Northwest Michigan fruit update – June 22, 2021

The weather has cooled considerably, and tart and sweet cherries are starting to color.

Weather report

Happy summer! As usual, the weather is unusual again this week. We had extreme heat and dry conditions two weeks ago, then the weather cooled last week and we received some much needed rain. The conditions cooled even more yesterday, June 21; the morning was relatively warm, but conditions cooled throughout the day. By late morning, temperature was 51 degrees Fahrenheit. In addition to the cool conditions, the wind speeds were high with gusts reaching into the mid-30s mph, which made the day feel even colder. At the Northwest Michigan Horticulture Research Center, the Michigan State University Enviroweather station recorded 0.18 inches of rain. We have accumulated 1,283 growing degree days (GDD) base 42 and 778 GDD base 50. These accumulations are ahead of our 31-year averages: 1,090 GDD base 42 and 619 GDD base 50.

MSU state climatologist Jeff Andresen reported that today, June 22, will be unseasonably cool and breezy, and cool again for the overnight tonight into Wednesday. Thursday, June 24, will be the warmest day this week, and daytime highs should hit into the 80s F. Rain is predicted to move into the state on Thursday evening through Saturday, and there is a chance of more significant rainfall during this period of time than we have had in past weeks. There is also a prediction of warmer than normal weather as we move into July.

View Andresen’s weekly weather report.

We also want to note that there was an issue with the Enviroweather system on Sunday into Monday morning (June 20-21), and the northwest stations were not recording rainfall. However, this issue was addressed, and all stations were recording properly by 11 a.m.

Crop report

Sweet cherries size increased during the past week and are at about 15-21 millimeters. Tart cherry fruit size also increased last week, and fruit size is up 4 to 17 millimeters Montmorency. Both sweet and tart cherries are coloring, and we have begun sampling for spotted wing Drosophila (SWD) in ripening fruit.

Apples are sizing and are at 29-30 millimeters.

Pest and disease report

Diseases

The Sunday night into Monday rain triggered a moderate apple scab infection period (Figure 1). The Northwest Michigan Horticulture Research Center recorded a span of “wet” for 17 hours. This moderate infection level is the highest we have seen for scab infection all season. Most other wetting events resulted in a light level of infection. With the recent rains across the state, both southwest and the Ridge are still catching scab spores and have not called an end to primary scab.

Graph of Apple scab model output.
Figure 1. Apple scab model output from the Northwest Michigan Horticulture Research Center on June 20-21, 2021.

The RIMPro models also indicated a high risk for apple scab infection. The good news is that there are very few mature spores left, and primary scab season should be over with the next rain event. Please check out the RIMpro apple scab data below:

The recent rains also resulted in a moderate cherry leaf spot infection (Figure 2). Again, we have not seen tremendous disease pressure with the dry conditions, but we did have three leaf spot infection periods on the following recent dates: June 13-14, 18 and 20-21. With the predicted cool 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 conducted at the Northwest Michigan Horticulture Research Center.

Graph of Cherry leaf spot model output
Figure 2. Cherry leaf spot model output from the Northwest Michigan Horticulture Research Center on June 20-21, 2021.

Insects

We are still catching American plum borer this week, but numbers are down compared to weeks past. We captured an average of 0.67 moths this week. Peach tree borer numbers were down to zero this week; last week, we caught an average of 13 moths compared to zero moths this week. We have caught no greater peach tree borer yet this season.

We are still catching a few oriental fruit moth this week, but numbers are down compared to last week’s catch. Codling moth numbers remain higher at the Northwest Michigan Horticulture Research Center this week; we caught an average of five moths per trap, which is down from last week’s average catch of nine last week. Most growers have set biofix for codling moth around mid-May for this pest.

Again this week, the San Jose scale trap counts were zeros and we caught no males in any of the traps. This situation was similar where we are trapping at grower sites, and we caught very few moths in commercial blocks. MSU entomologist Larry Gut also suggests that the peak male flight may be compressed this year, which will reduce the number of applications growers may use to combat San Jose scale.

Rose chafers are still here this week, and we have found the beetles feeding and skeletonizing leaves. Young trees are particularly vulnerable to the beetles as they can quickly defoliate the few leaves on new trees. We remind growers that the beetles have an aggregation pheromone that attract the beetles to each other; hence, the beetles tend to congregate in hot spot areas. Orchards adjacent to hay fields tend to get pressure from rose chafers after they are cut.

Additionally, rose chafer management can be difficult because the beetles can re-infest an orchard quickly after a spray has knocked the population down. Most materials that growers use for rose chafers provide quick knockdown efficacy of this pest, but residuals do not last long particularly in wet weather. Furthermore, this pest can reinfest blocks quickly – we encourage growers and consultants to continue monitoring problem blocks for this pest.

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 moderate 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.

Graph of SWD model prediction
Figure 3. SWD model prediction for the Northwest Michigan Horticulture Research Center with the bloom date of May 15, 2021.

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