MSU Extension

Fruit development continues and all tree fruits are beginning to size with many apples now in the 15 to 20 millimeters stage. Some June drop has started with more expected in the coming week or two. Growers have been cautiously thinning apples.

Disease update

Pressure from diseases in tree fruits has been minimal due to lack of rainfall. All primary apple scab ascospores should be mature and nearly all have been discharged. Rainfall amounts have been very light in the past two weeks, which might not have been adequate to release the last of the primary spores. I caught 33 spores per rod with the rain in the morning hours of Sunday, June 4, so primary scab is still on.

All Michigan State University Enviroweather stations had only four or five hours of wetting, so this was not an infection period. It was a pretty nice rain between 0.2 and 0.3 inch, but all in the pre-dawn hours, so perhaps all mature spores were not discharged. Primary scab is not yet over. I won’t know until we have one more nice daytime rain and that might be awhile as this week looks to be clear and dry.

It’s becoming easier to find scab on the fifth, sixth and seventh leaves on growing shoots. These would be from the long infection period that began on April 29 and lasted for 60 to 70 hours of wetting, depending on Enviroweather station location. Before you consider reducing fungicide rates, be sure you are free from primary scab.

There is a little bloom present in newly planted trees that are behind in development compared to other established apple trees. This bloom was subject to a fire blight infection risk with the rain on June 4. We seem to be in for a stretch of dry and a bit cooler weather, which lessens the potential fire blight risk unless trauma situations occur.

According to the MSU Enviroweather MaryBlyt prediction model, there were blossom blight infections with rain events May 15 – 18. Symptoms from these infections started showing up about a week ago, so be on the lookout for strikes in your blocks.

It does look like we are in for a bit of a dry stretch, which makes it safe to feather young trees and other orchard tasks with little risk for trauma blight infections. If you have blocks with active blight, remove the strikes as you see them but stay out of those blocks for pruning or any other activity that potentially injures trees unnecessarily.

Occasionally, some powdery mildew can be found in apple susceptible apple cultivars but, overall, mildew infections are low so far.

Bacterial canker in sweet cherry and plums is present as expected from the cold snap in early May. The best tactic is to boost the health of infected trees so they can wall off the invasion.

Insect update

Insect activity has moved forward a bit with the warmer weather last week. Plum curculio adult activity is still ongoing and some new egglaying evidence can be found in the last few days. Activity in apples could still be of concern until fruits get to 25 millimeters. Plum curculio could remain active in stone fruits for another 10 days.

Trap numbers continue in varying levels for codling moth and we should be near 50 percent emergence for first generation moths in the Grand Rapids, Michigan, area. First larvae will be hatching now as well and cover sprays are important in blocks over the threshold of an accumulation of 5 moths per trap.

A regional biofix was set for May 15 (250 GDD50) with 235 GDD50 accumulated since the biofix. Blocks with very low pressure and below threshold could target 350 GDD50 for a first cover spray to prevent stings, estimated for June 12 or so.

Obliquebanded leafroller are mostly in the pupal stage. A few stray moths have started to emerge. A regional biofix has not been set and no management is needed in this stage. Traps should be in place to accurately monitor and set a biofix.

Male San Jose scale flight continues and numbers have declined as expected. A regional biofix was set for May 21 (318 GDD50), 175 GDD50 since the biofix. Yellowish crawlers generally are present 300-350 GDD base 50 after the first adult catch of either generation. Crawler emergence is estimated for June 13 to 17 for the Ridge. Cover sprays will be needed in blocks where San Jose scale was an issue on fruit in 2016.

First generation flight oriental fruit moth is declining. Egg hatch should just past peak, and cover sprays are still crucial. A regional biofix was set for April 28 (307 GDD45) with 470 GDD45 since the biofix. Continue to monitor traps and maintain cover sprays as rainfall warrants. Apples with more than 70 moths per trap per week also need cover sprays. Trap numbers are expected to increase in 10 to 14 days for the start of second generation and lures should be changed out soon.

All stages of European red mite can now be found. Predator mites are also present. MSU Extension advises monitoring for European red mite and beneficial mite populations. The threshold for June is 2.5 mites per leaf.

Various species of aphids can be found in all tree fruits, but overall numbers seem to be low this season. Continue to monitor for all aphid species in all tree fruits as well as the beneficials that attack them. Woolly apple aphids can be found in known hot spots at this time.

White apple leafhopper nymphs are present in low numbers. We should be at peak hatch, but they seem to be hard to find overall. Watch non-bearing trees for high populations of leafhopper and aphids and manage where necessary to maximize shoot growth.

Spotted tentiform leafminer tissue feeders can be found in low numbers in unsprayed trees. They are a minor pest in cared for blocks.

Various borer insects are active in commercial orchards. The American plum borer flight continues and should nearly be over. Both species of peachtree borer flight is beginning. There is no dogwood borer flight in apples.

Be on the lookout for black stem borer in sick trees or top-worked trees. Monitor with traps for adult activity. Trunk sprays in stone fruits should be applied right now. Dogwood borer trunk sprays should wait until the first week in July and timed for peak adult flight.

Rose chafer are starting to be reported in blocks with sandy soils. Monitor for their presence, especially in peaches.

Traps with lures for spotted wing Drosophila need to be in place in susceptible crops like sweet and tart cherry, strawberry, raspberries, etc. Flight is becoming more regular and is earlier than ever for Michigan. Spotted wing Drosophila is not a pest of apple.

Overwintering adults of brown marmorated stink bug have been found more readily in 2017. Newly laid eggs are hatching with small nymphs present, but they are very difficult to find. They are not attracted to tree fruits as a food source. Management in tree fruits will target adults in summer when fruits are near ripening and become attractive as a food source. Consider trapping for them in blocks of tree fruits where you suspected damage in 2016.