MSU Extension

Weather and growth stages

Much warmer than normal weather continues and all tree fruits are sizing rapidly in response. Apples range from 8 to 22 millimeters, depending on variety and site. Peaches are out of the shuck and growing quickly – the overall peach crop appears to be a nice set. Sweet cherries are out of the shuck and also sizing quickly – some sites have a very nice crop and others are a bit light due to spring frosts.

Growing degree-day (GDD) totals for the Michigan State University Sparta Enviro-weather station are 1,622 GDD base 32, 844 GDD base 42, 675 GDD base 45 and 449 GDD base 50. On average, degree-day totals put this area seven days ahead of normal average accumulations.

Diseases

After a week or more of dry conditions, some rain moved back in late last week and with it, the risk for apple scab infections again. We continue to catch spores, but numbers are dwindling. Primary scab is still a threat and full fungicide covers need to be continued.

Most apples in the Grand Rapids, Michigan, area were past bloom with the recent rains and fire blight blossom blight was not a risk. For production areas farther north and along the lakeshore with open, viable bloom, the risk was very high last week for blossom blight and cover sprays were needed to prevent infection. Some initial blossom blight began showing up last week – this was from the rains around May 12. Symptoms from the most recent fire blight risk won’t show up for another week.

Insects

Plum curculio adult egglaying activity is being reported in all tree fruits over the last week with warmer weather. MSU Extension advises to continue monitoring and reapplying cover sprays as rainfall warrants.

Codling moth adult flight is now being reported across all blocks – very low numbers in some with light pressure and very high numbers in high pressure blocks. A Grand Rapids regional biofix was set for May 23 (302 GDD50) and 164 GDD50 have accumulated since that date – degree-days have accumulated very quickly with the very warm day and nighttime temperatures in the past week. Degree-days will continue to accumulate quickly in the Enviro-weather codling moth model, and early egg hatch will begin by June 5 in the general Grand Rapids area. The regional biofix is for blocks with normal to slightly high pressure to be sure coverage is adequate for worse case scenarios. Low pressure blocks might have a slightly later biofix and delayed first cover timings.

All stages of European red mite are now found in commercial apple blocks. The warmer weather really got their development moving and they need to be monitored closely for potential exploding populations.

More obliquebanded leafroller larvae are being found week to week feeding in terminals, and numbers continue to be very low in general. There were initial reports of pupation late last week, so traps need to be up to catch first adults, which could begin flying in a week or two.

San Jose scale males began their flight in earnest over the past few days – trap numbers are very high. Monitor known hot spots closely for activity. Crawlers are expected to emerge in seven to 14 days – begin monitoring for them in a week with black tape near infestations.

Oriental fruit moth first generation flight continues and should be declining as adult flight ends. Egg hatch should be well underway as we approach peak hatch (50 percent) in a day or two. Cover sprays in stone fruits need to be maintained at this time. A regional biofix was set for May 6 (308 GDD45) and 390 GDD45 have been added. Continue to monitor traps and maintain cover sprays as rainfall warrants. This is a key time for good coverage. Trap numbers should begin to decline steadily over the next two weeks as first generations winds down.

The following are minor apple insect pests to keep an eye on.

Apple grain aphids continue being reported in very low numbers and many winged adults can be found, indicating they will soon move to alternate food sources. Rosy apple aphids are being found in managed orchards as well as unsprayed trees. A few green peach aphids can now be found and black sweet cherry aphids have also made an appearance. Scouts should also note beneficial insects that might be present and feeding on aphid populations.

Initial reports of newly hatched white apple leafhopper nymphs started last week and more can be found now. Numbers continue to be low overall. Peak egg hatch is expected around the Grand Rapids area in seven to 10 days. Watch non-bearing trees for high populations of leafhoppers and aphids and manage where necessary to maximize shoot growth. Any adult leafhoppers present are likely potato leafhoppers that tracked here with the southern-based storms of last week.

American plum borer flight is declining in stone fruits. Lesser peachtree borers and greater peachtree borer flight is just beginning. No dogwood borer activity (frass or pupal cases in burr knots) has been found, but should begin anytime.

Apple thinning considerations

The Carbohydrate Model indicates there is likely to be less stress on apple trees this week as moderate temperatures and sunny weather are forecasted. Also, trees have a lot of leaves now that can produce carbohydrates, therefore reducing any shortage. Thinning will be less effective overall. Use standard to higher rates to get the job done.