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West central Michigan tree fruit update – June 18, 2019

Apples have sized past the optimal thinning window. Insect activity is picking up.

Nectria twig blight
Nectria twig blight caused by the fungus Nectria cinnabarina on Pacific Gala. This disease is often mistaken for fire blight, but is a minor disease of apple and is common to find this season. No management is needed. Photo by Dave Jones, MSU Extension.

Despite some intermittent sun, weather in the past week has remained cool and wet. Growing degree day (GDD) accumulations continue to lag behind five-year averages, and are unlikely to catch up to normal any time soon.

Growers are anticipating a late sweet and tart cherry harvest this season, and later fruit crops are also likely to be delayed, especially if current trends continue.

Growing degree day accumulations since Jan. 1 for west central Michigan.


Degree days base 42 F current

Degree days base 45 F current

Degree days base 50 F current

Benona / Shelby




Elbridge / Hart
















New Era




Shelby - East




Five-year average growing degree day accumulations for west central Michigan.


Degree days base 42 F

Degree days base 45 F

Degree days base 50 F

























Average of 5 years




Apples are sizing nicely with all of the rain. King fruitlets are 20-25 millimeters, with most sides in the 15-20 millimeter range. The optimal window for fruit thinning has closed, so growers that still need to thin are either trying one last shot at high rates in the hope of getting a result or are planning on hand-thinning. Generally, growers seem to have gotten good response with their thinners this season, having had two very nice windows at most locations to thin during the optimal timing between 8 and 12 millimeter fruit.

Vegetative growth is extensive this season due to the weather conditions, so varieties like Honeycrisp could end up struggling with bitter pit, especially where the crop is light. We are in the midst of the period of cellular division inside of fruitlets, so calcium sprays are critical during this span to help avoid problems with bitter pit down the line. No fire blight strikes on this year’s flowers or shoots has been reported so far this season, but numerous nectria twig blight strikes have been brought in to the Extension office and documented in the field over the past two weeks. At a glance, this disease can look somewhat like fire blight, which can lead to concern on the part of the grower. Traditional varieties like Rome are known hotspots for this disease, and we have also seen quite a bit of this show up this season on Gala. Retention of a stem from last season along with wet conditions during harvest and a hard winter are all likely contributors to this. Nectria twig blight is not a disease that typically requires any management, and this year is no exception.

Primary apple scab appears to be at an end in the region. Local spore rods have not detected ascospores in the last two rain events, and models for the region indicate that the last few spores were likely flushed out in the most recent two rain events. Growers will be transitioning in to summer fungicides this week if they have not done so already.

Tart cherries are starting to size up. The crop is highly variable in the region, and many orchards have poor fruit set this year. Some cherry leaf spot has been observed in orchards, but generally the sentiment among scouts and growers is that there is less than they expected given the wet start to the season. The cold weather during the frequent rains ended up limiting the number of infections, which certainly helped. Growers also did a good job of getting out and getting covered early this season. Most growers have moved past first cover timing and have already applied fungicide targeting powdery mildew. Growers who want to try an application of copper for cherry leaf spot this season are approaching the ideal window, after plum curculio sprays but before spotted wing Drosophila sprays, and in moderate weather conditions.

The sweet cherry crop looks heavy throughout the region, and trees look very healthy due to the lush growth from all the rain. Some bacterial canker has been observed, but does not appear extensive in the region. Growers are going to need to start thinking about American brown rot sprays as we start to get close to final coloration and ripening, especially in early sweet cherry varieties. Warmer weather on the forecast accompanied by intermittent rain and high humidity will highly favor the development of this disease.

Peaches are set heavily in most orchards this season, and are going to need to be thinned well to get acceptable size. Peach leaf curl has shown up on unsprayed trees in the past couple of weeks, but orchards that were treated generally look very good. The narrow set of infection parameters for this disease means that we don’t always get conditions that will trigger a peach leaf curl infection event every season, but the cold, wet start to spring this year meant that we had several infection events. This is a great year for growers to see the utility of early season peach leaf curl management.

Insect report

Orchards have been setting codling moth biofix since the first of June; dates generally ranging from June 17 in sites with a history. Some low pressure blocks have not captured this pest yet. The first application for codling moth is to be timed 100-250 growing degree days base 50 from biofix for the first generation, depending on product. Most growers are using materials that mean shooting for roughly the 250 mark, and a first material for codling moth is going out this week in orchards that biofixed for the first week of June. Warmer weather at dusk as this week progresses should mean that codling moth is more active.

Most orchards set oriental fruit moth biofix in late May or early June, with a regional biofix estimated for May 20. The first management application is recommended around 200 growing degree days base 45 from the first biofix. No shoot strikes from larval feeding have been observed yet.

Plum curculio activity was minimal last week due to the cold temperatures, but warmer weather this week should bring on another round of activity. Expect a late start to spring, meaning this pest may be active past its traditional management window this season. A material should be out for this pest this week.

Images from several sites and several samples indicate mullein bug has caused some damage this season. This is a sporadic pest of apple and can cause injury to fruits between bloom and about 10 millimeter fruit. If you see damage from mullein bugs in fruit right now, it is too late to do anything about it, but you are encouraged to read this article on mullein plant bug to familiarize yourself with optimal management of hot spots going forward.

Greater and lesser peachtree borer flight is ongoing, and trunk sprays of Lorsban have been going out over the past week.

The first four female Spotted wing Drosophila (SWD) were captured in Weare and Clay Banks Townships last Wednesday, June 12. All other sites in west central Michigan did not capture any SWD. Sweet and tart cherries are not yet at the developmental stage where we would consider management. It is time to manage once fruit is straw-colored and SWD has been captured in the vicinity.

Obliquebanded leafroller first flight will begin around 900 growing degree days base 42 from March 1. We are 100-200 degree days off from that mark in west central Michigan at this time, so flight of this pest could begin in hotspots in one to two weeks depending on weather. Traps should be up in blocks with a history.

European red mite and twospotted spider mite numbers are low due to the frequent rain events this season.

San Jose scale male flight is ongoing in the region. Products targeting crawler activity can be timed in late June.

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