Grand Rapids area tree fruit update – June 23, 2020

Average degree day totals in a year that is anything but average.

Apple mosaic virus
Apple mosaic virus. Photo by Phil Schwallier, MSU Extension.

Weather update

What a crazy year 2020 has been so far and for many reasons. Growing degree day (GDD) totals for the general Grand Rapids, Michigan, area are at normal averages in a spring that was anything but normal. Hot and humid one week and cool and dry the next line 2020 up as a year of extremes. The Michigan State University Enviroweather station at Sparta has accumulated 1,236 degree days base 42 and 721 degree days base 50. This is spot on with normal for base 42 and one day ahead of normal for base 50.

Soil moisture levels have declined quickly as expected and growers with irrigation should be running it now to stay ahead of deficits. We are post June drop in apples, but trees do not need additional stress or they will continue to drop fruitlets. You can track irrigation needs on MSU Enviroweather, which is linked to the Cornell NEWA irrigation model.

Crop update

Apples continue to grow quickly with the warm temperatures and most fall in the 18 to 25 millimeter stage depending on variety and site. There continues to be variability in fruit set— some lighter than expected and some heavier. Again, depending on variety and site. Sweet cherries have a light fruit set in general and some with U-pick are not offering it this year. Peaches have a very nice crop overall with some heavier or lighter depending cultivar and site.

Tree fruit diseases

Primary apple scab is over for 2020, but do keep a sharp eye for any lesions yet to show up from the last wetting periods. It was certainly a challenge to stay ahead of apple scab this spring with several long wetting events with very heavy rainfall totals. Tiny apple fruits are very susceptible to fruit scab at this time and all it takes is one or two lesions to provide secondary spores for further infections.

At this time, if fire blight has infected your apple trees it should be very easy to see. All blossom blight and all potential trauma blight has had enough time to present the classic flagging of shoots. Check blocks of lesser susceptible varieties (Red Delicious, etc.), especially if no blight management was applied. The risk was so high during bloom that even these low risk varieties might end up with blight this year.

If you have a fire blight situation, all potential trauma events can spread it further. Cover sprays ahead or immediately following hailstorms or high wind situations are called for until we get to terminal bud set. Eliminate fertilizer applications in blocks with active blight. Continue with your planned prohexadione-calcium (Apogee/Kudos) applications to reduce spread of blight.

Summer diseases—sooty blotch and flyspeck—need some more wetting hours before fungicides are needed to prevent signs of these slow growing fungal pathogens. Since it has been on the dry side since hours start adding up two weeks after petal fall, we need quite a bit more wetting to get possible disease expression. You can check out the model at MSU Enviroweather for information from the station closest to you.

Silver leaf is starting to be expressed in apples where infections are present. This is a slow growing fungal disease with no known cure. Remove infected trees as this fungus can spread via root grafts and through spores to weakened trees nearby.

Various types of virus symptoms are also showing up in apples, the most common to see is apple mosaic virus. There is no cure for plants infected with virus and tree removal is recommended. These viruses are not readily spread from tree to tree in a block. They are more likely to have been a result of dirty propagation techniques. Apple trees can live infested with virus, but they will have a reduced yield.

Tree fruit insects

Codling moth adult flight continues with some blocks reporting quite high numbers in the past week, which is a bit unusual. A regional biofix was set for May 25 with 472 degree days base 50 since then. This indicates we are nearing peak egg hatch from that biofix date. Cover sprays for codling moth in blocks over threshold in traps need to be maintained adequately over the next several weeks to prevent fruit stings. Managing first generation of codling moth well can greatly reduce the pressure from second generation.

Various species of aphids can now be found in all tree fruits including rosy apple aphids, green apple aphids and woolly apple aphid in apples, black cherry aphids in sweet cherry and green peach aphids in peaches. Warmer weather and rapidly expanding shoot growth are favorable for aphids. Beneficial insects such as ladybird beetle larvae, lacewing larvae and syrphid fly larvae are now easy to find in aphid populations.

Obliquebanded leafroller adults are flying, and numbers are on increasing as expected. A reginal biofix was set for June 16 (1,065 GDD42) for the general Grand Rapids area. We have accumulated 196 degree days base 42 since that biofix indicating we are nearing peak moth flight. Early egg hatch is forecasted for the first week of July and treatments need to begin at that time in blocks with a history of high obliquebanded leafroller numbers in recent years. In blocks with low pressure, scout for small larvae and delay applications until they are found. The degree day model calls for sprays about a week or 10 days earlier than one could visually see the tiny larvae, but this is key to staying ahead of high pressure populations.

San Jose scale adult male flight continues but appears to be declining rapidly. A regional biofix was set for June 1 (302 GDD51) with 352 degree days base 51 since biofix. Crawlers are expected to begin emerging any day now; the degree day model estimates June 24 or 25. Systemic management sprays should be timed for first crawler emergence to give time for uptake by plant tissues. Contact insecticide for San Jose scale are timed for peak crawler emergence, which is predicted for July 6 or 7.

Oriental fruit moth adults should begin to increase in traps this week. This is a good time to swap a new lure to be sure trap counts are adequately relating the adult activity. First generation egg hatch should be completed for 2020. Very little shoot flagging is being reported in stone fruits so far this season. A regional biofix was set for May 22 (323 GDD45) with 679 degree days base 45 since the biofix. The degree day model predicts early egg hatch for the second generation to be around July 11 or 12 when cover sprays will again be necessary.

All stages of European red mite can now be found in apples. Twospotted spider mites are also being found. We have reached the time when petal fall miticides start to break. Populations could grow quickly with the heat and lack of rainfall. Good scouting will be important this year to stay ahead of mite activity.

All stages of white apple leafhopper are present in very low numbers. Potato leafhopper are also present, mostly as adults but some of the nymphs I see could be potato leafhopper as well. Continue to monitor, particularly in non-bearing trees where pesticide covers are minimal.

Dogwood borer adults are flying. 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 borers and American plum borer, it is usually late June in stone fruits. For dogwood borer, it is usually around July 4 in apples.

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