MSU Extension

Weather report

Conditions have been extremely warm for this time of year. Daytime temperatures have felt more like July than May! Nighttime temperatures have also stayed quite warm—in the mid-60s. This warm weather has really pushed tree development along.

We had a surprise rain event on Friday, May 25, but the event went through quickly and did not record on most Michigan State University Enviroweather stations. We did have a rain event over the weekend, and the Northwest Michigan Horticulture Research Center Enviroweather station recorded 0.11 inch of rain. We also had a small amount of rain on Sunday, May 27.

These rain events did not provide much moisture for our dry conditions, but it was enough to cause problems with fire blight in blooming apples, particularly with the extremely warm temperatures and high epiphytic infection potential (EIP) values.

Cherry and apple crop report

Cherries are sizing and we have moved quickly through shuck split. Sweet cherries are so big that they are visible from the road. Apples are still at various stages of bloom throughout the region. We are past king bloom petal fall in most early varieties, and growers are applying Apogee at this time. We highly recommend Apogee this year with the tremendous amount of apple bloom and the high EIP values for fire blight. Growers are starting their thinning programs for the season, and we recommend making thinner applications before Apogee applications if growers are stretched for time.

The thinning window is likely to move very quickly with the warm temperatures, and growers should be diligent about thinning applications as the optimum window (8-10 millimeters) will come and go fast. The Cornell model is predicting stress and calling for a decrease in thinning rates by 15-30 percent in the next two days (May 29-30) with stress in the -34 to -49 range. By May 31, we will have less stress, and growers should apply standard thinning rates. Friday, the model predicts no stress (+3.81), and we recommend increasing thinning rates by 15 percent.

Apple carbohydrate thinning model for Northwest Michigan Horticulture Research Center

Change green tip and/or bloom date and click "Calculate" to recalculate results.

Note from the model developer (March 22, 2018)

The apple carbohydrate model simulates the response to weather of trees that are healthy with normal vigor and bloom, no significant water, nutrient or winter or spring freeze stress, and no significant carry-over stress from a previous year that will change tree responses. We are less confident in the model if temperatures are extremely cold or hot. Each orchard is unique, so use this tool, as any other, in the context of your own experience. For more information click on the "More Info" tab.

Cherry and apple pest report

In apples, although most of the region received some amount of rainfall over the weekend, the only Enviroweather station to report an apple scab infection was the Northwest Michigan Horticulture Research Center. Wet conditions in other areas dried quicker than the center, which had a 10 hour wetting period. According to Enviroweather and RIMpro, we are roughly halfway through primary apple scab at this time with the majority of scab spores predicted to reach maturity this week. Although many areas did not have scab infections, powdery mildew is a concern in these relatively dry conditions.

RIMpro-Venturia Outputs

• Bear Lake (Biofix 1 May)
• Benzonia (Biofix 1 May)
• East Leland (Biofix 1 May)
• Eastport (Biofix 2 May)
• Elk Rapids (Biofix 1 May)
• Kewadin (Biofix 1 May)
• Northport (Biofix 5 May)
• NWMHRC (Biofix 1 May)
• Old Mission (Biofix 1 May)
• Williamsburg (Biofix 1 May)

Fire blight was particularly concerning late last week, and over the weekend as many apple orchards were blooming during hot temperatures and multiple rain events. Many growers were concerned about fire blight following spotty rain on Friday, May 25, because many blossoms were open and very little water is needed to stimulate infection. Most growers were protected for fire blight going into the weekend’s rain and it was a tough decision to recover prior to rainfall on Saturday. However, if growers reapplied, this was likely a good strategy because temperatures were hot and new unprotected flowers opened.

The epiphytic infection potential (EIP) values remain high at this time, and although we have moved through apple bloom quickly, fire blight will continue to be a concern in the coming days. Monitor orchards for tag bloom as these flowers can be problematic for late fire blight infection. Some growers applied Apogee at king bloom petal fall and subsequent applications should go on about 14 days after the previous.

We have observed an uptick in apple pest activity at the center. The weekend’s warm evenings stimulated codling moth flight and the center’s first codling moth was found yesterday, May 28. Codling moth biofix is the first date of sustained catch; hence, we will check traps at the end of the week to determine if 28 May is our biofix. Egglaying begins 100 growing degree-days (GDD) after biofix and the treatment threshold is a cumulative catch of five moths in a trap. Codling moth activity has also been reported in other areas.

San Jose scale males began flying over the weekend, and petal fall timing in apples is the suggested timing to apply some systemic materials to target crawlers. This timing allows the material to be taken up and distributed in the plant tissues before crawlers emerge and begin feeding. Spotted tentiform leafminer (32 per trap) activity is ongoing in low numbers.

We have received reports of brown marmorated stink bugs emerging in residential areas this season. Brown marmorated stink bug emergence begins when day length reaches 13.5 hours—this timing corresponds with April 16. According to degree-day accumulations since April 16, brown marmorated stink bug egglaying should begin this week and scouting for eggs and nymphs in orchards should begin at this time. Although the population of this pest has been low in the northwest region in previous seasons, we suspect that some apple orchards may have had brown marmorated stink bug damage last season.

In cherries, Friday morning’s brief and spotty rain shower were not documented as cherry leaf spot infections in areas that recorded rainfall on Enviroweather; although there was enough rain accumulation to trigger the infection process, the moisture dried too quickly for an event to occur. Rainfall over the weekend resulted in leaf spot infections in many areas, but the periods of wet weather appear to have been too short for an infection in Northport, Kewadin and Williamsburg, Michigan. Growers have been preparing for rain in the forecast for Wednesday and Thursday.

According to label, chlorothalonil should not be applied after shuck split and both sweet and tart cherries started coming out of the shuck over the weekend at the center. Additionally, temperatures are predicted to be in the 80s until Friday and some materials (copper and Captan) can have phytotoxic effects in these hot conditions. Lastly, these relatively warm and dry conditions will be conducive for powdery mildew growth.

Cherry insect pests were very active in recent warm conditions. As anticipated, plum curculio adults were particularly active in the warm evenings over the weekend. We have observed plum curculio’s distinctive, crescent-shaped oviposition scars on sweet cherries at the center and we have received reports of damage on commercial farms. Warm evenings are in the forecast for today and tomorrow, and plum curculio will likely continue laying eggs in these conditions.

Some growers are also strategizing to target pestilent caterpillars (obliquebanded leafrollers and green fruitworm) in conjunction with plum curculio at this time. Determining how to manage these pests has been challenging as bloom happened quickly and many orchards still have bees present. Use materials with reduced risk to pollinators, particularly if bees are still in the orchard, and to use management practices that minimize harm to pollinators (i.e., make applications when bees are not actively foraging).

San Jose scale males took flight in sweet cherries over the weekend. We are monitoring in a commercial sweet cherry orchard and a block at the station with San Jose scale infestation; we found males in traps at both locations. Cherry foliage has developed very quickly in hot conditions and there is now substantial green tissue present that is needed for the uptake of systemic insecticides applied for scale control.

Lastly, there was a flush of lesser peachtree borers (four moths per trap) that emerged over the weekend and American plum borer numbers (seven per trap) have also increased since last week. We have not detected greater peachtree borers at the center at this time.