Grand Rapids area tree fruit update – June 16, 2020

June drop in apples is winding down. Fruit set looks good.

Apple scab on fruit
Apple scab on fruit from unsprayed trees. Photo by Amy Irish-Brown, MSU Extension.

Weather update

The rollercoaster temperatures continue for 2020. With cooler weather in place over the past several days, growing degree day (GDD) totals fell back slightly and now are about one day behind normal averages. Much warmer than normal temperatures this week will push us forward to ahead of normal over the next several days. The Michigan State University Enviroweather station at Sparta has accumulated 1,009 degree days base 42 and 559 degree days base 50.

Heavy rainfall last week diminished the soil water deficit that we had, but soils are quickly drying out again and with very warm weather and very little rainfall in the forecast, it is likely you will need to start irrigation systems this week to prevent stress to trees and the new crop. 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 14 to 20 millimeter stage, depending on variety and site. June drop is nearly over, resulting in a very nice fruit load for apples for the most part. Some growers who left check trees report similar crop load in non-thinned trees as thinned trees. 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.

Peaches have a very nice crop overall with some heavier or lighter depending cultivar and site.

Tree fruit diseases

On June 9, the remnants of Tropical Storm Cristobal blasted through the area bringing heavy rainfall to many areas. I caught seven apple scab spores per rod with this 2 inches of rain. There was another wetting event on June 11 and no additional primary scab spore were caught. I called primary apple scab over with that moderate to heavy infection on June 9 for all Enviroweather stations in the general Grand Rapids area. Locations in the west central Michigan counties are most likely done with primary scab as well, except for the Ludington area, which might need one more rain that far north to discharge remaining spores.

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. Unmanaged apple trees have full-on sheet scab at this time, indicating just how severe the primary scab season was for 2020.

Be on the lookout for fire blight that might have infected trees that were wounded by the high winds and, in some locations, hail from the storms on June 9. For the most part, blocks were clean of blossom blight, which lessens the bacterial load for a trauma situation but very strong winds and hail can still allow bacteria on the surface a way in to cause a trauma blight infection. If infection did occur, you can expect symptoms to begin to show up Thursday or Friday, June 18 or 19. Be prepared to cover for any trauma blight situations over the next several weeks or until terminal bud set.

Continue to apply your planned prohexadione-calcium (Apogee/Kudos). This is not the year to skimp on this very important fire blight management tool. In fact, you might want to increase your rates by a few ounces in the next application. There are a few reports of fire blight in apple varieties not typically very susceptible to it (Red Delicious and Northern Spy). This tells the story of how high the pressure was this year for blossom blight. The non-susceptible varieties do not get the antibiotic sprays, but the pressure was so high, they end up with some blight. I have only seen this happen a few times in my nearly 30 years with MSU Extension.

Tree fruit insects

Adult egglaying for plum curculio should be completed for the 2020 growing season. This pest really caused some fast and furious egglaying damage in some blocks and particularly along borders of woodlots. Plum curculio took advantage of the very warm nights right at the end of bloom to get a lot of work done.

Codling moth adults continue to fly and are likely at or just past their peak flight. A regional biofix was set for May 25 with 326 degree days base 50 since then. This indicates that about 20% of eggs are hatched with peak egg hatch about a week away. 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, and black cherry aphids in sweet cherries and green peach aphids in peaches. A few beneficials are being found feeding on aphid populations. Warmer weather and rapidly expanding shoot growth are favorable for aphids.

Obliquebanded leafroller larvae can still be found but many are now pupating, and management sprays will not be effective in this resting stage. Summer adults are beginning to fly sporadically and likely a regional biofix will be set this week to help with timing of the summer larvae management in high pressure blocks.

San Jose scale adult male flight continues. A regional biofix was set for June 1 (302 GDD51) with 213 degree days base 51 since biofix. Crawlers are expected to begin emerging in about a week; the degree day model estimates June 25 or 26. 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 a few weeks from now.

Oriental fruit moth adults are down this week again as first generation comes to an end. Egg hatch is also nearing an end for first generation. 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 497 degree days base 45 since the biofix. This is a good time to change lures for second generation adults which will begin to fly soon.

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 from Tropical Storm Cristobal that passed by a week ago.

Dogwood borer is just beginning to fly this week. Lesser peach tree borers and greater peach tree borer are flying. American plum borer is also flying. Borer sprays are usually timed for peak adult flight. For greater peach tree borer, lesser peach tree borer 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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