Northwest Michigan fruit update – July 13, 2021

Both sweet and tart cherry harvest is underway; rains have cracked sweet cherries but helped size tart cherries. Growers are moving through harvest quickly.

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

Weather report

We had a beautiful weekend for harvesting cherries. Daytime temperatures were in the mid- to high 70s and nighttime temperatures fell into the low to mid 50s, and conditions were dry. We did have a strange cold day on Thursday, July 8, when the daytime high only hit 60 degrees Fahrenheit at the Northwest Michigan Horticulture Research Center, and that cool weather was accompanied with rain. We received 0.41 inches of rain on July 8 and just under an inch of rain on July 7.

So far this season, we have accumulated 8.7 inches of rain at the Northwest Michigan Horticulture Research Center since Jan. 1. It is raining this morning (July 13) and is predicted to continue until later this afternoon when we will have cloudy conditions. More rain is in the forecast July 15, but the end of the week and into the weekend looks warm and dry.

Thus far, we have accumulated 1,807 growing degree days (GDD) base 42 and 1,136 GDD base 50. These accumulations remain ahead of our 31-year averages: 1,653 GDD base 42 and 1,015 GDD base 50.

Crop report

The region’s growers are still harvesting sweet cherries. The crop is on the lighter side, and we have cracks in some varieties. Again, brine cherries appear to have more cracks than canners. Trees with light crops also seem to have more cracks than bigger loads. However, some varieties still look great. We hand-harvested our Bentons on our high-density systems last Thursday and Friday, and the fruit is beautiful.

Tart cherry harvest has also begun this week across northwest Michigan. We harvested tart cherries in Antrim County for a research project July 12, and the quality of the fruit was excellent. We shook the trees without ethephon, and we left some fruit on the trees as a result. However, most growers have been making regular ethephon applications with the anticipation of harvest this week and into the coming weeks.

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.

On July 6-8, we had a high cherry leaf spot infection period where we had a wetting span of 38 hours. We have had some dry conditions this season to get a jump on cherry leaf spot infections, but we have also had rain events that have resulted in five leaf spot infection periods. We anticipate today’s rain will also result in a leaf spot infection period. Keep up your leaf spot programs as we recommend that trees keep their leaves into September to minimize chance of damage if we have a hard winter.

We have observed higher than normal levels of powdery mildew in both apple and cherry. This disease is favored by hot and dry conditions, both of which we have had this season. This disease often takes a back seat to leaf spot and apple scab, but this year, this disease is much more of a concern. There is a lot of mildew on new shoot growth in tart cherries, and as a result, powdery mildew makes the leaves brittle and are easily removed during the shaking process. Unfortunately, there is little growers can do to eradicate this disease once we can see mycelial growth on the leaves.

American brown rot is still a 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 spread 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 (SWD) 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. This rain event will not help the brown rot situation, and growers will have to protect fruit against this disease under wet conditions.

If growers do have brown rot in their orchards, we want to collect samples for to test for resistance to the SDHI fungicides. Michigan State University’s George Sundin will be in the region tomorrow to evaluate a disease project at the Northwest Michigan Horticulture Research Center, but we are looking to collect brown rot samples while he is here. Please reach out to Nikki Rothwell at 231-342-4094 if you would like American brown rot in your orchard sampled.

Insects

We did not catch American plum borer this week. We did catch an average of 6.3 peach tree borer numbers this week. We have caught an average of 3.5 greater peach tree borer moths this week.

We did not catch any oriental fruit moth this week. Codling moth numbers were all zeros 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 been collecting fruit for SWD sampling in an unsprayed block at the Northwest Michigan Horticulture Research Center. In these samples, we are quite surprised to see the number of obliquebanded leafroller larvae in this unsprayed tart cherry block. The larvae are still very small, but the number of larvae from these samples is quite high. In fact, we are seeing more obliquebanded leafroller larvae than SWD larvae. On average, we are finding 10-15 obliquebanded leafroller larvae per sample (750 fruit per sample).  

We have still caught very few SWD flies this year. Although we started the season with droughty conditions, it seems we have had enough rain, moisture, and high humidity to really build up the SWD populations. However, we do not see trap counts rising. In fact, we caught no adult flies in unsprayed blocks at the Northwest Michigan Horticulture Research Center last week or the start of this week.

As mentioned above, we have also been sampling for larvae if fruit in unsprayed blocks at the Northwest Michigan Horticulture Research Center, and we have found larvae in tart cherry fruit. We have been sampling daily for SWD larvae, and we have found the following numbers of larvae for July 10, 11 and 12: three, five and two larvae for the past three days. We did not find any larvae in today’s samples (July 13). We will sample for larvae for the remainder of the week.

Again, we have not been finding any adults in our traps but did find larvae. This finding is important because SWD traps have not provided us with any indication of the populations size of SWD. Growers should not be complacent because nothing is in the traps; our hypothesis is that the traps are not particularly effective at 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 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 about 1,200 GDD base 39.2 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. 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.

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