Grand Rapids area tree fruit update – June 2, 2020

Warm weather brought fast development of trees and insects.

Early fire blight symptoms
Be on the lookout for early fire blight symptoms later this week. Photo by Amy Irish-Brown, MSU Extension.

Weather update

Warmer than normal temperatures continue to push degree day totals forward and totals from Jan. 1 are nearly to normal average levels again after being 10 to 14 days behind. The  Michigan State University Enviroweather station at Sparta has accumulated 706 degree days base 42 and 333 degree days base 50 – these totals are both three days behind normal averages from Jan. 1 through June 1.

Crop update

Apples moved through bloom very quickly last week with very warm days and nights pushing development. Most apples are now at the 6- to 8-millimeter stage with a few early developing varieties closer to 10 millimeters and some late varieties with a little bloom still hanging on.

Tree fruit diseases

Light rain events have continued to discharge apple scab spores and numbers are declining rapidly. All apple sites in the general Grand Rapids, Michigan, area should now have all overwintering ascospores mature and ready to discharge. It will likely take one to three more rain events to discharge all primary apple scab spores and primary apple scab fungicide rates need to continue to be maintained for at least another week. Apple scab lesions from early season infections can be found in managed blocks, so be sure you are scouting very carefully in the next two weeks for any primary scab that might have snuck through before reducing to summer fungicide rates. There were some very challenging heavy rain events over several days that were hard to manage for and I wouldn’t’ be surprised to find scab this season.

Powdery mildew infections of new growing shoots can be found easily in unmanaged apple trees and are now starting to appear in managed blocks.

With bloom now over, the risk for fire blight of blossoms has decreased significantly. Some cultivars with ragtag bloom on 1-year-old wood could continue to be a possible blossom blight infection scenario. Newly planted trees that might be behind in development will also have later bloom that might be at risk for fire blight infection, although trees that went in the ground very, very early are more in sync this year than usual. Shoot blight is not very often common in Michigan conditions, but in the past few springs, the extreme warm temperatures in May and early June have changed that.

Shoot blight is something we need to watch for as long as tender tissue is present and if warm night temperatures and high relative humidity come into play. Of course, trauma blight is also something to keep in mind if storms develop with high winds and/or hail. Scout carefully for any signs of early blossom blight—it will likely begin to show up later this week.

Nectria twig blight is also beginning to show up and can easily be confused at first glance for fire blight. Be sure to identify correctly.

Tree fruit insects

Insect activity has very much increased with the warmer weather. Several insect biofix dates are being set, which are key to future management timings.

Plum curculio activity has been quite high in all tree fruits with the warmer weather that moved in a week or so ago. Do not delay plum curculio covers in apples and keep good material on for at least 10 more days. Plum curculio activity should quickly come to an end this year—they will quickly lay all the eggs they have on the very warm nights last week and in the week to come. Degree day models indicate activity is about 25% done and 10 to 14 more days should get us to 100%.

Warmer nighttime temperatures really flushed codling moth adults, which are now being trapped in all apple blocks—egglaying should be starting. A regional biofix was set for May 25 (233 DD50) with 100 degree days base 50 since that date. Egg targeting materials need to go on right now to have the most effect. Traditional egg hatch/small larvae timing materials should be timed to early next week or 3% egg hatch predicted for June 9 or 10 in blocks with higher pressure and those over threshold of five accumulated moths/trap. For low pressure blocks, use 350 degree days base 50 post-biofix for first spray, estimated to be June 16.

Various species of aphids all tree fruits can now be found. Rosy apple aphids are curling leaves in non-managed trees. Expect beneficial insects to catch up soon. Warmer weather and rapidly expanding shoot growth will likely be favorable for aphids to get established this year.

Small to medium sized obliquebanded leafroller larvae are present. Monitor known problem blocks closely for larval activity. There are a few small redbanded leafroller larvae present as well. Expect pupation of large obliquebanded leafroller larvae to begin soon. Traps should be placed this week for summer adults expected in mid-June.

San Jose scale adult male flight is beginning. A regional biofix was set for June 1 (302 DD51) with 8 degree days base 51 since biofix. Target sprays are only needed in blocks with known hot spots. The next timing is when crawlers appear in mid- to late June.

Oriental fruit moth adults continue to fly. Egg hatch is underway and cover sprays are needed in stone fruits to prevent shoot flagging. A regional biofix was set for May 22 (323 DD45) and 206 degree days base 45 have been now accumulated. Adults catches in traps will increase. Be sure trap liners stay clean. Peak egg hatch is estimated for June 10 or 11 in the general Grand Rapids area. Keep cover sprays on stone fruits to prevent shoot infestation and fruit injury once fruits are out of the shuck.

European red mite egg hatch is well underway in the Grand Rapids area. The warmer weather last week was perfect for mite development and immatures can be found in blocks with an European red mite history. Expect new adults to show up quickly this year with the heat. Petal fall miticides will be important for 2020.

White apple leafhopper hatch continues and nymphs are present in very low numbers. A few adult potato leafhoppers can be found. Continue to monitor, particularly in non-bearing trees where pesticide covers are minimal.

Some adult black stem borer activity began in the Grand Rapids area in the last week with warmer temperatures as expected. They seem to attack trees that are compromised with some other injury. Continue to monitor.

Dogwood borer could begin flying at any time. Traps with lures need to be in place as soon as possible. Lesser peach tree borers have started to fly. American plum borer and greater peach tree borer should start flying soon. Borer sprays are usually timed for peak adult flight. For greater peach tree bore and lesser peach tree bore, it is usually late June in stone fruits. For dogwood borer, it is usually around July 4 in apples.

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