MSU Extension Fruit & Nuts

Weather report 

For the first time this season, the past two days have felt like summer, and in many cases, it even felt HOT. In combination with the warm temperatures, the relative humidity has been high (82%)—today feels even muggy. High humidity will continue through the weekend into early next week. Today, June 17, is also breezy with winds reaching 20 mph gusts. These weather conditions make it difficult to spray to get ahead of the forecasted rains.

Growing degree day (GDD) accumulations will likely increase with the forthcoming warming trends, but we are still behind in total accumulations for the region. There have even been some instances in the southern part of the state that are now falling behind normal GDD temperatures as a result of the cool weather this spring. For 2025, at the Northwest Michigan Horticulture Research Center, we have accumulated 899.1 GDD base 42 and 495.7 GDD base 50. We are behind our 35-year average: 996.5 GDD base 42 and 558.4 GDD base 50.

Conditions feel dry, and the soil maps show that the soils are fairly dry throughout the state. We had 0.89 inches of rain on June 4, and another 0.33 inches June 8-9, but overall orchards are dry. There were some isolated rain showers near the research station today, and we could hear thunder to the north and noticed some darker skies also to the north of the station. Rain is predicted for tomorrow, and actual rainfall amounts could be substantial according to the forecast.

After the rain moves out tomorrow, Thursday, June 19 is predicted to be cloudy and cooler with some scattered showers in the forecast again for Friday. The weekend temperatures will warm, and we will continue to see significantly warmer temperatures into next week. The medium range outlooks call for an active storm track across the region, with above normal temperatures and rainfall.

Crop report

Growers are trying to estimate the tart cherry crop size in preparation for the Cherry Industry Administrative Board meeting tomorrow. The original estimate from the Michigan Cherry Committee two weeks ago was 50 million pounds for northwest Michigan. West central Michigan was also estimating their crop to be 50 million pounds; growers in southwest Michigan estimate their crop to be 15 million pounds for a total of 115 million pounds of tart cherries in Michigan for 2025. Growers in northwest are currently thinking the crop is smaller in this region, but some areas have set a good crop. We will inform growers of the estimates that will be given at tomorrow’s Cherry Industry Administrative Board meeting. Southwest Michigan growers are anticipating the start to tart cherry harvest around June 28.

Sweet cherries are sizing, and all varieties are 13-16 millimeters (mm) at the Northwest Michigan Horticulture Research Center, depending on variety. We have heard varied results of the size of the sweet cherry crop. Crop size seems to be variable between different varieties. Growers need to keep plum curculio out of these sizing fruits with the warm and wet weather in the forecast.

Gala and Honeycrisp fruit size jumped since last week when they were about 14.5-15 mm; this week we measured Gala at 21 mm and Honeycrisp at 22 mm. We are approaching the window when apples are too big to thin. Some growers are still thinning apples, and some have already applied one thinner and going in with another to achieve optimal crop load. The crop load in apples is variable, and thinning has been a challenge this season. The carbohydrate thinning model shows a good window for thinning with the warmer conditions, but many growers are already reaching the fruitlet size that are too big for traditional materials to be effective. Accede is a new material that may thin even larger fruit into the 20-25 mm range.

Disease report

Fire blight. We have had reports of apples that still have bloom in the area. Likely, these are lesser planted varieties, such as cider apples, which we still have one in full bloom just north of Suttons Bay. We also have new apples that are flowering. If you have time or the labor, remove flowers from newly planted apples to reduce the potential for a blossom infection. The fire blight model is showing high epiphytic infection potential (EIP) numbers for the next few days with the hotter temperatures. The EIPs from the fire blight model at the Northwest Michigan Horticulture Research Center are as follows for today through Saturday, June 21: 107, 217, 218, 172, 209 and 172. There is isolated rain in the forecast for the remainder of the day (June 17) and for much of the day tomorrow. If you have bloom in the orchard, Michigan State University Extension recommends Kasumin or streptomycin to protect these flowers.

Two treatments of the Apogee and Actigard combination provided excellent suppression of fire blight. Treatments of 2 ounces of Apogee and 1 ounce of Actigard per acre applied starting at king bloom petal fall and then again 10 days later suppressed fire blight lesion development. This strategy is critical to successfully control shoot blight, especially in high density orchards.

Apple scab. We are close to calling the end of primary scab season. According to the Michigan State University (MSU) Enviroweather scab model, we are at 100% mature spores and 96% dispersed spores. Scab spores were not caught after the last rain in the Grand Rapids region, but this rain was pretty light and much of it came at night. They are also waiting to call the end of primary scab until after tomorrow’s rain moves through the state. We had a moderate scab infection on June 8, and a light infection on June 9.

Use materials such as Inspire Super, Cevya, SDHI fungicides or the new fungicide Axios. All materials have shown excellent efficacy in MSU scab trials. These single site fungicides should be tank mixed with protectants, such as Captan or EBDCs. EBDCs have long preharvest intervals (PHI): 77 days prior to harvest. Growers with early apple varieties should keep an eye on this PHI.

Here are the following links for RIMpro outputs from June 3, 202:

• Williamsburg
• Benzonia
• East Leland

Powdery mildew is showing up on terminals in apples at the station (Photo 2). Once the white mycelium is evident on leaves, this disease is difficult to eradicate. If powdery mildew is showing up in the orchard, use fungicides to protect new tissue from infection. This disease does not need rainfall for activity—hot and humid conditions are optimal for powdery mildew infections. For powdery mildew control, use single site fungicides at this timing: SHDIs (Miravis, Tesaris, Merivon), QoIs (Flint Extra) and DMIs (Rhyme). Many of the materials used for scab control will also control powdery mildew.

Cherry leaf spot is more evident in the untreated trees in our cherry leaf spot efficacy trial. We have had three leaf spot infections since mid-May: one moderate on May 15-18, and two low infections on June 3 8. With the rain in the forecast for tomorrow, I anticipate we will have another infection period for this disease, particularly as the weather is predicted to be warm. The daytime high for June 18 is forecasted to be 70 degrees Fahrenheit. At 70 F, the cherry leaf spot fungus only needs 6 hours to cause an infection.

Growers should be covering their canopies prior to tomorrow’s rain. Captan is an excellent material for leaf spot, and it continues to perform well in all cherry leaf spot efficacy trials even when used alone throughout the season. Copper materials are also rated excellent for leaf spot control, and we have new data on the efficacy for leaf spot management comparing the different copper materials. Cool weather is a great time for copper use against leaf spot to minimize the potential for phytotoxicity to leaves. According to Jeff Andresen, the weather will cool slightly for the remainder of the week, but temperatures are predicted to rise over the weekend. Next week is predicted to be hot with potentially more moisture, a perfect combination for leaf spot infection.

Pest report

American plum borer. We caught an average of 5.6 moths this week, which is up slightly from last week where we caught an average of 3.7 moths.

Lesser peachtree borer were caught for the fourth week, an average of 12.6 moths per trap compared to 30 moths last week. In the past, we would target trunk sprays to overlap American plum borer and lesser peachtree borer, and even greater peachtree borer to maximize the trunk sprays.

Black stem borer. We caught an average of four black stem borers again this week.

San Jose scale. We caught more males this week (an average of 12) compared to just over one last week. This insect has behaved a bit odd this year. Male catch is very low in many orchards where we are trapping, but the Trevor Nichols Research Center has captured lots of crawlers despite catching few males in traps.

Plum curculio should be controlled at this time in cherry. At the Northwest Michigan Horticulture Research Center, we have observed sweet cherries at 13-14 mm, and plum curculio will oviposit into this fruit. Growers have reported plum curculio stings in cherry fruit, and we have oviposition damage in sweet cherries at the Northwest Michigan Horticulture Research Center. These insects are out and active, particularly with this warm weather. This pest can sneak up on cherry growers whose main focus is spotted wing drosophila, so growers should be aware that plum curculio is active in orchards now. Because of the cold spring, this pest’s activity may be extended as it likely was less active when it was cool. With the warm weather and wet weather in the forecast, some of the slow moving or less active females will be looking to oviposit into stone fruits.

Codling moth. This pest has increased a lot since last week. We did have some warmer evenings, so codling moth activity likely increased with the warm weather. This pest flies at dusk. We caught 68 moths in our apple block near the woods at the Northwest Michigan Horticulture Research Center. The other two traps only had seven and two moths. Setting a biofix might be hard for this pest with the swings in temperatures and the fluctuating codling moth activity. The first date of sustained captures of males in traps is used to set a biofix for the codling moth degree-day model, which uses accumulated degree-days for predicting key pest life stages and helps target the proper timing for management. The action threshold for codling moth is an accumulation of five to seven moths in any one trap.

We did catch codling moth at the Northwest Michigan Horticulture Research Center two weeks in a row, so we are going to set our biofix for June 3. If we set biofix for June 3, we are at the timing for a material targeting eggs, such as Rimon, as we are at 100 GDD from biofix. Rimon (novaluron) is an insect growth regular that has strong ovicidal activity, thus codling moth egg laying is the optimal timing for this material. Rimon applied at codling moth biofix plus 100 GDD also provides excellent control of obliquebanded leafroller, and suppression of plum curculio (sublethal effects on subsequent generation). Growers that are not trapping on their own farm can use the biofix we set here at the Northwest Michigan Horticulture Research Center. Many growers use the traditional timing for controlling codling moth larvae. Most insecticides registered for use against codling moth are aimed at killing larvae, and thus are typically applied beginning at 250 GDD post biofix. Depending on the weather, we should be near 250 GDD post biofix next week. “Codling moth management options for Michigan apples” is a good article on codling moth.

Rose chafer. We saw our first rose chafer this week on a new sweet cherry planting at the Northwest Michigan Horticulture Research Center (Photo 1). These beetles are in the family Scarabaeidae and are related to Japanese beetles. Like Japanese beetles, rose chafers feed on a long list of host plants including several ornamental flowers, trees and shrubs, annual vegetables, tree fruits, grapes and berry crops. We have received a few reports of rose chafer activity and feeding damage on tree fruit foliage and developing fruits. 

Rose chafer adults are 0.5 inch long, tan to dull light green in color with dark, long and spiny legs (Photo 1). Adults emerge from the soil in late May and into June; the adults can live for three to four weeks, and this pest has a single generation per year. Male beetles are attracted to females, and the beetles congregate on host plants to mate and feed. The beetles are the most damaging as they feed on plant foliage, and in particular the leaves of tree fruits and grapes grown in our region. Feeding can be especially damaging in young trees and vines. The feeding damage is also the most obvious sign that rose chafers are present in the orchard and/or vineyard. These beetles cause a type of injury known as skeletonizing where beetles feed on the leaf tissue between the large veins; this damage is similar that of Japanese beetles. In high populations, rose chafers may feed on developing fruits. 

Orchards and vineyards adjacent to grassy areas can have a higher abundance of rose chafers, particularly if the soil is sandy. This is because mated female beetles select grassy areas to deposit eggs. Those eggs hatch and the larvae are C-shaped white grubs that feed on grass roots through the summer. As the temperature cools in autumn and winter, the larvae overwinter deep in the soil, and in spring, they move up to feed on grass roots just below the soil surface. These larvae pupate and emerge in the following May-June timeframe.

Rose chafers often reappear in hot spot locations; hence, growers with rose chafer challenges are familiar with this pest and which locations may need to be treated. Management can be questionable, however, because the insects are only present for a short time and they can re-infest an area quickly after an insecticide application. The feeding damage and/or population size of this pest may not warrant an insecticide, particularly on older trees or vines with ample leaves present. However, there is no established management threshold for this pest.

Many insecticides will knock down the beetle population effectively, but most are only rated as fair or good because of the beetles’ mobility and potential to re-infest. In tart cherry, Imidan is rated good for rose chafer; however, we remind growers that this material is among the most effective for spotted wing drosophila, so use other products to control rose chafer if you plan to use Imidan in your tart cherry block. Sevin is rated good in both tart and sweet cherry. Some pyrethroid and neonicotinoid insecticides provide good knock-down of this pest, and the product Assail should provide some residual control due to its systemic properties. Refer to the Michigan Fruit Management Guide for additional information on insecticide efficacy for specific fruit crops. 

Spotted wing drosophila. We caught our first spotted wing drosophila in traps today in northwest Michigan. They have been catching this pest in traps in southwest Michigan for a few weeks. In an MSU trial, they found a spotted wing drosophila larva in fruit, and they only trapped two female flies this past week. This pest can infest fruit even if trap numbers are low—we have seen this in many years where we trap few adult flies in an unsprayed block but end up with significant infestation. However, our fruit in the northwest is still too green for spotted wing drosophila to oviposit, so we are at low risk for spotted wing drosophila infestation at this time.

Thinning

As mentioned above, we are at 21-22 mm in size in both Gala and Honeycrisp, so the thinning window is closing at the Northwest Michigan Horticulture Research Center. The carbohydrate model is predicting fruit will be easier to thin than it has been when the weather was cool. However, many orchards have fruit that is too big to be thinned with traditional materials. Additionally, although the weather is looking better for thinning, the accumulation of GDD of 4 degrees Celsius since bloom is well over the 200-250 window that is optimal for thinning. The model is recommending increasing the thinner rates by 30% because the fruit will be more difficult to thin.

MSU has worked with the new thinner, Accede. Accede is primarily intended for later thinning treatments (fruit size 15-18 mm) when earlier chemical thinners are no longer as effective. It works best as part of a season-long thinning program, not as a standalone solution. Michigan trials have shown it to be particularly effective in the "rescue" thinning window, when fruitlets are approximately 20 mm in diameter. Accede can be used when the king fruit's average size ranges from 8-25 mm.