Northwest Michigan fruit update – July 2, 2019

Nikki Rothwell<rothwel3@msu.edu> and Emily Pochubay, Michigan State University Extension - July 03, 2019

Managing tree fruit diseases is the primary concern for regional growers with the continued high levels of rainfall and humidity.

Growing degree day accumulations as of July 1, 2019 at the Northwest Michigan Horticulture Research Center
Year	2019	2018	2017	2016	2015	2014	29 Yr. Avg.
GDD42	1085	1419	1361	1385	1302	1279	1366.2
GDD50	598	903	785	823	754	775	811.5

Weather report

Wet conditions continue for this soggy 2019 season. Last week, we had rain events on Monday, Tuesday and Friday (June 24, 25 and 28), and rainfall totals at the Northwest Michigan Horticulture Research Center were just over 1.25 inches. The rain continued the morning of July 2 and was predicted to stop raining by the afternoon. These conditions have been perfect for disease development, and we have no shortage of disease symptoms showing up in apples, cherries and grapes. Growers have been trying to be diligent about disease control, but conditions have favored rapid pathogen growth.

In addition to the wet weather, the daytime temperatures have been higher than in past weeks. We hit highs of low to mid-80s seven times last week. Nighttime temperatures have also been slightly warmer: in the low to mid-60s. Again, these warmer and wetter conditions are conducive for rapid disease development. We have accumulated 1,085 growing degree days (GDD) base 42 and 598 GDD base 50. We are still well behind our 30-plus-year averages: 1,366 GDD base 42 and 811 GDD base 50.

Crop report

Some varieties of sweet cherries are starting to turn color. We are seeing high bird pressure in early ripening varieties, and the birds seem particularly damaging this year. Tart cherries are still mostly green, but the occasional fruit is starting to turn straw colored.

Apples have increased in size at the Northwest Michigan Horticulture Research Center, and most varieties jumped by 10 millimeters. The thinning window has closed for the season. At the station, Gala thinned well, but Honeycrisp trees still have too much fruit. Once the wet weather passes, we will hand thin Honeycrisp.

Strawberry harvest has begun in northwest Michigan, and quality is looking good. Wet weather is a concern for rots in strawberry. However, strawberry fruit size is adequate with all the moisture this season.

Pest report

On Friday, we reported that the RIMpro models suggested we are at the end of primary apple scab season in the Benzonia and Williamsburg areas. The East Leland RIMpro model is suggesting the remaining spores discharged in the July 2rain, indicating the end of primary for the East Leland area. It was a long primary apple scab season due to cooler conditions this spring and early summer. This season’s wet weather has been especially favorable for fungal pathogens and growers will need to keep a close watch on the possibility of scab development. Moving forward, it will be critical to monitor for scab lesions that could begin showing up on leaves and/or fruit as growers lighten up on fungicide programs following the end of primary.

Over the last week, new symptoms such as flagging terminals and ooze have continued to show up in orchards with ongoing fire blight infections. Fire blight seems to be especially problematic in Gala and some Jonagold blocks in the region. At the station, we have observed new symptoms and ooze in Gala, and according to the fire blight interactive predictor model on Enviroweather, these symptoms are from an infection that occurred June 24. June 24 was also the first day we observed fire blight at the station; we cut infected branches and applied copper on that day. On July 1 we found new fire blight symptoms and ooze at the station and made a second copper application; we have also used prohexadione-calcium (i.e., Kudos, Apogee) this season to help prevent the fire blight bacteria from progressing further into branches and the central leader of trees.

Since our codling moth biofix of June 8, we have accumulated about 343 GDD base 50 and we are approaching 350 GDD base 50, which indicates that about 20% of eggs have hatched; 350 GDD is also the timing for the delayed treatment strategy. We have had several warm evenings over the last week and codling moth have been active in these conditions; our trap numbers reflected increased activity with another good flush of moths in the traps. Applications targeting this pest would have been well-timed over the weekend when many areas reached 250 GDD base 50.

Yellow leaves became noticeable in cherry blocks recently. Some of the yellow leaves are a result of virus, but we have also observed cherry leaf spot infected leaves that are yellowing and falling from trees. At the station, there are a few blocks with very serious leaf spot infections where some of the trees will likely defoliate before the crop is ripe.

Weather conditions have been ideal for American brown rot development. There has been a tremendous amount of bacterial canker on the fruit of sweet cherries this year. Additionally, birds have been targeting sweet cherries showing any pink or red color. With the high amounts of rainfall, cracks in cherries have been showing up. These damaged fruit will grow brown rot very quickly, particularly in the warm and wet weather we have experienced lately.

We began testing fruit for spotted wing Drosophila (SWD) infestation this week. We will provide regular updates on fruit susceptibility to SWD throughout the season. Preliminary data show that fruit become infested by SWD when they lose their green color, but higher levels of infestation occur once fruit turns red. Trap catches of SWD continue to be low this season with only one female found in a trap at the station this week.

San Jose scale male numbers on traps have continued to decline this week. We have not observed crawlers at this time, but peak crawler emergence (about 600 GDD base 51) is estimated to occur this week.

Obliquebanded leafroller flight has begun with a good flush of activity that likely occurred over the weekend. We will check traps again this Friday, July 5, to determine biofix—the first date of sustained catch.

American plum borer and lesser peachtree borer activity is ongoing and we had our first detection of greater peachtree borers this week.

Traps for cherry fruit flies have been deployed, but we have not detected this pest at this time.

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