MSU Extension

Weather and growth stages

Finally – some very warm weather. It’s hard to believe just 10 days ago it was snowing during full bloom of apples and now temperatures are in the 80s. Apples are sizing quickly now with warm days and warm nights and fruit set appears to be normal to heavy in some sites. Peaches are in shuck split. Sweet cherries are mostly at petal fall, but some late sites still have a bit of bloom.

Growing degree-day (GDD) totals for the Michigan State University Sparta Enviro-weather station are 1,349 GDD base 32, 641 GDD base 42, 493 GDD base 45 and 302 GDD base 50. On average, degree-day totals put this area two to four days ahead of normal average accumulations.

Diseases

A nice stretch of dry weather has lessened the ongoing disease pressure in general, but wetter weather for later in the week will bring more potential for apple scab infections as we are still in primary scab with a few more spores to be released yet. Do not reduce your apple scab fungicide rates yet.

For any apple varieties with open, viable bloom, fire blight blossom blight will need to be covered for upcoming rain events – the MaryBlyt model indicates very high risk for infections if bloom is present with rain in the next four or five days. Storms that bring high winds or hail will need to be watched for the possibility of trauma blight as well.

Insects

Warm weather makes humans happy and it makes insects develop – both have been true in the past few days. Many apple growers have their rented hives removed and petal fall applications are being made. The following are key apple insects to watch closely.

Codling moth adult flight has been reported regularly and steadily with the warmer nighttime temperatures. A Grand Rapids, Michigan, regional biofix was set for May 23 (302 GDD50) and 20 GDD50 have accumulated since that date. Monitor traps twice per week until a sustained flight is set in your blocks. The regional biofix is for blocks with normal to slightly high pressure, so be sure coverage is adequate for worse case scenarios. With very warm temperatures, degree-days will accumulate quickly in the Enviro-weather codling moth model and egglaying will begin by May 26 or 27, which is the timing for insect growth regulator materials (Rimon). Early egg hatch could begin quickly with warm weather, which is estimated for June 4-6. Earlier spray timings would be encouraged for high pressure blocks. Low pressure blocks will most likely have later spray timings.

Plum curculio initial activity is now being reported in known hot spots and warmer sites. Petal fall applications should include materials for plum curculio over the next few days. MSU Extension advises to start looking for their damage in near woodlot edges in early-setting fruits like plums and sweet cherries.

All stages of European red mite can now be found in commercial apple blocks. Petal fall is a great time to target mites. If you are using a miticide that requires the addition of oil, keep in mind the oil can enhance any thinners that get added by 10 percent or so.

More obliquebanded leafroller larvae are being found week to week feeding in terminals. Numbers are very low in general. Continue monitoring known problem blocks closely for larval activity. Adult flight is most likely a couple of weeks away. This warm weather is very nice for B.t. to work very well for all leafroller species.

Oriental fruit moth first generation flight continues and should be reaching a peak. Early egg hatch (8-10 percent) is now occurring and cover sprays in stone fruits are needed to prevent flagging of new shoots. A regional biofix was set for May 6 (308 GDD45). Since that date, there has been an accumulation of 200 GDD45 using Sparta Enviro-weather station data. Oriental fruit moth mating disruption needs to be up now in stone fruits to be most effective. Peak egg hatch is estimated to occur around Memorial Day, May 30.

Overwintering San Jose scale are present with no activity being reported. Traps for adult males should be in place and they should begin to fly any day now. It’s best to monitor known hot spots for male activity – they are very small and poor flyers. Males typically fly during bloom and crawlers emerge 10 to 14 days after.

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

Adult activity of black stem borer has increased a bit with warmer temperatures. Continue monitoring sites with suspected activity.

Apple rust mite are present now as adult females. Monitoring is needed in blocks with a history of apple rust mite issues; Red Delicious is a favorite variety for rust mites.

Apple grain aphids continue being reported in very low numbers. Rosy apple aphids continue being found with some leaf curling found in unsprayed trees – none found yet in managed blocks. A few green peach aphids can now be found and black sweet cherry aphids should make an appearance at any time. Scouts should also be taking note of beneficial insects that might be present and feeding on aphid populations.

Initial reports of newly hatched white apple leafhopper nymphs are starting to come in. They overwinter as eggs and typically egg hatch begins at petal fall and peaks at the first cover timing. The earliest nymphs are often found on the underside of older leaves. Potato leafhoppers are likely to show up as adults in the southern-based storms heading our way later in the week.

A few spotted tentiform leafminer sap feeders were found in unsprayed apples. This is a minor pest in general.

Borer pests to monitor include American plum borer, now flying in low numbers. Lesser peachtree borers and greater peachtree borers should begin flying at any time. No dogwood borer frass or pupal cases in burr knots have been seen. Mating disruption for all borer species can start being placed now.

Apple thinning considerations

The Carbohydrate Model indicates there could be moderate to high stress as we move into the rainy and cloudy weather predicted for the latter half of the week. This should lead to a good thinner response. Many varieties appear to be setting up well and normal thinner rates are appropriate. If trees are not setting heavily, consider mild to normal rates and be a bit cautious. Every year, nearly all commercial apple blocks need some fruit thinning. Chemical thinning is the most important application a grower makes – for this year’s crop and next year’s.