining spores to discharge. According to RIMpro, there are few spores remaining and at least one, but possibly more rain events will be needed before primary is over. Scab lesions started showing up in managed blocks last week from early June infections and symptoms of scab are predicted to show up later this week from infection periods during the second week of June. Overall incidence remains low at this time.
We have observed very few signs of fire blight infection in the region. This week, we found one Gala apple fruit with ooze at the station (see photo), but we have not found blighted flower clusters or flagging shoots.
Black stem borer trap catches have decreased this week and it appears that this first round of beetle emergence is winding down. As a reminder, once the beetles are inside of the trees, spray applications will not be effective for this pest. Beetles not impacted by treatment measures have bored new holes into trees where juvenile black stem borers will grow and develop in a gallery; the larvae will pupate and emerge as adults later this summer.
Our regional biofix for codling moth has been set for June 8. Traps were checked on June 7, June 10 and June 15; the first moths were found on June 10 and the second consecutive catch was on June 15. However, looking back at the weather, moths were most likely active on the evening of June 8, as we had a very warm evening across the region with no rainfall; the following day, June 9, was cooler with afternoon and evening rains. Since June 8, we have accumulated about 70 GDD base 50 and egg laying is ongoing. Growers planning to use ovicidal materials should time these materials for about 100 GDD after biofix in blocks where treatment thresholds have been met; we will likely hit that 100 GDD mark by the end of the week. Otherwise, larvicidal materials can be timed for 250 GDD in high pressure blocks and 350 GDD in lower pressure situations.
European red mite activity has slowed. Rosy apple aphids that have escaped control measures are protected in curled up leaves, and we have found lady beetles feeding on the aphids. We have observed an uptick in leafhoppers over the last week.
Growers tightened up spray programs for cherry leaf spot disease recently, and drier weather this week has been welcomed to help growers get caught up ahead of the coming rain for this weekend. Temperatures are predicted to get warm as we move toward the later part of the week into early next week, and the more traditional cooler weather materials (example: Syllit, coppers) will be riskier in warmer conditions. Additionally, we remind growers concerned with powdery mildew that these materials will not provide mildew control.
We have observed sweet cherries that have a water-soaked appearance; while we are unsure of the cause, we hypothesize that this symptom could be due to bacterial canker. We have also observed cherries with feeding damage. Undamaged, intact green cherries are typically not concerning for American brown rot development. However, fruit injured by canker and insects could be a concern for American brown rot. We have observed sporulating mummies and a few green cherries with active brown rot infections. As temperatures warm and fruit ripen, American brown rot could get a foothold and growers will need to be mindful of the potential for a continued wet season.
A total of five spotted wing Drosophila have been detected in our region so far this season. The first fly, a female, was found at the Northwest Michigan Horticulture Research Center on June 5, and the second fly, another female, was found in a trap in the Cedar, Michigan, area on June 6. Green cherries are not susceptible to oviposition, but it is not too early to consider management strategies for this pest, particularly as growers concentrate on plum curculio control at this time. We will continue to check traps and report numbers later this week.
Plum curculio activity has been sporadic, and the beetles have been taking advantage of periods of warmer, drier weather. We have observed ovipositional scars in cherries and apples at the station and have received reports of plum curculio damage in hot spot areas of commercial blocks. According to degree day accumulations, we are near peak plum curculio activity. Warmer conditions in the forecast could trigger a flush of activity.
We are still finding large obliquebanded leafroller larvae. These larvae will pupate soon and pheromone traps will be deployed later this week to monitor for adult flight.
San Jose scale male flight seems to have peaked last week, and numbers are down this week. Crawlers are expected to begin emergence at about 400 GDD base 51 and we will hit that target by the end of this week-early next week. Peak crawler emergence is estimated for about 600 GDD base 51, and contact materials targeting crawlers should be timed for peak emergence.
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