Northwest Michigan fruit update – June 2, 2020

Bloom is coming to an end and growers are focusing management efforts to protect the developing fruits.

European red mites on apple
Photo 1. European red mites on apple at the Northwest Michigan Horticulture Research Center on June 1, 2020. Photo by Emily Pochubay, MSU Extension.

Weather report

As is typical of Michigan, the weather has been all over the board this past week. We have had warm and cool temperatures, rain, wind, and growers have been juggling spray activities amidst these wild weather patterns. Over the past week, daytime temperatures ranged between 55 and 87 degrees Fahrenheit—swings ranging over 30 F. Traverse City, Michigan, hit a new daytime high record temperature last Tuesday, May 26, where the thermometer hit 92 F, which was higher than the previous record set in 1978.

High temperatures hit across the region on Monday, Tuesday and Wednesday (May 25-27), and most Michigan State University Enviroweather stations recorded temperatures in the mid to high-80s. Temperatures cooled on Thursday and into Friday, and the daytime high on Saturday, May 30, was 55 F at the Northwest Michigan Horticulture Research Center.

In addition to the heat at the start of last week, the relative humidity was particularly high, which made the heat seem more intense. We have accumulated growing degree days (GDD) rapidly over the last week, and we are at 503 GDD base 42 and 252 GDD base 50.

Growing degree days (GDDs) through June 1, 2020

Year

2020

2019

2018

2017

2016

2015

30 year average

GDD42

503

452

669

633

696

547

650.2

GDD50

252

199

390

297

371

342

332.4

Rain is expected to move into the region tonight and potentially Friday night, but the weekend is predicted to be seasonable and dry. As mentioned, humidity was high over the past week, and the Northwest Michigan Horticulture Research Center recorded multiple days of rainfall. We had the following rainfall amounts on May 24, 25, 26, 27, 28 and 29: 0.21 inch, 0.02 inch, 0.33 inch, 0.25 inch, 0.87 inch and 0.02 inch.

This rainy weather was followed by two dry days, and we recorded rainfall again on June 1: 0.04 inch. Rainfall seems particularly variable this season. Some growers have reported rainfall on their farms while the nearest Enviroweather station did not record any rain. Additionally, some areas, like Kewadin and Northport, have not recorded much rain this season. Bear Lake has recorded a good amount of total rainfall for this season but has been dry recently. This variable rainfall has been a challenge for growers to make decisions on re-applications and preventative fungicide sprays.

Review Jeff Andresen’s weather report below for June 2, 2020, below.

Crop report

Fruit development sped up with the warm conditions over this past week. Most of the tart cherries in the region are in the shuck. Sweet cherries are sizing, and they are 8-9 millimeters at the Northwest Michigan Horticulture Research Center. We will know better about the crop size over the coming week.

Educators in other regions of the state think their crop size is larger than they originally thought after some of the cold temperatures in April and May. Because conditions were so cold leading up to some of the frosty evenings, that situation may have been helpful in saving some of the crop. We will report on the crop size in the coming week.

Pest report

Wet weather last week was conducive for disease development with a few long infection periods. The region also had scattered rain on Saturday, May 30, that was and wasn’t recorded at some Enviroweather stations. Where this rain was picked up by Enviroweather, apple scab and cherry leaf spot infections were not recorded as conditions dried quickly. However, in areas that remained wet for longer, there could be potential for localized infections during these variable rains.

Scattered rains and hot temperatures were concerning for fire blight infection. However, as conditions cooled over the weekend, the epiphytic infection potential (EIP) values decreased. Many orchards are at or quickly approaching petal fall, but there are a few blossoms remaining and tag bloom could be a concern for fire blight infection. Plant growth regulator (PGR) programs can help to slow vegetative growth and the opportunity for shoot blight, if infection occurred or if severe, trauma blight inducing weather comes to the region.

MSU plant pathologist George Sundin joined our northwest Tree Fruit Update last week, May 28, to discuss different strategies to minimize fire blight. If you would like to review the fire blight topics discussed, the recording can be viewed on our YouTube channel, NW MI Hort and IPM.

Apogee and/or Kudos are excellent tools for minimizing fire blight. We had a pretty bad fire blight infection in our Gala block at the Northwest Michigan Horticulture Research Center, and we did not use Apogee to minimize fast growing shoots. We also saw this similar situation in grower orchards where Apogee was or was not used—this material minimized fire blight in orchards compared to orchards where it was not used.

Primary apple scab is ongoing, and orchards continue to look scab-free at this time. The three RIMpro models are showing that the region is past peak spore discharge, but there are still spores needing time to mature before they are ready to discharge. The most recent wetting event that began late last night, June 1, dried quickly for most areas (except Bear Lake) and did not result in an infection. Forecasted warm weather will continue to facilitate spore maturation and a few more good rains will be needed before primary apple scab ends.

Wet conditions last week favored cherry leaf spot infections. We have not yet observed lesions on tart cherries, but a few lesions without conidia were found on very early leaves of sweet cherries at the station. As noted previously, Enviroweather stations that recorded rainfall on May 30 did not record cherry leaf spot infections as conditions dried quickly. Similarly, most areas did not have an infection following overnight rains that began on June 1. Lesions from the very high infection that occurred from May 25-29 (depending on location) could begin showing up later this week.

Hot temperatures will favor powdery mildew development. First cover timing in cherries is the time to include a material for this disease.

In apples, codling moth began flying over the last week, and we found five moths in a trap placed in one of our apple blocks adjacent to the wood edge. We have received reports that codling moth is active in commercial orchards as well. After setting biofix (i.e., two consecutive catches), the treatment threshold in blocks without disruption is a cumulative catch of five moths in a trap.

We set biofix for oriental fruit moth for May 25, although hot spot areas may have set biofix a few days earlier. Moth catches continue to be low at the station. As of today, we have accumulated 173 GDD base 45, and eggs hatch has begun.

European red mite egg hatch peaked over the last week and motiles are visible and active on apple leaves (Photo 1). Treatment may be needed if prebloom mite management did not occur this season or if mites are over threshold. Monitoring guidance and a binomial sampling chart can be found in this MSU Extension article from 2014, “Managing mites in apples;” however, this article is a few years old and while monitoring strategies haven’t changed, chemistries and labeled miticides may have. MSU entomologist John Wise et. al provided this article, “Miticide options for controlling mites in fruit,” with updated chemistries earlier this year.

Black stem borer has been active for three weeks, and activity increased this week with a total of 41 beetles at the station. This pest is attracted to and primarily attacks weak and/or stressed trees. Hence, the best way to mitigate black stem borer is to address the cause of stress. Treat for this pest before they bore into the tree and insecticides cannot reach them.

Spotted tentiform leaf miner numbers decreased this week, but the traps along the wood edge continue to catch the greatest numbers of moths.

The first flight of male San Jose scale began in the last week. We have traps in both apples and sweet cherries at the station where this pest was been observed during dormant scouting. The next opportunity for San Jose scale management targets crawlers. If you are using a systemic material (for example, Movento), apply the application before crawlers are active to provide enough time for the material to work its way through the tree. Additionally, according to the label, this product should not be applied prior to petal fall on pome and stone fruits as “sufficient leaf tissue must be present for uptake and translocation.”

Evidence of plum curculio activity has not been found in apples at this time, but we have observed oviposition scars in sweet cherries (Photo 2). With the exception of a couple cooler nights, recent warm and moist weather has been favorable for this pest. Growers have started protecting vulnerable fruit from plum curculio. Please see the updated article on plum curculio by Wise and Rothwell.

Plum curculio oviposition scars
Photo 2. Plum curculio oviposition scars at the Northwest Michigan Horticulture Research Center on June 1, 2020. Photo by Emily Pochubay, MSU Extension.

American plum borer flight is underway and trap numbers remain consistent with last week; we found an average of one moth per trap. No lesser peach tree borers have been detected at this time.

Spotted wing Drosophila (SWD) traps are up, and we will begin checking traps and reporting numbers this week. The first detections of SWD in the state occurred last week in southwest Michigan where one male and one female were caught. The Isaacs Lab at MSU has also started to catch SWD in very low numbers in southwest Michigan.

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