Grand Rapids area tree fruit update – Aug. 17, 2022

Abundant rainfall slows some pests and enhances others.

San Jose scale crawlers on Gala apple fruits.
San Jose scale crawlers on Gala apple fruits. Photo by Amy Irish-Brown, MSU Extension.

The abundant rainfall has been great in July and August – we are blessed. This is a reminder to not forget that this ample rainfall can influence pests as well. While it’s good for suppressing mites and perhaps other insects, it is also good for promoting summer diseases and fruit rots.

Tree fruit diseases

Summer disease management in apples should be considered in areas where the Michigan State University Enviroweather model indicates adequate wetting hours for disease expression. The wetting hour totals vary greatly from station to station this year—be sure to check the summer disease model on Enviroweather for the weather station nearest you for guidance. If you want to read about how to use this model, see About Enviroweather's Sooty Blotch and Flyspeck Progress or give me a call and we can go over it together.

Fruit rots could be enhanced with higher rainfall that washes away fungicide cover sprays. Temperatures and relative humidity levels have been moderate to average but are still favorable for fruit rots when fungicide residues are lacking. High heat and high humidity greatly favor fruit rots.

There is a fungicide table for summer diseases and fruit rots on page 107 of the Michigan State University Extension bulletin E154, Michigan Fruit Management Guide.

Tree fruit insects

The big story to be aware of this week is the announcement from the Michigan Department of Agriculture and Rural Development (MDARD) about the finding of live spotted lanternfly in Michigan. The details are that it was found in Oakland County nursery stock and while it is an isolated area, this insect has shown a propensity for quickly expanding its range. It is likely we will have to deal with it sooner rather than later. Please take the time to look over the news releases and informational sheets I’ve listed below. If you see anything suspicious, please contact me right away.

Spotted lanternfly will be mainly a late season nuisance pest in tree fruits – they can develop very high numbers and exude honeydew – that lovely substrate for sooty mold to grow on, much like woolly apple aphid. They do suck sap from plants, but there are no reports of them killing fruit trees at this time.

Links for more spotted lanternfly information:

I was able to set a new biofix for second generation codling moth on July 19 (1330 DD50). We’ve accumulated 542 degree days base 50 since. This indicates peak hatch is occurring and cover sprays are critical to protect fruit from infestation. I expect second generation to be a concern for at least two more weeks in low pressure blocks. High pressure blocks will have to stay covered for at least three more weeks. I know you are tired of spraying and getting geared up for harvest, but with the heavy rains of late, don’t get too stretched out on codling moth.

The Grand Rapids regional biofix for obliquebanded leafroller was set for June 14 (1076 DD42) with 1771 degree days accumulated since. Adult moth numbers are low compared obliquebanded leafroller’s glory days in the 1990s and many blocks are well below a threshold for treatment. However, this insect has been slowing building back its numbers in some locations and we need to pay attention. Because this insect has many hosts and the males can fly great distances, pheromone trapping is good for setting a biofix, but not so much for determining what the population is in the orchard. The threshold I use is one larva infested shoot tip per tree. Others use four infested shoot tips per 100 randomly sampled.

Regardless, it is harder to manage this summer generation as we get close to harvest dates. Consider adding B.t. materials with cover sprays to help knock down the numbers in problem blocks – B.t. is an old material (2022 is its 50th anniversary), but it can help to keep low to moderate populations from becoming future high populations.

The Grand Rapids regional biofix for oriental fruit moth was set for May 12 (312 DD45) with 2157 degree days base 45 accumulated since. The third generation of oriental fruit moth is now actively flying. The degree day model uses 2250 to 2280 DD base 45 to indicate early egg hatch for this generation which can be an issue in late season peaches and possibly in apples. Your general cover sprays for codling moth should be lined up this year to manage oriental fruit moth – this isn’t always the case. In general, oriental fruit moth is not a major late season apple pest for our area, but if you have been doing mating disruption for codling moth and use limited to no cover sprays for codling moth, this oriental fruit moth can sneak up on you if you aren’t monitoring for it.

Woolly apple aphids are still too easy to find in just about every block you look in. I see good control in blocks where growers slowed down and used more water than they like to, but coverage is key with managing this pest.

A regional biofix for San Jose scale for the general Grand Rapids area was set for May 20 (288 DD51 with 1549 degree days accumulated since the regional biofix. In high pressure areas, adult males were trapped in much higher numbers for second generation than for first, which is unusual. Second generation crawlers are present and should reach a peak by the end of this week. This is typically the generation we see settling on fruit with a red halo around them, near the calyx end. However, there were several reports of San Jose scale on fruit in their first generation this year which indicates higher populations are present. When targeting scale, we need to slow down and use more water to deliver management products to all plant surfaces where this tiny insect resides.

All stages of European red mite are present and recent heavy rains did help to wash some adults away, but new eggs quickly hatched in high pressure blocks. This is the first year in the past 10 or so that bronzing from red mites is rather common. Predatory mites continue to be found in European red mite populations. Two-spotted spider mites are also present in some blocks, as they have been for the past few years – likely due to hotter summers. Summer miticides should be considered in blocks over the August threshold of 15 mites per leaf. For varieties that have 30 days to go until harvest, management should be considered this year. Most blocks have a good fruit set, even heavy in some blocks. You will need all the carbohydrates you can get to finish this crop well and mites can severely curb the carbs when they are in high numbers.

Adult Japanese beetles continue to be active but are quickly disappearing as they complete their adult life cycle for 2022.

Apple maggot adult emergence continues and a jump in trap catch was observed with recent rains. Traps need to be in place if you want to monitor apple maggot effectively for decision making, or, you should be adding preventative cover sprays for apple maggot. I am noticing high apple maggot infestation in non-sprayed apples – higher than usual.

I have been getting a few reports, samples, and pictures of damage due to brown marmorated stink bug, but overall, this late season invasive pest appears to light so far for 2022. There are hot spots, of course, but this pest hasn’t been as prevalent as we expected it to be. The Wilson Tree Fruit Entomology lab is sampling brown marmorated stink bugs for potential parasitoids and other beneficial organisms that might be at work for us.

Finally, as we approach harvest dates for apples, keep in mind preharvest intervals for all spray applications. Please refer to the product label or guidelines from your end buyers and processors. There are several tables in the back of E-154 as well. Look for the Days Between Final Spray and Harvest tables on pages 322 to 327.

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