MSU Extension Fruit & Nuts

Weather report 

We had a hint of summer weather last week, which seems like a long time ago with the recent cold temperatures. Daytime highs were the following from May 11 through May 17: 73.7 degrees Fahrenheit, 79.9 F, 67.4 F, 82.2 F, 82.3 F. However, on May17, the temperature dropped almost 30 degrees, and the daytime high was 56.4 F. The following day, May 18, was even colder, and the daytime temperature never hit 45 F.  

In addition to the cold temperatures, today, May 20, we have windy conditions, and wind gusts are up at 30 mph, making the cool temperatures feel even colder. Temperatures are predicted to warm through the weekend, and conditions will dry. Much of the winds for today will come from the east. The 2025 season is shaping up to be similar to our average temperatures, but the weather seems to be colder than usual for this time of year. In fact, Jeff Andresen, Michigan State University (MSU) climatologist, predicts that this week will be 10-20 degrees colder than normal. The 2025 accumulations are as follows: 452.7 growing degree days (GDD) base 42 and 216.5 GDD base 50; our 35-year averages are 443.5 GDD base 42 and 208.7 GDD base 50. 

The region received rainfall at all MSU Enviroweather stations in the north. This rainfall varied by day and by totals. Kewadin had the least rain with a total of 0.67 inches of rainfall (May 13-18). The East Leland station recorded the highest rainfall totals at 1.79 inches in those same five days. All other Enviroweather stations recorded amounts in between. These rainfalls have been welcomed as the region has not had significant rain since the first of May. There is more rain in the forecast, but the forecasts keep changing. Growers should monitor weather reports because more rain and increasing temperatures into the weekend will likely result in disease infection periods. Again, rainfall events may not be adequately captured by Enviroweather stations if the rains are spotty and isolated. 

The medium-range forecast is still calling for cooler than normal temperatures to persist through the remainder of May. However, for the month of June, temperatures will become above normal, and the forecasts are predicting precipitation to be normal to below normal for that month. 

Crop report 

With the warm temperatures last week, we literally blew through bloom. We had that stretch of six warm days, and crop development moved quickly. Even though bloom was fast, we did have good pollination weather within that brief window. We will know more about the crop in the coming weeks once all cherries come out of the shuck. Tart cherries are all in petal fall at the Northwest Michigan Horticulture Research Center, but there is still open bloom on the trees. There seems to be a spread in bloom in tart cherry, likely as a result of the warm and cold weather. Sweet cherry crop is in various stages depending on variety. We have cherries out of the shuck, and we have some sizeable fruit on the trees. All sweets are at petal fall or beyond at this time. 

Apples hit a more problematic window for pollination as they were coming into bloom when the temperatures started to cool. With the current cold conditions now, bee activity is extremely low, particularly with the current winds. At the Northwest Michigan Horticulture Research Center, Gala are at king bloom petal fall, and again, there seems to be quite the spread in bloom within this variety. At the Northwest Michigan Horticulture Research Center, we also have low return bloom in Gala, which is somewhat unusual. Honeycrisp are at 80% bloom.  

For reference, the growth stages collected at the Trevor Nichols Research Center are as follows:  

• Apples – petal fall (Enterprise, Golden Delicious, Red Chief, Crimson Gold), first set (Red Delicious, Empire, JonaMac). 
• Blueberries – bloom 
• Cherries – shuck split (May 14, 2025) 
• Peaches – shuck off (May 19, 2025) 
• Pears – petal fall (May 9, 2025) 
• Plums – petal fall (April 25, 2025) 

Disease report 

In general, again this week, growers are going to be challenged in making disease control decisions because the weather forecasts continue to change. As mentioned above, we had a stretch of warm weather, which would both result in increased green tissue growth and fungal growth. The weather turned over the weekend, and we had much more rain than predicted. There is more rain in the forecast for tomorrow (May 21) and Friday (May 23), but the percentages are lower now than they were earlier in the day. With these recent, unpredictable weather patterns, growers should plan to cover again as the temperatures warm and the chance of rainfall increases. Remember that fungal development and growth is slowed with cooler conditions, and it will take more hours of a wetting event to reach an infection event. Also keep an eye on the relative humidities as they are predicted to be high over the next three days. Relative humidity over 80% will continue a “wetting event” and increase infection potential for many fungal diseases.  

Apple scab. Depending on where the orchard is in northwest Michigan, growers are at petal fall to bloom. Either way, there is green tissue on most varieties of apples, and growers should be covering this tissue before the next rain is in the forecast. Optimal conditions for apple scab are wet and warm. With warmer average daily temperatures, fungi require less time to grow and initiate infection compared to cool temperatures. In the case with apple scab, this pathogen takes 18 hours of wetness to cause infection at about 43 F compared to 6 hours at about 61 F—apple scab infection is dependent on the length of the wetting period and temperature.  

We had a heavy apple scab infection period at the Northwest Michigan Horticulture Research Center on May 18 where we had a span of wetness for 40 hours at an average temperature of 48.5 F, and just under a half inch of rain. Currently, the model predicts that we have 86% of apple scab spores that are mature and 53% of the spores are dispersed. The forecast is predicting 96% of spores will be mature on May 26, and 69% of them will be discharged. In most years in northern Michigan, we call the end of primary scab in the second week of June. The RIMpro models are also showing that we had an extreme infection period around May 18. The model is also predicting lower risk with cool temperatures in the forecast. 

Here are the following links for RIMpro outputs from May 20.  

• Williamsburg 
• Benzonia 
• East Leland 

Powdery mildew. This disease is more problematic in warm conditions, so the recent cooler weather is good for slowing powdery mildew. Sulfur is a good material early, but do not spray sulfur during bloom or during apple thinning. Tight cluster through petal fall is a critical time for powdery mildew control, and single site fungicides should be used: SHDIs (Miravis, Tesaris, Merivon), QoIs (Flint Extra) and DMIs (Rhyme).  

American brown rot. American brown rot is favored by warm and wet conditions. These conditions are not in the forecast, and most growers have or will move beyond bloom in the coming days, so the window for American brown rot bloom sprays is or has closed for most orchards.  

Cherry leaf spot. Bract leaves have fully emerged at 250 GDD base 42, and we are well beyond that time frame (452.7GDD base 42), so leaf spot programs should have begun in orchards. Most orchards are at petal fall or soon will be at that stage, so the best options for early season control are captan and chlorothalonil. Captan is one of the best materials for leaf spot overall, and it is more resistant to rain wash off than chlorothalonil. However, chlorothalonil is a good to excellent leaf spot material early and in drier conditions. Remember that chlorothalonil cannot be reapplied within a 10-day reentry window, even if growers are using an every-other-row spray strategy. As with scab, the leaf spot fungi will grow slower at colder temperatures, and it will take more hours of wetting to cause infection. Similarly to apple scab, it would take 28 hours to cause a leaf spot infection at 46 F, and five hours to cause infection at 63-65 F.  

Pest report 

With the warm weather last week, we saw an increase in insect activity. However, now with the recent cold weather, the activity will slow considerably. At the Trevor Nichols Research Center, they caught their first San Jose scale male this week, and the first lesser peachtree borer moths. Their codling moth catches increased, and they caught 20 per trap compared to last week when they had 8.7 moths per trap. Their oriental fruit moth catch also went up to 51 per trap compared with 42 per trap last week. They are still catching plum curculio. Here at the Northwest Michigan Horticulture Research Center, we trapped more black stem borers this week but no codling moth.  

With the variability in weather, we are likely to have an extended insect season. There are always insects in the population that move earlier in the season and those that move later, which will stretch out the need to control pests. Pests like black stem borer and plum curculio will likely show peak activity during the warm periods and drop off when it is cool. Trapline data will reflect trap catch caught by week, so it will not capture the snapshot of the insect activity. Plan on insect control applications for trapped pests when the warm weather resumes.  

American plum borer. We caught an average of seven months per trap. Wait to control this pest when we start catching lesser peach tree borer, but we have not done so yet. 

Black stem borer. We did catch black stem borer at the Northwest Michigan Horticulture Research Center again this week, and in one trap we caught 18 beetles and only two in the second trap. However, that high number of 18 suggests that the population is relatively high in our small blocks of apples, and that they responded to the warm temperatures last week. Again, this pest activity will rise and fall with temperatures. Be ready to apply trunk sprays for this pest in the next warm stretch, particularly if growers have had issues with black stem borer in the past.  

San Jose scale. We did not catch San Jose scale in traps this year. As mentioned above, they caught a small number of males at Trevor Nichols Research Center. If growers missed oil at the delayed dormant timing, recent data show that Esteem 0.86EC at 16 ounces at pink provided excellent control of San Jose scale in a particularly high-pressure block. Centaur 70WDG at a rate of 34.5 ounces at the pink timing provided good control of this pest. We are past the pink stage in apples, but the cool weather has likely limited male San Jose scale flight. Growers with past issues of San Jose scale in orchards should consider a San Jose scale spray when the temperatures warm.  

Plum curculio. Plum curculio becomes active with overnight temperatures above 65 F, and they do prefer moisture or humidity to increase activity. With the recent warm weather last week and now the wet weather this week, this pest will become active the next time we have warm weather. Again, this beetle pest activity will be dictated by the weather, and we will see increases and decreases as temperatures rise and fall. This pest will be a threat to cherries coming out of the shuck when the temperatures warm. Be ready to make a plum curculio spray when temperatures rise above 60 F.  

The rotation of Avaunt and Neonicotinoids is critical to control plum curculio early in the season. Avaunt needs to be used prior to the neonicotinoids because Avaunt must be ingested to work, and Neonicotinoids (Voliam flexi, Leverage, Actara, Assail, Belay) have an anti-feedant characteristic. Avaunt also works primarily by lethal activity, but ingestion is the important means for delivering this insecticide. Neonicotinoids are highly lethal via contact for first several days after application, and as these translaminar materials move into plant tissue, they protect via oviposition deterrence and anti-feedant modes of activity. Growers should save organophosphates and pyrethroids for spotted wing drosophila timing in cherry. 

Plant growth regulators 

Pro Gibb. The tart cherry trees at the Northwest Michigan Horticulture Research Center have four leaves expanded and are not yet ready for GA at this time. Additionally, plant growth regulators do not work in cold weather, so growers should be ready for warmth and leaf growth and expansion for Pro-Gibb applications in the coming weeks. 

Apple thinning. If growers have frost damage in apples, Michigan State University Extension does not recommend bloom sprays as we are unsure of the crop load. At the Northwest Michigan Horticulture Research Center, we did sustain damage in both Honeycrisp and Gala, and we are waiting to see about pollination and set before making thinning decisions. If growers have no frost damage in apples and decent pollination weather, we recommend the nibble thinning approach to crop load management. The concept of nibble thinning is to thin a little of the crop at every opportunity until the crop load has been reduced to the desired target level. This means to thin starting early and planning multiple applications. 

Start thinning early at full bloom, then at petal fall, then again at 6 millimeters (mm) and 10 mm and more if needed. Often, we let the early thinning windows (full bloom, petal fall and 6 mm) pass by because we are unsure of bud health or fruit set. A frost event or some other early trauma makes us want to wait and see what fruitset will be before thinning. We would like to delay thinning until more time has passed and there is more information to judge frui tset and thinning needs, including frost injury, bee activity, pollination and fertilization. At this point in the 2025 season, we had some cold weather on April 29-30 that may have caused damage, and we have not had good pollination weather with these cold temperatures.