MSU Extension Apples

Apple growth seemed to happen very quickly this season and it’s setting up to be a nice crop so far. Growing degree-days have moved from a week or so behind normal to now a few days ahead of normal. For the Sparta Enviroweather station, degree-days are as follows up until June 11, 2018: 1,042 GDD42, 870 GDD45 and 620 GDD50. Many apples are in the 20 to 25 millimeter stage, depending on variety.

If you still want to try to get chemical apple thinning done, you need to really get aggressive and even then that might not work well as fruits are getting very much past the stage where chemical thinners will have any activity.

Tree fruit diseases

Primary apple scab is still on for the general Grand Rapids, Michigan, area. We reached the 1,300 GDD32 mark over the weekend and now we need a daytime rain or two to get all final spores discharged. With the recent rain, I caught 36 spores per rod. There were some light showers that moved through some areas, but very light rains (0.01 inch) that lasted only an hour did not release any additional spores, and it looks like the monitoring equipment did not even run.

Regarding fire blight, we are past nearly all bloom being over—some varieties still have a little ragtag bloom present. Very little fire blight strikes are being seen in the Grand Rapids area. Keep an eye on the weather for any storm fronts that might bring hail or high winds. Trees are still very much susceptible to trauma blight until we get to terminal bud set in July.

Tree fruit insects

Plum curculio adult activity has declined and apples are getting too large for further egglaying activity in the Grand Rapids area. Continue to monitor along woodlots for recent damage.

Codling moth adult trap numbers have leveled out. Egg hatch should be well underway. A regional biofix was set for May 25 (313 GDD50) and 328 GDD50 have been accumulated since that date. We should be nearing 20 percent egg hatch and cover sprays are critical in blocks over threshold of five moths accumulated per trap for first generation.

Obliquebanded leafroller large larvae and pupae are present in orchards. Numbers seem higher this year than in recent years. A few adults showed up in traps over the last weekend. A regional biofix was set for June 11 (1,042 GDD42) with 30 GDD42 accumulated since then. Peak adult flight is about a week away. First egg hatch is expected in the last week of June, around June 26. Use the Enviroweather obliquebanded leafroller model to high pressure blocks. Low pressure can scout for larvae and spray.

Very few aphids are present overall in Grand Rapids orchards. Continue to scout for all aphid species and be on the lookout for predators.

San Jose scale male flight has declined greatly in the last week. A regional biofix was set for May 25 (286 GDD51) with 310 GDD51 since then. Crawlers are estimated to emerge at 405 GDD51 from first flight or around June 16 or 17 for the Ridge.

All stage of European red mite are now present. Continue to monitor. The threshold for treatment of European red mite is 2.5 mites per leaf. The presence of predators should be taken into account.

Numbers are fairly low for white apple leafhopper. If you find adult leafhoppers, they are potato leafhoppers. Peak egg hatch typically occurs around first cover.

A few spotted tentiform leafminer mines are being reported, but spotted tentiform leafminer is a very random pest these days. Continue to monitor.

First generation oriental fruit moth adult flight is over and second generation flight should begin soon. Egg hatch should also be ending. A regional biofix was set for May 10 (272 GDD45) with 624 GDD45 accumulated since then. Second generation flight should step up in a week or so—this is a good time to change lures. Expect new larvae again in late June.

American plum borer, lesser peachtree borer and peachtree borer are all flying. No dogwood borer flight yet. Trunk sprays in stone fruits can begin at any time. Look for dogwood borer frass and pupal cases in burr knots. Mating disruption for dogwood borer needs to go up right away.

Black stem borer trap numbers seemed to have declined. Evidence of adult activity can be found in injured trees. Monitor blocks for burrowing damage.