Northwest Michigan fruit update – July 28, 2020
Midsummer apple pest activity has started. Spotted wing Drosophila numbers are rising as cherry harvest is in full swing.
We had a good run of summer-like weather this past week. The weather has been favorable as most of the growers in the region are in the full swing of harvest. Daytime temperatures were warm over the weekend and hit the mid- to high 80s. However, nighttime temperatures were moderate and were in the mid-60s. We did have some rainfall last Wednesday, July 22, where the Northwest Michigan Horticulture Research Center’s Enviroweather station received 0.31 inches. We also had some rainfall on Sunday, July 26, where the station recorded 0.12 inches of rain.
We have accumulated 2,064 growing degree days (GDD) base 42 and 1,372 GDD base 50. Our 25-plus-year averages are 2,084 GDD base 42 and 1,330 GDD base 50.
As mentioned above, we are in the middle of tart cherry harvest. Fruit quality is still excellent at this time. Most of the sweet cherries have been harvested in the region.
In apples, the first apple maggot fly was found in our trap line yesterday, July 27. This initial detection is later than usual for this pest. According to degree days, this pest first emerges around 900 GDD base 50; the Northwest Michigan Horticulture Research Center is currently at 1,372 GDD base 50, well beyond the 900 GDD mark. We are approaching the timing for peak emergence of this pest. Flies emerge from the soil and seven to 10 days later the flies are sexually mature and begin laying eggs. Due in part to no or low catches, some growers used products for other orchard pests that would have also been effective against apple maggot flies as a precaution.
Although there was not a good distinction between first and second generation codling moth activity, we are at about 1,106 GDD base 50 post-biofix (June 1) and second generation codling moth are likely flying at the Northwest Michigan Horticulture Research Center; trap numbers were in the single digits but above threshold this week. Second generation adults are estimated to emerge at about 1,060 GDD base 50, which would have occurred Sunday, July 26. Second generation egg hatch begins about 1,200-1,250 GDD base 50 post-biofix, which is currently estimated for the first week of August.
Twospotted spider mites and European red mite populations are high at the Northwest Michigan Horticulture Research Center. We received reports that populations are also high on some farms despite efforts to manage mites.
The second flight of black stem borer has been underway for three weeks with reports of spikes in the number of beetles found in traps this week.
San Jose scale male flight has also started again. While management does not target this life stage, we can use trap numbers to assist in estimating population density and when the second- generation crawlers will emerge.
The sooty blotch and flyspeck model on Enviroweather indicates we have accumulated a total of 226 wet hours since June 12, the estimated 10 days after petal fall when the model initiates. Symptoms from sooty blotch and flyspeck are estimated to appear when the model reaches 240 hours. Fungicide applications reset the clock on the model; hence, if a fungicide was applied after petal fall, fewer accumulated wet hours have likely occurred at this time. Visit Enviroweather to read more about this model and how to use it.
In cherries, spotted wing Drosophila (SWD) numbers in traps have continued to rise, which seemed to be related to wetter and cooler weather. The Northwest Michigan Horticulture Research Center removed traps from commercial blocks last week as most growers were harvesting blocks where traps were located. We have received reports that larvae can be found in some commercial blocks, particularly in fruit remaining after harvest. Many growers were fortunate to harvest before this pest overwhelmed spray programs.
Rains caused some cracking in sweet cherries and brown rot also got a foothold during the wet and humid conditions. In addition to brown rot on sweet cherries, we have also found brown rot infected tart cherries at the Northwest Michigan Horticulture Research Center. Infected fruit this season will be the source of inoculum for next year.
Wet weather has been a concern for continued development of cherry leaf spot disease. Although some wetting periods have been relatively shorter recently (i.e., seven hours on July 19, 11 hours on July 22, and 14 hours on July 26), there was still sufficient time for infection to occur. Our preliminary data from the cherry leaf spot efficacy trial this season showed that trees treated with a season long Captan program provided significantly better control compared with many other programs. Hence, Captan used for post-harvest applications may be a better option than our traditional strategy of using a fungicide containing chlorothalonil.