MSU Extension Fruit & Nuts

Weather report

The most notable weather event in the past week is the significant amount of rainfall we received. However, our rainfall totals in northwest Michigan are far less than other areas in the state. We received about 1 inch of rain at the Northwest Michigan Horticulture Research Center; other Michigan State University Enviroweather stations recorded slightly more or less rainfall than the Northwest Michigan Horticulture Research Center (Table 1), but our overall totals were well below other regions (2-6 inches less rainfall than other areas that received huge amounts of rain). Much of the state has been in some stage of drought, and this rain was much needed. However, some sweet cherries have cracked as a result of the duration of the rain event.

There is more rain in the forecast for today, June 29, and potentially some thunderstorms and isolated showers through Thursday. According to MSU state climatologist Jeff Andresen, the best chance for rain in our area is today. In addition to the ongoing rain events, humidity levels have been extremely high. Temperatures were in the low to mid-70s degrees Fahrenheit last week.

Thus far, we have accumulated 1,447 growing degree days (GDD) base 42 and 888 GDD base 50. These accumulations remain ahead of our 31-year averages: 1,274 GDD base 42 and 748 GDD base 50.

Andresen reported that rain showers will move out by Thursday, and Friday, July 2, will by dry and cool. Conditions will remain dry into early next week. The medium-term forecast is predicting conditions to be warmer than usual in July.

View Andresen’s weekly weather report.

Crop report

The biggest concern at this time is the cracking of sweet cherries that has come from the long wetting period. As mentioned above, 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, but our sample size is still quite small. We thought sweet cherries would be able to resist some cracking as we have had such a dry season so far and fruit size was extremely small. Some growers hypothesize that was the duration of the long wetting period more so than the actual rainfall amounts.

As noted in the above rainfall table, the region did not receive a tremendous amount of rain compared to areas like Grand Rapids and Detroit. We have observed some cracking even in tart cherry, which typically do not crack as a result of excess moisture.

Pest and disease report

Diseases

The recent rain events triggered a heavy apple scab infection period (Figure 1). The Northwest Michigan Horticulture Research Center recorded a span of wet for 95 hours. This heavy infection level is the highest we have seen for scab infection all season. The challenge has been for growers to get back into cover up again before the next rain event. Hopefully, the weather predictions are correct, and conditions will dry this week and remain dry next week.

The RIMPro models also indicated a high risk for apple scab infection. The good news is that with this long wetting period, we are calling the end to primary scab. Please check out the RIMpro apple scab data below:

• Benzonia RIMpro
• East Leland RIMpro
• Williamsburg RIMpro

We have observed new fire blight shoot blight strikes in our pathology orchard at the Northwest Michigan Horticulture Research Center. We have also been hearing of strikes in regional orchards from crop consultants. The potential for fire blight to spread at this point in the season is low according to MSU plant pathologist George Sundin.

The recent rains also resulted in a high cherry leaf spot infection (Figure 2). Again, we have not seen tremendous disease pressure with the dry conditions, but we did have four leaf spot infection periods on the following recent dates: June 13-14, 18, 20-21 and 24-28. With the predicted cooler weather in the forecast, copper applications are an excellent option for this time of the season. Copper formulations have performed excellent in the efficacy trials at the Northwest Michigan Horticulture Research Center.  

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 week. We did catch an average of 6.3 peach tree borers this week. We have caught one and our first greater peach tree borer 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 2.3 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 have caught very few spotted wing Drosophila (SWD) flies this year so far. We hypothesized that the hot and dry weather was keeping populations or SWD activity low. However, even with the cooler weather and recent rains, our numbers in traps remain low. Additionally, the SWD numbers across the state are also low. Each MSU lab group that is trapping has only captured a few flies here and there; there is no consistent catch, and numbers are very low.

Despite the low trap counts, growers should be using the SWD model on Enviroweather. This 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 F post bloom.

Figure 3 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. Growers should be using the SWD model for their individual farms to help predict when SWD infestation may begin in their blocks.