Grand Rapids’ apple trees are entering king bloom this week, the Michigan State University (MSU) Extension said. However, a thunderstorm forecasted for today along with lower temperatures could prove detrimental to the crop.

Integrated pest management educator Lindsay Brown said in the update that “bud development advanced rapidly from last week”. Apple buds are currently at full “pink”, signaling that the first bloom, also called “king”, is close.

With cold temperatures forecasted for this week, MSU advised local growers to refer to critical spring temperatures for tree fruit bud development stages. 

Frost fans and micro-sprinklers can be useful tools to mitigate frost damage, but they can also do more harm than good, MSU warned.

“Be sure to check wind speeds and the temperature inversion potential for your area before using frost fans. Keep a close eye on wind speed and avoid sprinklers if the wind speed is above 10 mph,” Brown explained regarding lower temperature concerns.

Growers on alert for apple scab 

Apple scab effects on fruit.

As blooming stages approach, Brown advised growers to keep pesticide exposure to a minimum, as pollinators are crucial to achieve full bloom.

Cool weather and rainfall have kept Grand Rapids growers on alert for apple scab. The disease typically leads to fruit deformation and premature leaf and fruit drop.

“The rainfall this weekend will be a high-risk time for apple scab with over an inch of rain anticipated over four days. Please ensure that the developing tissue is covered at that time,” Brown added.

Excessive moisture promotes the release of sexual spores, known as ascospores, which cause apple scab. Rainfall also helps to propagate the disease. These spores migrate through wind and moisture, infecting newly-emerged blossoms or leaves. 

Southeast Michigan growers are experiencing similar conditions, with the sector bracing for apple scab risk on Wednesday.

“We are expecting highly variable weather conditions in the coming week and the models could change. Just between yesterday and today the model changed from no scab risk to actionable scab risk, so make sure to check frequently,” MSU Southeast Michigan educator Derek Plotkowski, Ph.D. said in a crop update.

From April 20-21, a low of 28.9°F caused some minor damage in flowering fruit trees, Plotkowski added in the update.

Despite overnight cold weather, the area saw above-normal temperatures and is seeing bloom in many stone fruits and early apple cultivars.

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