MSU Extension Fruit & Nuts

Weather report

Summer is here! The weather certainly has felt more summer-like over this past week. The sunshine has been endless, and even if the mornings start out cool, the afternoons have been warm and pleasant. The temperature differentials during the day are noteworthy with the cool starts and warm ends. We have also had some good conditions for spraying; this change also feels different from the windy start to the spring; the calmer winds have been good for both working and spraying. The forecasts have been interesting as the daytime highs have been higher than many of the predictions, so, even with cools starts to our days, we have had warm weather in the afternoons.

The days have also been extremely sunny, and these conditions have increased soil temperatures over the past week. The weather has been notably dry, and growers have appreciated that change in weather after an extremely wet April. At the Northwest Michigan Horticulture Research Center, we have accumulated 645.7 growing degree days (GDD) base 42, and last week at this same time, we had accumulated 507.2GDD base 42—that is quite the jump in the past week. This week, we have accumulated 319.3GDD base 50, and we were at 233.4GDD base 50 last week. Our 36-year averages are as follows: 652.7GDD base 42 and 330.4GDD base 50. The forecasts are predicting warm weather this week with Thursday, June 4 predictions estimating daytime temperatures into the 80s.

We had a dry week, and there is slight chance of rain in the forecast. Jeff Andresen’s forecast showed a higher potential for rain in northwest Michigan on Friday into Saturday with up to a half-inch of rainfall. However, the local forecast tools are showing the percentages of rain to be much lower, and most of the predictions are less than 50% chance of rain. Andresen showed a slow-moving front for potential rainfall coming from the west that will hit the state on Friday and may bring some isolated thunderstorms and rain, but the chance of rainfall is further south. The 6–10-day outlook is showing warmer than average temperatures and normal precipitation for northern Michigan. The map of this outlook looks much redder (warmer) than anything we have seen yet this season.

Crop report

Crop development has moved quickly this week with the warm conditions. Tart cherries moved out of the shuck, and the fruitlets are at 9 millimeters (mm). We have a light crop of tarts at the Northwest Michigan Horticulture Research Center, but there is more fruit in the tops of the trees than low, so as the fruit sizes and colors, the crop size may increase with time. Balaton fruit is also at 9mm.

Sweet cherries are just larger than the tarts, ranging from 10-11 mm depending on variety. Our sweet cherry crop varies across varieties, and this situation is likely the result of where the trees were when the cold conditions hit in April.

Apples at the station are also sizing. There is some variability in development and crop across our blocks, but in general, our Galas range from 5-7 mm and Honeycrisp is similarly sized. Growers are starting their thinning programs with the warm weather in the forecast (more thinning recommendations at the end of the report). Start thinking about NAA applications for return bloom in Honeycrisp. Chayce Griffith, PhD, is recommending these treatments for growers to try:

• 10 ppm NAA at 30, 45, and 60 DAFB
• 150 ppm ABA at 30 DAFB
• 10 ppm NAA at 30, 45, and 60 DAFB + 150 ppm ABA at 30 DAFB (so NAA and ABA are tank- mixed at 30 DAFB)

Disease report

Apple scab. We are not calling the end to primary scab, but we are getting closer to that goal. We had no scab infections over the past week, and the last infection period was heavy on May 24. With the recent forecast, the next potential scab infection could be on Friday, June 5. Keep an eye on the forecast and protect the tissue from infection prior to the next rain event. This situation is a hard call because the forecasts are predicting less than 50% chance of rain now, but this could be the last rain event to disperse scab spores. MSU Enviroweather’s scab model is showing that 100% spores are matured and 84% of them are dispersed. If we do have a rain event, the model is predicting that 93% of the spores could be dispersed by June 7. Again, growers will have to watch the forecast closely to see if the chance of rain on Friday increases.

Fire blight. With the recent warm temperatures, the fire blight model is showing that the EIP could hit 234 by Thursday. This number is well above the recommended EIP of 70 that growers use to apply an antibiotic for fire blight control. However, conditions are extremely dry, and most apples are at petal fall or beyond. Growers with cider apples or other varieties that have tag bloom should watch the forecast for rain on Friday—EIPs will still be high as we approach the weekend, and bacterial counts will be high, and any amount of rainfall will wash the bacteria down into the blossoms. 

Cherry leaf spot. Like apple scab, we have not had the weather conditions conducive for leaf spot infection. The dry weather reduces the potential for fungal infections, and the cherry leaf spot model shows that the last infection period was also May 24. Watch the forecast for rain.

Powdery mildew. Unlike other fungal pathogens, powdery mildew thrives in hot and dry conditions. This recent weather pattern is perfect for powdery mildew as this fungus grows well when temperatures are between 60-80 F. This fungus does not need rainfall to infect as humidity and/or dew can result in a powdery mildew infection. Once this disease infects the leaves, a white mycelium appears on the foliage-once this mycelium is visible, there are no fungicides that can eradicate powdery mildew.

This disease must be treated preventatively. We have had more reports of PM in recent years, and there have been concerns about resistance issues. Kerik Cox, PhD, Cornell University, has been working on powdery mildew resistance, and there is little evidence about fungicide resistance in the pathogen that causes powdery mildew. Powdery mildew may also be more evident recently as our weather conditions have been hotter than in past decades.

In cherry, many growers rely on chlorothalonil before shucksplit. This fungicide will not control powdery mildew, so growers must actively manage for this disease with different materials. Our previous work has also shown that the first cover timing is optimal for controlling powdery mildew for the remainder of the season—growers should not delay a powdery mildew application (or tank mix for powdery mildew and cherry leaf spot). Luna Sensation, Merivon, Torino, Pristine, Flint Extra and Cevya are all rated excellent for powdery mildew.

Apple powdery mildew has also been on the rise in Michigan orchards. Growers need to treat for this disease earlier in the season, and because we do not have hot conditions at tight cluster, many growers may have not put a material targeting PM into the tank until later in the season. Luna Tranquility, Merivon, Inspire Super, and Miravis are all rated excellent in the E-154 guide.

Insect report

Our insect counts have increased markedly this week following a long stretch of warm weather.

Codling moth. We caught our first codling moth at the research station last week, May 26. We caught a lot of codling moth this week, especially for the Northwest Michigan Horticulture Research Center where trap counts have historically been low. Our trap counts for this week were as follows: 1, 37, and 56. Because we have sustained catch for codling moth, we are setting the biofix for May 26, 2026. Based on the Enviroweather codling moth model, egg-laying will begin on June 3, and growers targeting codling moth egg-laying with Rimon should begin these applications on this date. If growers are targeting the larval stage, they have more time. Use the Northwest Michigan Horticulture Research Center biofix timing if you are not trapping codling moth on your own farms.

Plum curculio. We have had many reports of plum curculio in sweet and tart cherry orchards. Apples are also sizing, so growers need to be on the lookout for plum curculio in apple orchards. Activity increases in warm weather, and with the extended cold period this spring, they have recently become active. Growers should look out for the moon-shaped oviposition scar, and most orchards have hotspots for plum curculio activity, likely near a woodlot where they are moving in from overwintering. Plum curculio also seem to lay in wait for cherries to come out of the shuck, so exposed fruit needs to be protected.

Avaunt works primarily by lethal activity, but ingestion is the important means for delivering the poison. Neonicotinoids are highly lethal to plum curculio via contact for the first several days after application, but as these systemic compounds move into plant tissue, they protect fruit from plum curculio injury via their oviposition deterrence and anti-feedant modes of activity. The rotation of these two modes of action is critical to successful plum curculio control. Avuant needs to be used prior to any neonicotinoids because Avaunt must be ingested to work effectively, and the neonicotinoids have that anti-feedant characteristic. If needed use Avaunt first followed by neonicotinoid insecticide.

At this time in the season, cherry growers should be using the neonicotinoid and indoxacarb (Avaunt) chemistries as these materials are not effective at controlling spotted wing Drosophila. Save Imidan and the pyrethroids to target and potentially overlap these materials to control late season plum curculio and early spotted wing Drosophila emergers. Imidan and pyrethroids are rated excellent for spotted wing Drosophila, and neonicotinoids are only rated as fair against this pest. To make the most out of a cherry insecticide program, the more broad-spectrum materials, like pyrethroids and Imidan, should be used later in the season to do double duty on these two pests.

Obliquebanded leafroller. We have had many reports of obliquebanded leafroller larvae in both apple and cherry orchards. Obliquebanded leafroller is a threat to apples as a direct pest; the larvae feed on leaves and fruit in summer and fruit in fall. In cherries, obliquebanded leafroller does not feed on fruit, but they can cause damage to cherry leaves, which can result in defoliation. The greater concern of obliquebanded leafroller in cherries is the threat as a contamination pest, as they can be found in cherry tanks if present in trees at harvest.

If growers are seeing abnormally higher than normal obliquebanded leafroller larvae in their orchards, targeting them while they are small is recommended as this pest is much harder to control once the larvae are larger in size. If growers do not treat now, they should set obliquebanded leafroller traps to monitor for adult flight to set a biofix to target summer generation larvae. Sometimes the summer generation larvae overlap with cherry harvest, and as mentioned above, the larvae can end up in tanks if the populations are high enough. Cherry growers with past obliquebanded leafroller issues should treat now to be sure they are controlling this pest.

Black stem borer are still relatively low at the Northwest Michigan Horticulture Research Center. However, this warm weather will increase activity and adult flight. If growers have had problems with this pest in the past, they should be controlling it now.

Horticulture

Thinning. This weather is conducive for thinning, and this is the task on many growers’ minds at this time. Apples are ~7 mm at the Northwest Michigan Horticulture Research Center as of Monday and likely larger today, so growers should take advantage of this warm weather for thinning. This is an excellent article about thinning apples in Michigan. Conditions for optimal thinning are listed in the following table.

The first line on the chart says that all thinners work best under warm conditions, and the forecast is predicting warm weather. In northern Michigan, we typically struggle to find a window for thinning because it is cool, but since it is June, we have an opportunity to thin under more optimal conditions—let’s do it!

The carbohydrate model shows we are going to have fairly significant stress on the trees (-40 for the six-day weighted average today, and the model is even suggesting decreasing your thinning rate by 30% because of the conditions. Remember that the optimal thinning is between 8-10mm fruit, and we are close here at the station. If the weather conditions continue to be warm, fruit will grow quickly, so growers should thin as soon as they can to catch the warm window weather and to ensure their fruit is in the optimal thinning range. Once fruit is 20-25mm, they become much more resistant to thinning.