Northwest Michigan fruit update – July 6, 2021

Sweet cherry harvest is underway, and many growers will begin tart cherry harvest early next week.

Weather report

The weather has been very summer-like for the last week. Most daytime highs are in the mid- to upper 80s degrees Fahrenheit. Nighttime temperatures have also remained quite warm recently (low 70s), but nighttime temperatures did fall into the 50s late last week. The region received varying amounts of rainfall last night (5 July), but the Enviroweather station at the Northwest Michigan Horticulture Research Center only received 0.06 inches of rain. More rain is in the forecast for this evening into tomorrow (July 6-7).

Thus far, we have accumulated 1641 growing degree days (GDD) base 42 and 1,025 GDD base 50. These accumulations remain ahead of our 31-year averages: 1,462 GDD base 42 and 880 GDD base 50.

Crop report

The region has begun sweet cherry harvest. As mentioned last week, we had a wetting event that lasted 95 hours, and this duration of rain and high humidity cracked a lot of sweet cherries. Brine cherries appear to have more cracks than canners. Trees with light crops also seem to have more cracks than bigger loads. We have observed some cracking even in tart cherry, which typically do not crack as a result of excess moisture. Many growers are applying ethephon in tart cherries this week with the anticipation of harvest beginning later this week, over the weekend and into the beginning of next week. Growers have been cautious with ethephon rates with the recent heat. Growers with a light crop are also reducing their rates.

The 95th National Cherry Festival kicked off on Saturday, and we have local sweet cherries for this year’s event! We get that question all the time here at the Northwest Michigan Horticulture Research Center. July 6 is also Industry Day, which is typically held here at the station, but this year, the Elks Lodge is hosting the event. Congratulations to Gary Kaberle, Cherry Industry Person of the Year!

Pest and disease report

Diseases

Primary apple scab season is over for northwest Michigan. If growers were successful in controlling primary scab, there should be little concern for fruit scab. However, if scab was able to get a toe hold on foliage, fruit must continue to be protected.

We have had four cherry leaf spot infection periods on the following recent dates: June 13-14, 18, 20-21 and 24-28. The forecast is also predicting rain for tonight into tomorrow, and growers should be protected for this next wetting event.

American brown rot is the major concern at this time, particularly if sweet cherries are cracked. This disease favors warm and wet conditions, and it is a fast growing fungus that spreads throughout an orchard in 24-36 hours under optimal conditions. In addition to the favorable weather conditions and recent cracks in the fruit, growers are limited on the fungicides they can use to control American brown rot. In our recent fungicide screening, we have documented that Indar (fenbuconazole) is no longer controlling American brown rot, and growers should not depend on this product for controlling brown rot this season. Without Indar, growers are left with Merivon, Luna Sensation and Flint Extra.

The best material is Merivon for American brown rot control, but the label does not recommend tank mixing Merivon with an EC insecticide, which may be needed as we move into optimal weather conditions and ripening fruit for spotted wing Drosophila oviposition. We have no data as to how prevalent fruit issues are with the Merivon/EC combinations, but BASF does not recommend a tank mix of these products. Our best hope is that the weather will dry soon to allow us to get through sweet cherry harvest.

Insects

We did not catch American plum borer this pest this week. We did catch an average of 6.3 peach tree borer numbers this week. We have caught an average of 2.3 greater peach tree borer moths this week. Remember that if you are spraying to target greaters, which can be more problematic in Balaton, the preharvest interval is 21 days for trunk applications.

We are still catching a few oriental fruit moth this week. Codling moth numbers remain low but we did catch an average of 1.6 moths in our traps at the Northwest Michigan Horticulture Research Center this week. In northwest Michigan, we often do not see a distinct first and second generation of codling moth but a continual catch of moths throughout the season.

We found obliquebanded leafroller larvae in our unsprayed tart cherry block. The larva was a first instar. This is the first larva of the second generation we have found this season.

We have still caught very few spotted wing Drosophila (SWD) flies this year. Likely the early drought limited their population from building, and with the recent rains and humidity, we expect the numbers to increase. We caught one female on Saturday, July 3, and two females on Monday, July 5. We have also been sampling for larvae if fruit in unsprayed blocks at the Northwest Michigan Horticulture Research Center, and we found two larvae in tart cherry fruit. We did not find any adults in our traps today but did find larvae.

This finding is important because SWD traps do not indicate when fruit can be infested. Again, no adults in traps today but larvae in fruit. Growers should not be complacent because nothing is in the traps; our hypothesis is that the traps are not particularly effective in catching adults, particularly if fruit are at a ripeness where they can be infested. Fruit can be infested with SWD even if the trap counts are zero.

The numbers of flies are rising in southwest Michigan. One MSU researcher caught 54 flies in one of her traps, but most traps are zero to five flies; average weeklong trap counts for this researcher in southwest Michigan is 10 flies per trap. They are checking unsprayed tart cherries every day (as we are at the Northwest Michigan Horticulture Research Center), and they saw SWD larvae in fruit on July 1. They are conducting a similar study in blueberry, and they have found one larva in unsprayed blueberries late last week.

The SWD model is based on Montmorency crop development, and it monitors when the crop is susceptible to SWD oviposition/potential infestation. The flies are likely present through much of the growing season, but fruit is susceptible to egglaying at around 1,200 GDD base 39.2 F post-bloom. Figure 1 shows that we are at high risk of SWD infestation based on the full bloom date of May 15 at the Northwest Michigan Horticulture Research Center. Based on finding larvae today, we are at 1,392 GDD base 39.2 from full bloom. The model is more conservative, and we assume fruit can be infested at 1,200 GDD based on lab results. Growers should be using the SWD model for their individual farms to help predict when SWD infestation may begin in their blocks.

SWD model prediction chart.
Figure 1. SWD model prediction for the Northwest Michigan Horticulture Research Center with the bloom date of May 15, 2021.

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