Grand Rapids area tree fruit update – May 28, 2019

Apple bloom is coming to an end.

Weather and crop development

The warmer weather over the past weekend moved tree development forward, but not by leaps and bounds—just normal development. Many apple varieties have moved into the petal fall stage, but there is still bloom present in some cultivars. Peaches are still in the shuck, as are tart cherry and sweet cherry.

Degree day totals for the general west Michigan area are about five or six days behind normal averages through May 27. From Jan. 1 through May 20, the Michigan State University Sparta Enviroweather station has accumulated 563 degree days base 42 and 264 degree days base 50.

Tree fruit diseases

There have been more days in May with some amount of rainfall than without, but overall the total rainfall amounts are in the normal, average range. We have had many apple scab wetting events, but very few that have been wet long enough to become infections. I continue to catch primary apple scab spores, but numbers are starting to decline as expected. We should reach 100% mature spores in the next week or 10 days and then will need a rain or two to completely discharge them all. Some scab lesions from the April 29 infection can now be found in managed blocks (about a week after they were found in unsprayed trees). I can also see secondary spores (conidia) on my spore monitoring rods.

Powdery mildew infested terminals have been found in unsprayed apples. Continue to monitor for this pathogen. Warmer weather will favor it and new tender leaf tissue and future apples are exposed for infection at this time. Keep fungicides on for mildew.

The MaryBlyt model was calling for the possibility of fire blight blossom blight infections coming into the past weekend, but in many cases the temperatures did not get as warm as predicted, so the risk was lower than expected but susceptible bloom still needed to be protected. Temperatures are more on the cool side this week, which is not as favorable for blossom blight, but keep an eye on the MSU Enviroweather site and the station nearest your orchards for further fire blight model details as forecasts are updated.

Tree fruit insects

As we move into the petal fall stage, there are several key pests that need to be managed: plum curculio, obliquebanded leafroller, European red mite and rosy apple aphid. Other pests that might need to be addressed might include San Jose scale, oriental fruit moth and tarnished plant bug. Here’s what’s happening right now that you should be monitoring for.

First reports of adult plum curculio egglaying scars on plums, sweet cherries and a few apples are coming in for the Grand Rapids, Michigan, area. Cool nights will slow their activity. They prefer nighttime temperatures over 60 F for best activity.

First reports of codling moth adult males in high pressure blocks started about a week ago. Other blocks have now had some flight as well. Set your biofix at sustained flight. Expect trap catch in low pressure blocks to be delayed, especially if temps at dusk are not above 50 F.

Medium-sized obliquebanded leafroller larvae are present. There seems to have been an uptick in feeding activity. No pupating larvae have been found yet. Scout for their presence and add management sprays at petal fall for blocks with high numbers. The threshold is one larva per tree. Traps for adult flight go up in early to mid-June (1,000 GDD42).

Aphids have had some slight increase in numbers, but nothing out of the ordinary at this time.    

Warmer weather and rapidly expanding shoot growth will likely be favorable for aphids to get established. Keep an eye on non-bearing trees where routine cover sprays are lower; aphid populations can get out of hand in these blocks quickly.

Overwintering San Jose scale are still present. Adult male flight began with the warmer weather over the weekend. A tentative regional biofix was set for May 25 (211 GDD51) with 40 GDD51 accumulated since. This biofix is tentative until we can see what traps do over the next week.

Initial activity of oriental fruit moth adults began in the Grand Rapids area last week with very low numbers in traps. A regional biofix was set for May 17 (288 GDD45) with 145 GDD45 accumulated since that date. This model indicates we should have reached the 10% egg hatch threshold and cover sprays to prevent shoot tip damage should be applied in stone fruits.

Some egg hatch of European red mite is being reported in the area, but activity continues to be very slow with cooler temps recently. Continue to monitor and expect warmer weather this week to activate this pest and petal fall sprays should target it.

Some activity of adult black stem borer was reported in the Grand Rapids area in last few days. If flight is sustained, then trunk sprays should be started soon.

First generation spotted tentiform leafminer adult flight is declining. Egg hatch is at a peak. No sap feeders are being found yet but should be visible soon. This is another minor pest where management is needed only if damage was high the previous growing season.

First hatch of white apple leafhopper should be occurring, but nymphs have been very hard to find. As with aphids, keep an eye on non-bearing trees where routine cover sprays are lower; hopper populations can get out of hand in these blocks quickly.  

Some tarnished plant bug adults can be seen on weeds in the orchard floor in light numbers. No evidence of fruit feeding is being found or reported. Continue to monitor, especially after mowing, which can push them up into the tree canopy.

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