MSU Extension Fruit & Nuts

Weather and phenology update

Conditions have remained colder than normal in the Grand Rapids, Michigan, area. Over the past week, highs were mostly in the 40s to 50s (degrees Fahrenheit) with low overnight temperatures in the low 30s. As of May 2, the Michigan State University Sparta Enviroweather station has accumulated 229.6 growing degree days base 42F (GDD42). This is below average, which is 286.7 GDD42, and approximately one week behind normal.

Rainfall and soil moisture continue to be above average, and fields are muddy. After mostly dry conditions last week, significant rainfall accumulation occurred over the weekend. An average of 0.91 inches of precipitation was recorded at weather stations across the region, ranging from 0.53-1.15 inches Saturday night and Sunday.

Phenological development continues to be slow. Apples have reached approximately tight cluster growth stage, with some early varieties and blocks at open cluster. In peaches, first pink was observed on Monday at a few sites. Tart cherries are at approximately half-inch green or slightly more advanced. In sweet cherries, first bloom was observed yesterday.  

Over the next week, a warmer trend with significant rainfall expected. Temperatures are expected to be in the low 60s for the remainder of the week with overnight lows in the 40s. Early next week, a trend of even warmer conditions is likely, with highs reaching into the 70s. Over today and tomorrow, approximately 0.5-1.0 inch of precipitation is expected across the region. Partly cloudy conditions are likely for the remainder of the week through the weekend. Showers are expected Friday afternoon with an accumulation of about 0.1 inch. With all this warm weather and precipitation, phenological development is likely to move fairly quickly over the next week.

Cold temperature concerns

As buds develop, they become more sensitive to cold temperatures, and more susceptible to frost injury. You can find a table of critical spring temperatures for tree fruits from Michigan State University Extension. Keep in mind the temperatures referenced for apples are averages based on Red Delicious, which is more sensitive to cold than many other varieties. Actual thresholds will be dependent on variety, specific trees and location. Risk increases in the case of a temperature inversion, in which calm, clear conditions and a rapid decrease in wind speed result in layering of atmospheric zones, with heavier cold air sinking to the ground level. Potential for inversions can be monitored using the Enviroweather Temperature Inversion Potential Tool. In addition, the Meteogram for the Enviroweather ‘Sparta 20m Tower’ site to monitor inversion conditions that may have occurred.

At this time, there is no indication of cold temperature risk in the forecast for the coming week. As a result of earlier cold events, some king damage has been observed in locations across the Grand Rapids area. Most of this has been attributed to low temperatures on the mornings of April 16 and 17. Recorded low temperatures ranged from 24.6F in the coldest locations to 27.1F. This damage was somewhat unexpected; at this time, the bud stage was approximately green tip, with critical temperatures of 18 for 10% mortality and 10 for 90% mortality. A short temperature inversion on the morning of April 16 may have contributed to colder temperatures and explain the observed king bud damage: temperatures of 24.8-25.0 were recorded over approximately 2 hours at the Sparta 20m weather station.

Cold temperatures occurred again last week on Wednesday and Thursday mornings. On April 27, temperatures recorded across the region ranged from 26.4-29.3 F, and wind was sufficient to prevent inversion conditions. On April 28, low temperature did not fall below 28 F; a slight inversion was observed, but lasted less than an hour and occurred before the coldest part of the  morning. At this time, we do not anticipate significant damage from these cold events. Overall, there is little concern about the potential crop, there are plenty of healthy laterals and kings remaining. This is especially the case for the 2022 season because of the heavy bloom and potential crop we’re observing across the region.

For these updates, we used averages for 1997-2021 from the Michigan Automated Weather Network (MAWN) to represent normal conditions. Weather data was gathered from MSU Enviroweather.

More information and reports on normal weather conditions and departures from normal can be found on the NOAA Climate Prediction Center website, NOAA U.S. Climate Normals website, NOAA Climate Normals Quick Access Page (which may be searched by region) and Midwest Regional Climate Center website.

Tree fruit diseases

There have been several rain events in the general Grand Rapids area since first green tissue appeared in mid-April. However, there have not been many wetting events that have resulted in apple scab infections. The apple scab ascospore numbers have been very low – until this past weekend. Ascospore numbers increased dramatically with the rain event on April 30 with an average of 957 spores per rod. This is a significant increase and right on time with the crop development stage of tight cluster when spore number typically jump up. From now until the end of primary scab, fungicide coverage is essential in apples to prevent infections. I expect spore number to reach some high numbers over the next two weeks with warmer weather forecasted. For the Sparta area, using the degree day model for apple scab maturity, there are 250 DD32 accumulated since Mac Green Tip on April 13, indicating about 13% ascospore maturity. May 8, this will jump to 30% spore maturity, increasing the inoculum pressure overall.

Warmer weather on tap will increase the likelihood of favorable conditions for powdery mildew. Mildew operates best in warm and high humidity (not necessarily wet) conditions. Apple growth stage will bring lots of lush, tender leaves and flower buds, ripe for all kinds of fungal invaders like mildew and scab. Keep your fungicide programs tight over the next few weeks for best success.

Tree fruit insects

Cool weather has kept insects inactive, and this will likely change greatly a week from now. I estimate pink timed insecticides are about a week away in the Grand Rapids area. Of course, you need to time that with your pollinator services, but do not be in a rush to apply pink sprays too early. Insects you might be targeting at pink in apple include Green Fruitworm, Redbanded Leafroller, Spotted Tentiform Leafroller, Obliquebanded Leafroller, Rosy Apple Aphid, San Jose Scale, Tarnished Plant Bug, and perhaps European Red Mites (oils). This is a lengthy list, but very few of these insects are troublesome pests in the modern apple orchard so early in the growing season. Please consider your scouting reports and block history to make decisions on the pink spray composition best for your orchards. Rosy apple aphid is probably the main concern at the pink time, and it is always important to target them before they roll the leaves. Looking ahead even more, the weather forecast is for continued lower than average temperatures for much of May. This could mean a long stretch of bloom where very few insecticides can be used. You know your blocks best.

Some insects to be thinking about now as you plan for monitoring and management.

Black stem borer - A few adults were active about ten days ago, but no flight since then. The warmth forecasted in the next week will get them moving again. BSB prefers temperatures above 60 F for flight.

Green fruitworm – This is a minor pest in tree fruit, but we still see some damage from larval feeding in any given year. Adult flight has been ongoing for about two weeks in very low numbers.

Redbanded leafroller – Adult fight is very low in number. Continue to monitor this minor tree fruit pest.

Spotted tentiform leafminer – An uptick in adults trap numbers, but still quite low overall. This pest has not been a target in apple orchards for many years.

Obliquebanded leafroller – A few overwintering larvae were found two weeks ago on green tissue. With warmer weather on tap, consider Bt sprays in problem blocks over the next few weeks.

Aphids – A few apple grain aphids continue to be found in non-managed apple terminals and a few were noticed last week in managed blocks.  Rosy apple aphids eggs have been found and should begin hatching in the next week to ten days.

San Jose scale - Overwintering scale are present. Management sprays should be considered for orchards where scale were present on fruit in the 2021 harvest and dormant oil is first management choice to consider. This pest rarely requires whole orchard applications to manage in apples. Careful scouting is key to identify hot spots to target spray.

European red mite – In the egg stage and not active in such cool temperatures. Monitor for overwinter egg mortality. If eggs look pale or clear instead of bright red, they are not viable.

Climbing cutworm – I have found two terminals chewed away in non-managed apples – likely from climbing cutworm. This is a minor pest overall, but we tend to see more damage in a cool, drawn out spring such as this year.

Pear psylla - Adults and newly laid eggs are present in pear – an early start is key for season long pear psylla management.