Grand Rapids area tree fruit update – Aug. 11, 2020
Late season insects and diseases should not be overlooked
Degree day accumulations continue to have the Grand Rapids area slightly ahead of normal averages for the first weeks of August. As of the end of day on August 10, degree days totals for the Sparta weather station are: 2671 base 42 (4 days ahead of average) and 1773 base 50 (7 days ahead of normal average). The warmer than average night-time temperatures have been pushing degree days base 50 ahead of normal.
Recent rainfall in the evening of August 10 was quite variable across the region. Totals ranged from 0.25 inch to 1.25 inch. The line of storms brought some wind, but not nearly as bad as the winds in southern Michigan. No hail has been reported in the Grand Rapids area from this line of storms. Soil moisture levels are still inadequate for most orchard sites around the general Grand Rapids area and irrigation systems should be running routinely if you have them in place. Irrigation needs can be monitored on MSU Enviroweather which is linked to the Cornell NEWA irrigation model using the MSU Enviroweather station nearest your location – they two systems are linked together.
Apples continue to put on size nicely even with the dry conditions. Blocks with irrigation have the size advantage. Some hand thinning is taking place where cropload was heavier than first anticipated. Fruit finish is very good considering the cold spring conditions. There are blocks and varieties with russet and frost marks, but much less than anticipated.
This is the time to get tissue sampling done in tree fruits. Be sure to follow instructions for taking proper samples from the lab you use for testing.
The 2020 predicted apple harvest dates are published. You can find the article by clicking this link, “Predicted 2020 Apple Harvest Dates”. We can look to early harvested varieties to tell us how accurate our predicted harvest dates are. Some Jersey Mac harvest started in the last 5 to 7 days. Zestar is about 4 or 5 days later than average. Paula Red harvest is being talked about to be harvested near or even a bit later than last year’s dates in the last week of August. Typically, the early harvested varieties can swing with the summer weather – hot summer equals earlier than predicted harvest and cool summer equals a bit later development. Later varieties (Golden Delicious and later) do not seem to be as influenced by the summer weather and are less likely to change much from the predicted dates. The warm nights are not good for color development and perhaps you can use harvest management sprays to hold fruit longer to tap into some cooler weather a few weeks later. These warm falls have been a challenge when it comes to fruit color.
Tree fruit diseases
Continue to maintain a general protective fungicide program for any potential apple scab or fruit rots. 2020 has been a banner year for fungal pathogens and there seems to be more issues with fruit rots and summer scab than usual. You should be adding more that captan to the tank to stay ahead of rots and summer diseases. MSU Extension is starting to see the first expression of summer diseases, – sooty blotch and flyspeck – on unsprayed or low maintenance blocks. The model that predicts when to apply fungicides for summer diseases is showing great variability for the fourteen weather stations around the Grand Rapids area. And while you can use this to fine-tune your summer disease management applications, I get the sense that this could be a very favorable year for summer diseases and rots, so do not let your fungicide program get too stretched out.
Tree fruit insects
A general reminder for all the late season pest management: pay attention to pre-harvest intervals for pesticides as we approach harvest for early apple varieties along with peaches, plums, and other tree fruits.
Adult flight of second generation codling moth should be past its peak for most blocks and trap numbers seem to be declining as expected. We are likely just past the peak egg hatch period for second generation codling moth and cover sprays are critical to prevent fruit stings over the next two weeks at least – don’t stretch sprays.
Apple maggot adults continue to emerge and seem to have spikes when heavy rainfall occurs. Be sure to maintain good insecticide covers for the next several weeks to prevent fruit damage.
Japanese beetles are nearly over for the season, but they can still be found feeding and mating in various tree fruit crops.
Summer generation adult obliquebanded leafroller flight is slowing and early egg hatch for summer larvae should be well underway. The new larvae will be very difficult to see until perhaps later next week when they get to their second or third instar. If you have had an issue with OBLR this season, target the small larvae in the next few days to stay ahead of them.
Second generation adult males of San Jose scale are flying and likely just past their peak emergence now. I expect the second generation crawlers to emerge in the next few days in the Grand Rapids area. If SJS has been visible on fruits at harvest in the past year or two, consider targeting the crawlers by the end of this week for best control. In extremely high pressure situations, it is thought that there is a third generation of SJS adding to the fruit damage and a late spray around Labor Day might be needed in later harvested apple varieties.
Second generation activity of Oriental fruit moth should be completed with third generation adult flight starting. The OFM model suggests that early egg hatch (8 – 10%) will occur around August 20 using the Sparta weather station data. This generation can attack late peaches and other stone fruits, but also can cause damage to apples that might look a lot like codling moth. In blocks with a long history of mating disruption for codling moth, it is a good practice to have at least one ORM trap up per 10 to 20 acres to keep an eye on potential activity. When routine covers sprays for CM are reduced or eliminated in a disrupted block, OFM could be left to increase to damaging levels.
As I predicted in late May, 2020 has been a great year for European red mites as well as other mite species. A mild winter was good for egg survival and the hot weather in late May really pushed populations to explode early in the season when predators were not yet active. In fact, mites seem to be having a great year on many plants this year, not just fruit trees.
No late summer feeding damage has been reported from brown marmorated stink bug nymphs or adults yet but I expect to start hearing of it soon. The MSU Extension statewide trapline continues to report very low numbers of nymphs or adults and very few in the Grand Rapids area. In those areas with a history of damage, once you trap nymphs, management should be considered in apple blocks.