MSU Extension Fruit & Nuts

Weather update

In the past week, the Grand Rapids, Michigan region continued to experience significantly cooler temperatures than normal. The Michigan State University Sparta Enviroweather station has accumulated a total of 323.7 growing degree days (GDD) base 42 since Jan. 1. Despite the cool temperatures, the region is still approximately 10 days earlier than the average for April 26. Dry conditions also continue and are becoming more severe. Only about 0.1 inch of precipitation was recorded in most locations across the Grand Rapids region in the past five days and a total of only 4-6 inches since Jan. 1.

A hard freeze took place early in the morning on both Wednesday and Thursday, April 21 and 22, last week. Low temperatures dropped to approximately 23-24 degrees Fahrenheit across most of the region on Wednesday morning. Conditions were less severe on Thursday, with lows recorded between 25-28 F. Considerable damage has been observed in flower bud evaluations across the region. Extent of damage is greatly dependent on variety and location.

More sensitive varieties, such as Red Delicious, suffered the worst injury, with nearly 100% mortality of king blossoms, but healthy laterals present in most flower clusters. In other varieties and many locations, damage was much less severe. In some cases, very little king damage has been observed. In most places there appear to be plenty of healthy lateral blossoms present to support a full crop. Full extent of the damage will become clearer over the coming weeks.

Warmer weather is expected this week, as a subtropical air mass moves into the state from the southwest. The next few days will be warm and humid with scattered showers, but very little accumulation expected. Friday morning will be chilly (temperatures in the low 30s) and then conditions are expected return to warmer and wetter than normal conditions.

Apple bud development across the region is between open cluster and full pink. The very first blossoms began opening in earlier varieties over the past two days. King Bloom in most varieties is expected in the middle of next week. Peaches in the region are at approximately 20% to full bloom and cherries are at full bloom.

Trees are starting to look a bit sickly and yellow. You can add some foliar nutrition to your tank to give trees some extra love, especially as we move into bloom, but we really need some heat to pull them out of this.

Pest update

Continued cold weather kept overall insect activity low over the past week, and this trend is likely to continue based on the forecast. The cool weather flyers took a short break last week, and they will start back up this week—speckled green fruitworm, redbanded leafroller and spotted tentiform leafminer are all very minor pests in the Grand Rapids area and we track them nonetheless. Small overwintering obliquebanded leafroller larvae continue to be found in a known hot spots feeding on terminals. Evidence of climbing cutworm damage in low input apples was found late last week.

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 in sporadic numbers and a sustained flight has not been determined. It is expected that oriental fruit moth flight will become established this week with warmer temperatures. Mating disruption dispensers for oriental fruit moth and codling moth can go up at any time—they should be in place by apple bloom timing.

Some have applied pink sprays to apples already and others are getting that job done this week. Pests to target with pink applications in apple include San Jose scale, obliquebanded leafroller, mites and aphids. While insect activity continues to be very slow to get started in 2021, with bloom starting to open, 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.

It has been over a week since we have had measurable rainfall with dry and cool conditions in place. The forecast for the coming week to ten days includes warmer and wetter conditions which will bring apple scab back into play. High spore discharge numbers are expected in the next two to three weeks—be cautious of stretching protectant fungicides too much. With a good deal of fresh, green tissue showing in apples, 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 at bay in apple blocks. Cool and dry (low relative humidity) weather so far has likely reduced mildew infections. With warmer and potentially wetter conditions in the forecast, be sure to add something in for mildew over the next few cover sprays.

Some apple bloom is beginning slowly in the general Grand Rapids area. Open bloom brings the risk of blossom blight from the fire blight bacteria. When running the forecasted temperatures through the MaryBlyt model, the risk is mostly moderate due to cooler temperatures. For April 28, the risk bumps up to high with the addition of rainfall. Maryblyt uses a baseline average temperature of 60 F to start building the EIP (bacteria numbers) on open bloom. Current forecasts have the average temperature barely above 60, so this is an overall low risk scenario.

At this time, only the highest risk apple blocks should consider fire blight management applications where there is open bloom. This would include high risk varieties and any block that had issues with blight last season. Save the big antibiotics for the high risk or full infection potential in the future.

You can track potential fire blight risk of blossoms 24/7 using your nearest MSU Enviroweather station. At this time, please refer to the old Enviroweather website for fire blight, as the new site’s model is not quite right yet.

Apogee or Kudos applications should 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.