Grand Rapids area tree fruit update – July 21, 2020

Summer pest and crop concerns for apples.

Red sphere trap
Red sphere trap and lure for apple maggot. Photo by Amy Irish-Brown, MSU Extension.

Weather update

Growing degree day (GDD) accumulations put most of the Grand Rapids, Michigan, area at or slightly ahead of normal averages for mid-July. As of the end of day on July 19, degree day totals for the Sparta Enviroweather station are 2051 base 42 (four days ahead of average) and 1329 base 50 (five days ahead of normal average).

Soil moisture levels are now adequate for most orchard sites around the general Grand Rapids area but seem to dry out quickly with high summer heat and some very windy days. You can track irrigation needs on Michigan State University Enviroweather, which is linked to the Cornell NEWA irrigation model using the MSU Enviroweather station nearest your location – the two systems are linked together.

Crop update

Apples continue to grow quickly with the warm temperatures and recent rainfall. It is becoming much easier to see apples even where it was thought the crop was light a month ago. Peaches have a decent crop overall with some heavier or lighter depending cultivar and site. The earliest peach cultivar harvest started last week. Continue to irrigate peaches as needed to help with fruit size in this critical time.

The 2020 predicted apple harvest dates are published.

High heat and dry soils will cause stressed trees to show they aren’t doing well. We continue to see trees that are in various decline due to winter injury, drought or wet feet or a combination of several stressors.

Tree fruit diseases

Continue to maintain a general protective fungicide program for any potential apple scab or fruit rots.

Fungicide applications to target summer diseases – sooty blotch and flyspeck – are needed in some areas, but not yet in others. The wetting hours in 2020 for this model have been extremely variable from station to station, so be sure to check out the model online at MSU Enviroweather for information from the station closest to you to help with decision making as to timing for summer disease fungicides. The table below is a summary of the various weather stations around the general Grand Rapids area. Please note that the stations that have over 240 hours of wetting should already have summer disease fungicides on, and Grand and Standale are nearly ready for second applications. Aetna-Fremont and Belding should be applying summer disease fungicides now if not already. Kent City is getting close with 167 hours of wetting. Alpine, Conklin, Reeman-Fremont, Sparta and Sparta North have to add some more wetting hours to satisfy the model.

It’s very interesting how similar the rainfall is at the Sparta station and Sparta Tower, but the hours of wetting are very different. Weather monitoring equipment is not fail-proof and models are only as good as the data we feed into them. Models are guides. The summer disease model says that after 240 hours of wetting (and wetting must be at least 4 hours of heavy dew or rain to get added in), the diseases can start to be visible on fruit surfaces. We recommend applying fungicides at about 200 hours of wetting to prevent these disease expressions. Then, we add another 2 inches of rainfall and another 200 or so of wetting and the next spray is needed.

Summer diseases 2020 – Grand Rapids area.As of July 20, 2020, 12 p.m.


Estimated PF Date

Start of model (PF+ 10 Days)

Accumulated wet hours since start of model

Cumulative rainfall since petal fall

Aetna - Fremont

May 26

June 6




May 24

June 5




May 25

June 5




May 24

June 6




May 26

June 6




May 25

June 5



Kent City

May 25

June 5




May 25

June 6




May 25

June 6



Sparta Tower

May 25

June 5



Sparta North

May 25

June 5




May 23

June 3



Tree fruit insects

First generation codling moth has ended and only high pressure blocks where trap numbers do not decline to zero need to continue with cover sprays at this time. This is a good time to change out lures ahead of second generation flight, which should be starting any day if not already. As of July 19, the Sparta station data indicates we are at 1096 degree days base 50 since the May 25 biofix. If you get a week or two with zero trap catches, it is best to set a new biofix for second generation when you begin to catch moths regularly again. If you catch continually, then it is likely that early second generation egg hatch will occur in these higher pressure blocks around July 24, 25 or 26 or between 1220 and 1250 degree days base 50 from the first generation biofix. You will need to apply insecticides for generation two egg hatch this coming weekend in high pressure blocks and by July 30 or 31 in lower pressure blocks.

Apple maggot adults started flying after the heavy rain event on July 9 and 10 as expected. We reached 900 degree days base 50 from January first on July 2 when overwintering larvae should be ready to emerge from the soil. Using baited yellow or red sticky traps is better than with no bait. When using red spheres, keep in mind that females ready to lay eggs are very attracted to them and if you catch a high number of flies, you should react quickly. I’ve read that five flies per trap per week of a good threshold to use – over that amount on red baited traps indicates it is time to spray. If you are doing good trapping along woodlots where flies emerge from, you should have a handle on whether you are over threshold or not. In low pressure blocks, perimeter application might be all that is needed. In high pressure blocks and favorite apple maggot varieties, such as Honeycrisp and Ginger Gold, whole blocks sprays might be better.

Japanese beetles were also driven from their overwintering sties in the soil and sod with rains on July 9 and 10 when they seemed to appear overnight. They are very active and will continue to damage all fruits for at least another four weeks. Keep in mind that Japanese beetles are very strong flyers and will quickly move in from neighboring hot spots as soon as your insecticide cover sprays start to fade.

Various species of aphids can now be found in all tree fruits including green apple aphids and woolly apple aphid in apples, black cherry aphids in sweet cherries and green peach aphids in peaches. Warmer weather and rapidly expanding shoot growth are favorable for aphids. Beneficial insects such as ladybird beetle larvae, lacewing larvae and syrphid fly larvae are now easy to find in aphid populations.

As a general rule for the Grand Rapids area, obliquebanded leafroller egg hatch should be complete as we’ve crossed the 1000 degree day base 42 mark since the regional June 16 biofix. Always use your own biofix whenever possible to fine tune your management programs. Various sizes of obliquebanded leafroller larvae are being found – some are getting quite large and should pupate soon, leading to the summer generation adults. If you are finding more than one larva per tree, management should be considered right away to knock numbers down as much as possible to avoid high larval numbers at harvest.

The first generation of San Jose scale activity is coming to an end and sprays might not target as well at this time. Second generation adult males should begin flying at anytime – traps should be places near known infestations in apple blocks to have the best chance at catching them. Looking forward, I expect the second generation crawlers to emerge in mid-August where they can again be targeted with insecticides. In recent years, we’ve been getting a third generation that typically ends up in the calyx end or on the surface of apples. It is likely this could happen in 2020 as well with degree days being slightly ahead of normal.

Second generation oriental fruit moth adults should be nearing their peak flight this week. In stone fruits, you should have at least one application on to target second generation and could be nearing the second application to prevent larval feeding in shoots and fruits. Oriental fruit moth can also cause damage to growing shoots of young apple trees or nursery trees as summer insecticide covers get too stretched and we have had some heavy rainfall of late. With peaches, be mindful of preharvest intervals.

All stages of European red mite continue to be found in apples. Twospotted spider mites are also being found. Mite populations have quickly grown this season with some bronzing already noticeable in some apple blocks.

Brown marmorated stink bug damage has been reported occasionally in apples over the past month. We think much of this feeding is exploratory feeding by overwintering adults as they move across an orchard seeking better food choices. By the time the damage becomes visible, brown marmorated stink bugs have long moved on and are no longer in the orchard. There are some singleton nymphs now starting to be found in traps in known hots spots for brown marmorated stink bugs. Once you trap nymphs, management should be considered in blocks with a history of brown marmorated stink bug damage.

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