MSU Extension Fruit & Nuts

Weather report

Cool weather continues in the northern part of Michigan. This year’s start to the season seems much colder than the warm spring we had in 2024. Overall, the weather has been cool in northern Michigan, and there is a marked line through the middle of the state that shows cooler weather with normal growing degree day (GDD) accumulations in the north compared to above average GDD accumulations in the south. Although it has seemed cooler than normal in the northwest region of Michigan, our GDD accumulations are similar to our 35-year averages, which kind of makes the 2025 spring more normal.

Yesterday, April 28, was considerably warmer than any day we have seen so far this year. The Michigan State University (MSU) Enviroweather station at the Northwest Michigan Horticulture Research Center reported a maximum daytime temperature of 76.5 degrees Fahrenheit. Nighttime temperatures were also high, and there may have been records broken for overnight temperature highs for late April.

Eventually, temperatures cooled slightly overnight but the daytime high today, April 29, is supposed to be in the mid-70s. However, temperatures will drop throughout the day to below freezing overnight and into Wednesday. The weather forecast predicts an overnight temperature of 29 F. Frost is highly probable throughout our region overnight into the morning. Jeff Andresen, MSU climatologist, predicted that it is questionable if there will be an inversion to run frost fans, but be aware that winds may die down in the evening, making an inversion more likely.

In addition to warm conditions, the northwest region had stormy weather overnight April 28-29. We had high winds, lightning and thunder, and there were trees down throughout the region this morning. We recorded just over a quarter-inch of rain accumulation overnight and into this morning; all other northwest Enviroweather stations reported similar amounts of rainfall. More rain is in the forecast for May 1, and the forecast is predicting closer to a half-inch of rainfall. As mentioned in the weekly integrated pest management update, March 2025 is the second wettest March on record for Michigan, and much of the droughty conditions that plagued the state this past summer have been reversed or at least minimized by the replenished rainfall.

According to Andresen, the cooler weather will continue in the north for the remainder of the week into the weekend, but temperatures in the southern part of Michigan will remain warm; these conditions will continue the trend of warm to the south and cold to the north that we have seen so far this spring. The medium range forecast calls for warmer and drier than normal for the first half of May. Predictions for warmer weather includes northern Michigan, and our region should start to see some GDD accumulations in earnest in the beginning of May. Monday, May 5 (Cinco de Mayo!) is predicted to be 65 F.

Crop report

With the recent warm day and a half, we have seen movement in tree phenology. Montmorency tart cherry is at early bud burst, and Balaton is at late bud burst. Emperor Francis and Ulster are at the tight cluster stage, and Gold is at bud burst. Gala are at tight cluster and Honeycrisp are at early tight cluster. Apricots are just starting to bloom. Since Jan. 1, 2025, we have accumulated 223 GDD base 42 and 92.9 GDD base 50. Again, this is almost spot on with our 35-year averages: 199.4 GDD base 42 and 82.5 GDD base 50.

For reference, the growth stages collected at the Trevor Nichols Research Center on April 28 are as follows:

• Apples – full pink
• Cherries – first bloom (Montmorency and Hedelfingen) to full bloom (Balaton and Benton)
• Grapes – bud burst
• Peaches – full bloom
• Pears – first bloom
• Plums – petal fall

Disease report

Bacterial canker. Cool and wet weather is favored by the pathogen that causes the bacterial canker disease, so growers should wait for a stretch of warmer and drier weather to prune sweet cherry trees. There will be a good stretch to prune sweet cherries once this cold weather and the potential rain after May 1. Some growers are applying copper for canker control, but we have documented resistance in the canker pathogen to copper.

Apple scab. We have green tissue out on most varieties of apples across the region. As we have been reminding growers in past seasons, we should be getting a jump on apple scab control early in the season to keep scab at bay through primary scab to avoid having to fight this disease on fruit during the secondary phase of this disease later in the season. Although weather will cool for the next few days, next week is predicted to be warm and we are going to see tremendous growth in green tissue. Growers will need to apply fungicides to protect this new tissue from the apple scab pathogen. As George Sundin, PhD, says, protection is the key to a good season-long program to control apple scab, and it is critical right now to maintain thorough coverage of tissue.

The best choice for scab control early in the season is the broad-spectrum protectants, EBDCs and Captan. These fungicides should be used at full rates on a five-to-seven-day schedule with all middles sprayed to ensure proper coverage. Fungicides should be applied in advance of rains, and growers should keep the intervals tight. This protectant strategy should be followed at least until after second cover to minimize infection of newly emerging terminal leaves. Any strategy (i.e., lower rates, spraying alternate middles) that does not ensure full coverage will result in either unprotected tissue or tissue with fungicide at rates ineffective for control.

The MSU fruit team will be using the RIMpro model to help growers manage apple scab throughout the season. “How to interpret a RIMpro apple scab forecast when making management decisions for your operation” is a great article on how to use RIMPro to best predict scab in regional orchards.

Peach leaf curl. Peach leaf curl infections occur when wetting periods over 10 hours occur under cool conditions (46 to 53 F), with rainfall greater than 0.5 inch. This is an effective range of approximately seven degrees. This disease is remarkable as cherry leaf spot and apple scab can cause infections over a much wider temperature range — more than 30 degrees. Therefore, these rather specific conditions in peach leaf curl may not occur every spring, which explains why some years peach leaf curl does not show up on unprotected trees. Leaf and fruit buds are vulnerable for several weeks after swelling, as are new leaves produced on terminal growth into the summer. However, episodes of cool temperatures and extended wetting generally become uncommon as spring progresses.

To effectively spray for peach leaf curl, the best time is during the dormant season, ideally in early spring before buds swell. A single fungicide application, like copper or chlorothalonil, is often sufficient if applied thoroughly. 

Pest report

With the cool weather, we have not seen too many insects flying yet. However, this warm day will increase the likelihood of insect activity in the coming days. Again, as temperatures warm next week, be on the lookout for more insect activity. These are the insects that were caught in traps at the Trevor Nichols Research Center this week:

• Oriental fruit moth (April 7) – first catch this season, 2.7 per trap
• American plum borer (April 21) – none caught so far
• Red-banded leafroller (March 31) – increase over last week, 89 per trap (last week 51.3)
• Green fruit worm (March 31) – none caught this week (last week 2.6)
• Plum curculio (April 7) – same as last week, 2 per trap

Green fruit worm. We caught green fruit worm this week and last week. We caught an average of 7.3 in three traps last week, and an average of 18 moths this week. Many growers do not regularly control this pest, but we have seen this insect do some damage when we have small green fruit out there, and the larvae will feed on these tiny developing fruitlets.

Black stem borer. We did not catch black stem borer at the Northwest Michigan Horticulture Research Center, but we are trapping them at grower sites where they have had high black stem borer in the past. Females emerge in spring to find new hosts, boring a tunnel to create brood chambers in the tree. The brood chambers are where eggs are laid and larvae develop. The first flight of females in the spring typically occurs after the first two days with high temperatures above 68 F. This usually corresponded with the accumulation of 75 GDD base 50 F from Jan. 1. This usually occurs just before or during bloom in stone fruits and between 0.5-inch green and pink in apples.

Monitoring using ethanol traps are still the best tool for identifying the start of the flight and, therefore, timing of the first insecticide spray for black stem borer. At this time, we are at 92.9 GDD base 50, and we are at tight cluster in apples and cherries, so it’s a little early for most black stem borer activity. However, we are seeing some early emergers, so growers that have had issues should be trapping or following trapline numbers. We would not recommend spraying for this pest with the pending cold temperatures as all insect activity will decrease when it is cold. Wait until temperatures rise next week and check traps or trapline data to determine if an insecticide spray is warranted.