West central Michigan small fruit update — May 15, 2018

Low temperatures are delaying plant growth and development in most small fruit crops and rainy conditions are interfering with plant protection practices and disease management.

Blueberry flower buds damaged by hail at South Haven-Covert, Michigan. Photo by Jesus Barajas, Very Blue Farm, Covert, Michigan.
Blueberry flower buds damaged by hail at South Haven-Covert, Michigan. Photo by Jesus Barajas, Very Blue Farm, Covert, Michigan.

During the past several days, weather conditions in west central Michigan have been cold with periods of rain that, at some point, reached damaging conditions, especially around West Olive, Michigan. On Wednesday, May 9, rain accompanied by high winds downed trees around the area and could have caused some damage to blueberry fields. However, no damage has been reported by growers yet. We will conduct a damage assessment during this week. We have received at least one report of wind and hail damage at blueberry fields around Covert, Michigan. This damage was the result of weather conditions occurring over the weekend in that area (see photos).

In west central Michigan, weather conditions over the past seven days were characterized by low temperatures and rainy conditions. Average minimum temperatures in the area were 48 degrees Fahrenheit, and the maximum temperatures were, on average, 67 F. Precipitation during the same period varied from 2 to 3 inches of rain. The heaviest precipitation occurred in Allegan County.

Current growing degree-day (GDD) accumulations (base 50) varied from 210 GDD in Ottawa County to 295 GDD in Allegan County. This shows a difference of almost 100 GDD that is shown in plant growth and development in these two locations; blueberry fields in Ottawa County are five to seven days behind fields in Allegan and Van Buren counties. This difference in GDD accumulation also impacts the emergence of insect pests like the cherry and cranberry fruitworm.

According to the Michigan State University Enviroweather Cherry and Cranberry Fruitworm Phenology Models, the cherry fruitworm model traps should have been already deployed. We expect the first adult moths to arrive between May 16 and May 18 in West Olive and Fennville, Michigan. In Grand Junction, Michigan, the cherry fruitworm emergence should start this week in most fields (see table below). Remember, the day when the first moths are caught on your pheromone traps is the starting day for your GDD accumulation to predict when to place the first application against the cherry fruitworm (approximately 200 GDD).


First adults

First eggs

Current GDD accumulation as of May 15, 2018)

Grand Junction


West Olive

Cherry fruitworm

238 ± 30

432 ± 15




Cranberry fruitworm

375 ± 20

460 ± 20




For cranberry fruitworm, pheromone traps should be deployed at this time to correct the biofix of the GDD accumulation for the predictions of the first applications against cranberry fruitworm eggs.

On the issue of disease management in blueberries, cool and rainy conditions are suitable for some fungal diseases. Mummy berry and Botrytis twig blight are favored by a wet spring. MSU plant pathologist Annemiek Schilder reported up to 40 percent mummy berry apothecia germination in high pressure sites under these conditions. Thus, if you have not completed your early season spray program against these diseases, please do so as soon as the fields and the weather allow. Products recommended are Indar, Tilt, Proline and Quash.

For dose and more options, check the 2018 Fruit Management Guide, MSU Extension Bulletin E-154. 

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