Weather conditions in west central Michigan had a sudden turn for the worst during the past week, causing limited frost damage to berry crops, mainly blueberries and, to some extent, strawberries. The unexpected spring frosts that occured May 8 and 9, 2017, had some significant impact on blueberry varieties that were in advanced bloom stage, for example late pink bud and early bloom (25-50 percent bloom). Those bloom stages are less tolerant to freezing temperatures than early stages.

According to “A Pocket Guide to IPM Scouting in Highbush Blueberries,” Michigan State University Extension bulletin E2928, the late pink bud stage can tolerate freezing temperatures of 24 to 27 degrees Fahrenheit. However, the early bloom and full bloom stages tolerate only temperatures of 25 to 28 F.

During May 8 and 9, temperatures dropped down to 22 F in most blueberry fields across west and southwest Michigan. The length of the freeze/frost events was three to seven hours, depending on the location. Blueberry fields located in southern Allegan and Van Buren counties that were in advanced bloom stages, late pink to early bloom were the most affected. This included early-season varieties like Bluetta, Duke and others. Conversely, varieties like Jersey and Elliott, and other late-season varieties that were still at early pink stage, suffered significantly less damage.

A frost damage evaluation conducted immediately after the last event, May 9, in unprotected fields at Allegan, Van Buren and Ottawa counties showed the following percentages of flowers and buds damaged:

Allegan and Van Buren counties

• Elliott = 14 percent
• Jersey = 44 percent
• Bluecrop = 53 percent

Ottawa County

• Elliott = 6 percent
• Jersey = 33 percent
• Bluecrop = 34 percent

Although no substantial damage occurred at fields with Elliott or Jersey, there is a potential pollination problem later on since some flower structures inside unopened buds may have been affected (see picture). Some late pink buds that were dissected and examined with a dissection microscope revealed some stamens and the style were damaged.

Regardless of the variety or location, spring frost damage was not significant in blueberry fields that were frost protected. Also, since only a portion of the first bloom was affected, it is expected that the spring frost damage to blueberry production may not be significant.

Early-season blueberry insect problems

Cherry fruitworm and cranberry fruitworm have been emerging at Allegan and Van Buren County fields.

Below is a table with the predicted degree-days when we are expecting the emergence of cherry fruitworm and cranberry fruitworm, and the current degree-day accumulation for key weather stations in west Michigan.

* Adults already started emerging around Fennville and Grand Junction area.