Northwest Michigan fruit update – July 7, 2020

Hot weather has hastened cherry development, and harvest is right around the corner. Be cautious about ethephon applications in the heat.

American brown rot sporulating
American brown rot sporulating on a June drop fruit in cherry cluster. Photo by Emily Pochubay, MSU Extension.

Weather report

There is no doubt, it is summer in northwest Michigan. We have had extremely hot temperatures and it looks as though there is little relief in sight. Daytime temperatures have been in the mid- to high 80s, and Monday, July 6, even hit over 90 degrees Fahrenheit since Sunday, June 28. These high temperatures are somewhat unusual for northern Michigan, particularly with such a long stretch of hot weather in a row.

We have accumulated 1,420 growing degree days (GDD) base 42 and 869 GDD base 50. With the recent hot temperatures, we are almost spot on with accumulations from our long-term averages, which is a stark contrast to the start of the season when we were over two weeks behind normal. Our 25-plus-year average is 1,492.8 GDD base 42 and 900 GDD base 50.

Coupled with hot weather has been the lack of rainfall. We have had no rain at the Northwest Michigan Horticulture Research Center since June 23, where we received just under 1 inch of rainfall. There were reports of popup showers across the region, where some isolated areas received noticeable amounts of rainfall while other areas just had high winds but not even a trace of rain. Unfortunately, the forecast is not predicting any rain in the near future. The next best chance will be Friday, July 10, but the percentages are still relatively low on Michigan State University Enviroweather.

Growing degree days (GDD) through July 6, 2020

Year

2020

2019

2018

2017

2016

2015

30 Yr. Avg.

GDD42

1,420

1,239

1,589

1,493

1,525

1,383

1,492.8

GDD50

896

712

1033

877

923

847

900.4

Review Jeff Andresen’s weather reports.

Crop report

Cherries are ripening and coloring up with the recent stretch of warm weather. Sweet cherries have increased in size since last week. Ulster fruit has jumped from 14 to 23 millimeters while Golds increased from 12 to 17 millimeters, and Emperor Francis fruit grew 7 millimeters. Despite some of the growth in fruit size, size may be a concern in orchards without irrigation and in areas that have not even received any substantial rainfall in weeks. Montmorency grew 7 millimeters in the past week, while Balatons only increased by 3 millimeters.

Apple size also grew substantially last week, and Honeycrisp grew by 12 millimeters and Gala by 14 millimeters.

With the heat and cherry harvest approaching, growers are going to be putting on ethephon applications to best prepare for harvest. As mention on last week’s northwest IPM update webinar, growers should be cautious about applying full rates of Ethrel with the heat. We have seen damage with ethephon used in hot weather, particularly sweet cherries, and we have observed more damage in briners than canners. At this time, growers should use 25-33% of their normal rates.

Pest report

Yellowing leaves in tart cherry orchards became evident late last week and over the weekend. There are a number of potential causes contributing to the chlorosis including cherry leaf spot disease, virus and phytotoxicity. Most orchards had some level of cherry leaf spot infection coming into the last major cherry leaf spot infection event, which occurred during the week of June 22 with wetting periods spanning 34 to 153 hours depending on location. Substantial rainfall came to many areas, and unfortunately, fungicides may not have provided “excellent” control during this difficult, extended infection period.

Powdery mildew is also becoming more evident as conditions have been very favorable for this fungus and there will likely be some leaves lost where mildew has not been adequately managed. Fungicides will not eradicate existing powdery mildew at this time. Growers will need to be diligent with cherry leaf spot programs to keep leaves through the summer, particularly in orchards with heavy infections currently. Forecasts change quickly this time of year and we often experience spotty “surprise” rains. Hence, do not get stretched too far on programs to avoid getting caught by pop up showers.

In sweet cherries, cherry leaf spot and powdery mildew infections are not as severe. We are, however, beginning to find “June drop” fruits that didn’t drop out of fruit clusters that are infected with American brown rot and sporulating (see photo above). The blocks where we are finding sporulation are not intensively managed for brown rot, but this observation indicates that while weather has been generally dry, there is enough moisture/humidity to support this fungus’ development.

We have also received reports that there is a low level of brown rot showing up in some commercial orchards that have been managed for brown rot this season. The SDHI fungicides are currently the best option against brown rot as there is widespread resistance to the more traditional brown rot material, Indar. Practice good resistance management strategies by including a broad spectrum fungicide in your programs to minimize resistance development.

In apples, trauma blight symptoms have shown up in some apple blocks over the past week, particularly in blocks that are not receiving Apogee/Kudos treatments. Trauma blight incidence is low overall. Spotty rain showers yesterday brought windy conditions that could have been a trauma event for areas that received the stormy weather; symptoms from this recent event are estimated to show up on Friday, July 10.

Codling moth flight is ongoing and trap numbers took a slight jump with seven to 20 moths per trap. Egg hatch is also ongoing and according to GDD we are past the mark for peak egg hatch (i.e., about 550 GDD base 50).

Twospotted spider mite populations have grown substantially over the last week at the station and populations are also growing in commercial orchards. Mite populations grow quickly in hot temperatures. Additionally, mites will move from ground cover into tree canopies as the ground cover becomes less suitable due to weed management and mowing. Ground cover is also drying up due to a lack of rain which could drive the mites up into trees. Some insecticides can contribute to mite flaring so growers should continue to monitor populations.

Obliquebanded leafroller flight is ongoing with a slight increase in trap catches. The station set biofix for June 29 and we have accumulated 312 GDD base 42 since biofix. Egg hatch is estimated to begin around 400-450 GDD base 42 after biofix, which will occur on Thursday-Friday this week.

San Jose scale crawlers are at peak activity (around 600 GDD base 51 since June 1 biofix). If you plan to treat orchards with a history of damage, consider using a product with fast activity against this pest. Please refer to “San Jose scale crawler emergence beginning in Michigan tree fruit” for additional information on management.

Greater peach tree borer flight and lesser peach tree borers flight is ongoing with a slight increase in greater peach tree borer trap catches.

Most sweet cherries in the area are currently at high spotted wing Drosophila (SWD) risk, and all tart cherries are quickly approaching the high risk category. By the end of the week, July 10, all sweet and tart cherry orchards will be at high risk (Table 1). Trap catches were down last week compared with the previous two weeks, but the percentage of traps catching in the region was still above threshold. While hot daytime temperatures may slow activity, cooler parts of the day are still conducive for SWD activity. Continue to protect fruit and maintain good intervals to minimize the risk of infested fruit at harvest.

As noted in last week’s report, there are two 24 C Special Local Needs labels to help manage SWD close to harvest. These labels are for a three-day preharvest interval for Mustang Maxx in tart cherry and sweet cherry. Please note that a requirement of making applications under this label is to have the label in the possession of the user at the time of the application. Please review these labels for restrictions prior to making applications under them.

Table 1. Updated SWD model for July 7, 2020. Risk assuming flies have been caught.

Site

Crop

Bloom

Current GDD base 39.2 F from bloom to July 6

Forecast GDD base 39.2 F from bloom to July 10

SWMREC

Sweet cherry

1-May-20

1747

1915

Commerce TWP

Sweet cherry

4-May-20

1568

1740

Romeo

Sweet cherry

4-May-20

1563

1735

Sparta

Sweet cherry

7-May-20

1528

1690

Benzonia

Sweet cherry

20-May-20

1257

1411

NWMHRC

Sweet cherry

22-May-20

1257

1414

Elk Rapids

Sweet cherry

22-May-20

1193

1348

Old Mission

Sweet cherry

22-May-20

1204

1358

Williamsburg

Sweet cherry

22-May-20

1221

1377

East Leland

Sweet cherry

23-May-20

1139

1291

Commerce TWP

Tart cherry

10-May-20

1532

1704

Romeo

Tart cherry

10-May-20

1531

1703

SWMREC

Tart cherry

12-May-20

1654

1822

Hart

Tart cherry

17-May-20

1381

1540

Benzonia

Tart cherry

24-May-20

1157

1314

East Leland

Tart cherry

25-May-20

1093

1245

Eastport

Tart cherry

25-May-20

1121

1278

Old Mission

Tart cherry

25-May-20

1133

1287

NWMHRC

Tart cherry

26-May-20

1152

1309

Traps (0=flies not present, 1=flies present)

GDD 39.2 F from full bloom

SWD risk

0

GDD>1170

low

1

954>GDD

low

1

954<GDD<1170

medium

1

GDD>1170

high

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