Grand Rapids area tree fruit update – May 4, 2021

Cool conditions and an extended bloom period are expected this week.

Honeycrisp apples in bloom.
Honeycrisp in bloom on May 3, 2021, in Sparta, Michigan. Photo by Anna Wallis, MSU Extension.

Weather update

Weather conditions in the Grand Rapids, Michigan, region have been a little bit of a roller coaster. High temperatures recorded over the weekend were in the upper 70s (degrees Fahrenheit), with some locations reaching the low 80s. This was accompanied by sustained high winds and gusts over 20 mph. However, immediately preceding the hot weather, there was a minor frost event overnight from Friday into Saturday morning. Temperatures dropped to the low 30s and a strong inversion was observed from approximately 10 p.m. to 7 a.m.

Overall, the Michigan State University Sparta Enviroweather station has accumulated a total of 483 growing degree days (GDD) base 42 since Jan. 1, indicating the region is still approximately 14 days earlier than the average for May 3. Dry conditions also continue and are becoming more severe, with less than 0.5 inches of precipitation was recorded in most locations across the Grand Rapids region in the past five days.

Over the next week, very cool, dry conditions are predicted. Highs will hover in the mid- to upper 50s with lows in the 30s. Overcast conditions and wind are expected most days, with gusts up to 25 mph. Scattered showers are in the forecast on both Thursday afternoon and over the weekend, but little precipitation accumulation is expected. Irrigation will be necessary to supplement soil moisture over the coming weeks.

As a result of the warm weather, apples rushed into bloom. Most varieties are currently between king bloom and petal fall phenological growth stages. The bloom period is expected to be drawn out over the course of the week due to the cool temperatures in the forecast.

This week’s conditions may pose some challenges for pollination. Cool temperatures, wind and rain all reduce pollinator activity in apples. Unfortunately, the frost events on April 21 and 22 left many un-viable kings, which will influence fruit set as well as available pollen for pollination. Finally, the drought conditions this season have left little water for pollinators. Despite all this, active pollinators were observed over the weekend and Monday afternoon once the cool drizzle cleared. Honey bees will continue to fly when temperatures are in the 50s. In addition, many native pollinators tolerate temperatures 5-10 F lower than honey bees, as well as wind and rain.

Pest updates

A short spike of warm weather in the past week got a few insects moving, but overall insect activity is low for this time of year. Cooler than normal weather will be back in place for the coming week and that will delay insect activity. Dry conditions across the general Grand Rapids area have keep the risk for diseases lower than typical for April into May.

Traps should be in place for major pests such as oriental fruit moth and codling moth and San Jose scale. Oriental fruit moth adults have been flying for a few weeks in sporadic numbers and with the warmer weather, a sustained flight occurred over the past weekend. A biofix was set for the general Grand Rapids area for May 1. Mating disruption dispensers for oriental fruit moth and codling moth need to be in place no later than petal fall to be most effective.

Apples are in the peak of bloom this week and it’s time to start thinking of your tactics for petal fall applications that will be needed in a week or so. Pests to target with petal fall applications in apple include plum curculio, San Jose scale, obliquebanded leafroller, mites and aphids. There are very few pesticides that can safely be used when pollinators are present; be mindful of your bees and those in neighboring blocks before any insecticides are applied for the petal fall timing.

Rainfall events in the Grand Rapids area have been more off than on for several weeks, but as expected, apple scab spore numbers took a dramatic jump with the warmer weather rains of late. High spore discharge numbers are expected in the next week or two—be cautious of stretching protectant fungicides too much. With a good deal of fresh, green tissue and tiny apples present, the risk for primary scab is a concern with any rain event from now until at least the end of May.

Normally, tight cluster to petal fall is the time to start adding materials to manage powdery mildew in apple blocks. The recent warmer and somewhat wetter conditions might have increased the infection potential for mildew, but conditions are not ideal for it to become established.

Open apple bloom and warmer temperatures bring blossom blight due to fire blight to the forefront. Keep a close eye on your nearest MSU Enviroweather station to infect potential. Now is the timing for Apogee or Kudos applications to be used in high fire blight risk blocks—prohexadione calcium has kept fire blight at bay many times in the past and should be a routine part of your overall fire blight management program.

Trees continue to look a bit sickly and yellow. You can add some foliar nutrition to your tank, but we really need some heat to pull them out of this.

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