MSU Extension Fruit & Nuts

Weather report

Cool weather has continued in the northern part of Michigan, although we did have warmer temperatures during the weekend and into Monday, May 5. The daytime high on Sunday, May 4 was 62.9 degrees Fahrenheit, and the daytime temperature high on Monday, Cinco de Mayo, was 66.9 F. Temperatures are expected to be cooler today, Tuesday, May 6, and drop even more through the week. Daytime highs will only hit the mid-50s on Wednesday and Thursday this week. By the weekend, temperatures are predicted to warm, and the daytime high on May 12 is predicted to hit the low 70s. The warmer weather will move plant development along quickly, and growers will need to respond to make adequate and timely applications to protect new green tissue.

Overall, the weather has remained cool in northern Michigan, and there is still a marked line through the middle of the state that shows cooler weather and average growing degree day (GDD) accumulations in the north compared to above average GDD accumulations in the south. The southern part of the state is estimated to be two weeks ahead of normal, and the northwest region is a week behind normal. At this time, our GDD accumulations remain similar to our 35-year averages: 265.6 GDD base 42 and 113 GDD base 50. The 2025 accumulations are as follows: 259.5 GDD base 42 and 104.7 GDD base 50.

Last week, we had some cold overnight-morning temperatures across the northwest region. On April 29, the day was extremely windy and cold, and the wind remained quite high overnight, only dropping down to low wind speeds at 6 a.m. on April 30, when the tower at the Williamsburg Enviroweather station reported a slight inversion from 6-7 a.m. Looking at the overnight temperatures at all the Michigan State University (MSU) Enviroweather stations across the region, the East Leland station recorded the lowest temperatures, which dipped into 23-24 F from 5-7 a.m. Eastport also recorded temperatures around 25 F for 1 hour.

Based on those recorded overnight lows and the stage of development of the different tree fruit crops, there should have been little damage. However, we cut buds on Friday, May 2, and we observed damage in sweet cherries, tart cherries and apples. We hypothesize that the windy Tuesday leading into the cold overnight may have contributed to the elevated levels of damage reported. We are still assessing the damage at the start of this week.

According to MSU climatologist Jeff Andresen, there has been a unique weather phenomenon called a cut-off low that has been reported in Michigan over the weekend. A cut-off low is a low-pressure system that becomes detached from the main jet stream and can drift for days, causing persistent, often unsettled weather. These systems can bring rain, thunderstorms and even snow, and are known for being difficult to predict precisely. As a result of this weather pattern, the northwest region of the state was warmer and drier than the central and southern parts of Michigan. The southeast and south-central parts of the state had significant rainfall (over 12 hours of wetting) accompanied by cool temperatures over the weekend, whilst here in the north, we were dry and warmer over the same days. The medium-range forecast is predicting warmer and drier weather than normal through the third week of May.

Crop report

The crop development is moving along. Montmorency tart cherry is at late bud burst, and Balaton is at tight cluster. Emperor Francis and Ulster are at the white bud stage, and Gold is at early white bud. Gala and Honeycrisp are both at tight cluster. Apricots are starting petal fall.

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

• Apples - bloom
• Blueberries - late pink bud 
• Cherries - full bloom
• Grapes - bud burst (Aurora) to 1-3-inch shoots (Concord, Niagara, Chancellor, Foch)
• Peaches - full bloom (Red Haven) to petal fall (Coral Star)
• Pears - full bloom
• Plums - petal fall was recorded on April 25, 2025

Disease report

Apple scab. There is a significant amount of green tissue out on most varieties of apples across the region, and growers should be covering this tissue before the next rain is in the forecast. Fortunately for the growers in northwest, there is no rain in the 10-day forecast. As mentioned above, this weather pattern is significantly different to southern Michigan where they have received significant rainfall and are predicted to receive more in the next two days. The East Lansing Enviroweather station recorded just over an inch of rain on May 4 and 5, and the forecasts are predicting another inch today and into tomorrow, May 7.

Growers 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. Growers will need to apply fungicides to protect this new tissue from the apple scab pathogen. The best choice for scab control early in the season are the broad-spectrum protectants, EBDCs and Captan. If you have significant tissue, and powdery mildew has been a concern, add other modes of action to keep fast growing tissue protected.

The figure below is an excellent snapshot for managing scab throughout the season. As many orchards are at the tight cluster stage, there are a lot of options for adding single site fungicides to the tank along with the protectants. The SDHI fungicides are excellent as is Inspire Super and Cevya. Axios is a new fungicide with a new mode of action (FRAC 52), and it has excellent efficacy in apple scab efficacy trials. Try and alter modes of action for potentially a long scab season.

The Michigan State University Extension 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. There are significantly more spores mature this week than at this same time last week. With the maturing spores and predicted warmer weather next week, keep an eye on the weather forecast for rain events. The next daytime rain event will have huge numbers of spores that are mature and ready to discharge, and growers will need to be protecting green tissue. In addition to the high spore load, there will be significant green tissues exposed with the warm weather in the forecast—even more reason for growers to be aware of the next rainfall event.

Here are the following links for RIMpro outputs from May 6.

• Williamsburg
• Benzonia
• East Leland

Powdery mildew. There have been lots of reports of bad powdery mildew in apple orchards from last season. If mildew was a problem in the orchard, this disease will overwinter in the buds and can cause more infection this season. Start your mildew control programs early if you had problems with this disease in 2024. Also remember that apple powdery mildew fungal spores (conidia) cannot germinate in free water, and they prefer warm, humid conditions with relative humidity more than 60%. Again, there is no rain in the forecast for this next week, but relative humidities are predicted to reach into the mid-50s.

Apple leaves are most susceptible to powdery mildew for first few days after they open. Patches of white mycelium form on the underside of the leaf where the cuticle is less developed, and infected leaves tend to crinkle, curl or roll upwards along the edges, giving them a narrow appearance. Growers can start powdery mildew control as early as half-inch green. 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).

European brown rot. European brown rot is a disease of tart cherries caused by the fungus Monilinia laxa. The European brown rot fungus overwinters in infected shoot strikes and flowers from the previous season. European brown rot infection is favored by colder weather prior to and during bloom with extended wetting events, or rain followed by at least 18-24 hours with relative humidity above 90%. At this time, the weather conditions do not look to favor European brown rot infection.

Infection potential is also affected by cultivar; one possible reason for the susceptibility of Balaton is that M. laxa infects Balaton flower tissue more easily and thus does not require as long of a wetting period for infection. Fungicide sprays targeting European brown rot must be initiated at the white bud stage prior to a rain event. A second application follows usually one week later during bloom. Indar at 6oz rate is the material of choice.

American brown rot. Unlike its cousin, European brown rot, American brown rot is favored by warm and wet conditions; again, not the weather in the cards for the development of American brown rot since it predicted to be dry. However, if the weather forecast changes, growers should be using Rovral at 2 pints per acre; this is the material of choice since Rovral cannot be applied after petal fall, and we have limited materials to control American brown rot closer to harvest when this disease can take out sweet cherries quickly under optimal conditions. Rovral cannot be applied more than twice per season nor past petal fall.

Cherry leaf spot. The bract leaves are not out in the Montmorency tarts here at the station. As soon as those leaves open, the stomata, even on those tiny bract leaves, are susceptible to leaf spot. Again, with the dry weather in the forecast, growers may catch a break for leaf spot applications while the weather remains dry.

Pest report

With a few warm days, there is more happening in the insect world as they become more active in the orchard. At the Trevor Nichols Research Center, they have not yet caught San Jose scale, lesser or American plum borers or codling moth. They are still catching a few plum curculio. Here at the Northwest Michigan Horticulture Research Center, we caught a few spotted tentiform leafminer, and we caught one black stem borer.

Green fruitworm. We have caught green fruitworm for the last three weeks. We caught an average of 5.6 moths in three traps last week, and an average of seven moths last 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 catch our first black stem borer at the Northwest Michigan Horticulture Research Center, and we are catching higher numbers of this pest in apple orchards where they had been problematic in the past. The first flight of females in the spring typically occurs after the first two days with high temperatures above 68 F. This usually corresponds with the accumulation of 75 growing degree days (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.

Using ethanol traps is still the best tool for identifying the start of flight and, therefore, timing of the first insecticide spray for black stem borer. At this time, we are at 104.7 GDD base 50 compared with last week at this time where we had accumulated 92.9 GDD base 50. With the cold weather, we are at tight cluster in apples and cherries, so it is still 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. Growers should be targeting black stem borer sprays when temperatures warm this weekend and into next week. The cool weather today and tomorrow will not increase black stem borer activity, so wait to apply trunk sprays next week when temperatures hit above 60 F.

The MSU Tree Fruit Entomology lab has been conducting black stem borer efficacy trials. From the 2024 data, Assail with the adjuvant Widsepread had better control than Assail alone; the Widespread adjuvant did not improve efficacy when combined with Warrior II compared to Warrior alone. The data from 2024 had few attacks (just an average of about three holes in the UTC), and we will continue black stem borer insecticide options, including adjuvants with the insecticide materials.

San Jose scale. This pest will also likely be on the increase with the pending warm weather. As mentioned above, zero San Jose scale were caught at the Trevor Nichols Research Center trap line this week. This pest overwinters in the black cap stage. The scales are protected underneath this cap and insecticides cannot penetrate these coverings. At this delayed dormant timing, growers should be using oil in both apples and sweet cherries; the oil smothers the scales. However, oil can only be used at a delayed dormant timing because oil can cause phytotoxicity to expanding green tissue. This stage may be passed for many growers, especially as we see extensive growth with this predicted warm weather. If you 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. In short, if you missed your oil applications, target the pink stage with Esteem or Centaur.

Plant growth regulators

Retain in sweet cherry. The weather conditions are predicted to favor the efficacy of plant growth regulators (PGRs). Most PGRs work at temperatures above 60 F, which are in the forecast starting this weekend. Growers that are looking to have a better fruit set in sweet cherries, particularly the variety Regina, should consider Retain at an early bloom timing. Here is a good article on Retain use to increase fruit set in sweet cherry.