Grand Rapids area tree fruit update – June 9, 2020

Degree day totals are caught up to normal averages.

Apple scab lesions
Careful scouting for apple scab lesions is very important this week before reducing to summer fungicide rates. Photo by Amy Irish-Brown, MSU Extension.

Weather update

Warmer than normal temperatures continue to push growing degree day (GDD) totals forward. Totals from Jan. 1 are now at levels even with normal averages through June 8, which is quite amazing given we were 10 to 14 days behind less than a month ago. The Michigan State University Enviroweather station at Sparta has accumulated 866 degree days base 42 and 467 degree days base 50.

Crop update

Apples continue to grow quickly with the warm temperatures and most fall in the 8 to 15 millimeter stage. June drop is just beginning. There is a great deal of variability in fruit set at this time – some lighter than expected and some heavier. A bit of russet and frost marks can also be found in some blocks and various varieties. The apple crop potential is still very good for the Grand Rapids, Michigan, area.

Sweet cherries have a light fruit set, and some appear to be turning yellow and are expected to fall off. Peaches have a very nice crop with some cultivars heavier or lighter depending on bloom stage and site. It is still a bit early to make any crop load predictions for any tree fruits.

Tree fruit diseases

In the last rain event, apple scab spores numbers were very low (18). Keep in mind this rain event was during the night hours when less spores are naturally discharged. All Enviroweather station monitoring sites around the general Grand Rapids area are now over the 100% mature spore mark and all that is needed is a good rain or perhaps two to discharge the remaining spores. The rain precited for Tuesday night into Wednesday, June 10, could be heavy and a primary scab rate should be maintained at least through this rain.

Apple scab lesions from early season infections can be found in managed blocks. Before you reduce to summer fungicide rates, be sure you are scouting very carefully in the next week or so for any primary scab that might have snuck through. There were some very challenging heavy rain events this spring that were hard to manage for and I would not be surprised to find scab this season.

Powdery mildew infections of new growing shoots can be found in varieties highly susceptible to this disease. Cool conditions from tight cluster to pink were not conducive to mildew, but the heat and humidity during bloom and in recent days was and I can see it getting out of hand in some blocks.

Be on the lookout for fire blight that might have infected blossoms and is now starting to flag shoots. The pressure for blossom blight was extreme this season and now continues for possible trauma blight with the remnants of Tropical Storm Cristobal moving through the state. If you have any active fire blight in orchards, remove it if possible when leaves and trees are dry. Be prepared to cover for any trauma blight situations over the next several weeks or until terminal bud set. Continue to apply prohexadione-calcium (Apogee/Kudos) – this is not the year to skimp on this fire blight management tool. In fact, you might want to increase your rates by a few ounces in the next application.

Tree fruit insects

Adult egglaying for plum curculio seems to be declining and should quickly come to an end this year. Degree day models indicate activity is about 50% done and three to five more days should get us to 100%.

Warmer night temperatures really flushed codling moth adults, which are now being trapped in all apple blocks – egglaying is underway and early egg hatch should be starting, indicating cover sprays are needed now in blocks over threshold. A regional biofix was set for May 25 with 234 degree days base 50 since the biofix.

Various species of aphids in all tree fruits can now be found. Rosy apple aphids are curling apple leaves in some blocks. A few beneficials can now be found. Warmer weather and rapidly expanding shoot growth will likely be favorable for aphids to get established. Woolly apple aphid might be found soon in blocks with high numbers in fall 2019.

Various sizes of obliquebanded leafroller larvae are being found but most are getting large. Expect pupation of large larvae to begin any day if not already. Traps should be in place for the first flight of summer adults expected in the next week.

San Jose scale adult male flight is still in early stages. A regional biofix was set for June 1 (302 GDD51) with 126 degree days base 51 since biofix. Target sprays are only needed in blocks with known hot spots – the next timing is when crawlers emerge – estimated to be June 25 or 26.

Oriental fruit moth adults are declining in traps as first generation comes to an end. Egg hatch is near peak and cover sprays are needed in stone fruits to prevent shoot flagging. A regional biofix was set for May 22, 2020 (323 GDD45) with 374 degree days base 45 since the biofix. This is a good time to change lures for second generation adults. End of first generation egg hatch is predicted for June 20-22.

All stages of European red mite can now be found in apples with no miticides applied. Populations could grow quickly with the heat – good scouting will be important this year to stay ahead of mite activity.

Nymphs of various sized white apple leafhopper are present in very low numbers. A few adult Potato Leafhopper adults can be found. Continue to monitor, particularly in non-bearing trees where pesticide covers are minimal. Watch for potato leafhopper adults to arrive with Tropical Storm Cristobal remnants passing by this week.

Dogwood borer could begin flying at any time. Lesser peachtree borers and greater peachtree borer are flying. American plum borer is also flying. Borer sprays are usually timed for peak adult flight. For greater peachtree borer, lesser peachtree borer and American plum borer, it is usually late June. For dogwood borer, it is usually around July 4.

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