Northwest Michigan fruit update – July 20, 2021
Tart cherry harvest is in full swing in northwest Michigan, and growers are pleased there are more cherries out there than initially estimated.
The weather has been very summerlike, which has been helpful as we are busy with tart cherry harvest. Conditions have been a little unusual as it has been hazy with the jet stream pattern bringing in smoke from the fires out west. The hazy conditions have reduced overall temperatures, and today, July 20, the forecast is calling for cooler weather, low 70s with potentially 20 mph winds. Daytime temperatures have been in the low 80s and nighttime lows have only dipped into the high 50s to low to mid-60s. Although they are calling for higher wind this afternoon, conditions have been extremely still, and humidity is quite high in the past few days.
We have accumulated 1,975 growing degree days (GDD) base 42 and 1,246 GDD base 50. We also had some significant rainfall July 13-15. We had 0.84 inches on July 13 and another 0.93 inches on July 15. The rainfall helped size tart cherries.
Tart cherry harvest is underway across northwest Michigan. Some growers are finding that there is more fruit in the orchards than they predicted earlier in the season, which is good news for many growers. Tart cherry quality is excellent as size has improved with the recent rains. We can find wind whip on some cherries, and we have seen more brown rot in tart cherries than in typical years. Growers are moving through harvest quickly. We are concerned about beating up fruit if we do get strong winds today.
Pest and disease report
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 13 and 14, we had a cherry leaf spot infection period that was recorded on the Northwest Michigan Horticulture Research Center Enviroweather station where we had a wetting span of 10 hours followed by another 14 hours of wetting; the first wetting period resulted in a low infection for cherry leaf spot and the second one was a moderate infection period. We have had some dry conditions that have kept cherry leaf spot infection pressure low, but we have also had rain events that have resulted in nine leaf spot infection periods since the first of June. Growers should keep up their 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.
If growers have American 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.
We caught an average of 7.6 American plum borer moths this week, and our trap counts for peach tree borers doubled since last week; we caught an average of 17.6 peach tree borers in our traps this week. We have caught an average of 5.3 greater peach tree borer moths this week in three traps in cherries. We still have not caught a cherry fruit fly in our trapline at the station. The obliquebanded leaf roller count went up this week, and we caught an average of 8.7 moths compared to none in our traps last week.
We did not catch any oriental fruit moth this week in apple. Codling moth numbers were an average of two moths per trap, which is similar to what we have been seeing at the station for the whole season. We found one San Jose scale and no apple maggot in our traps this week. We did detect an average of 6.3 obliquebanded leafroller moths in our trap at the station. We also found zero black stem borers in our traps.
We continue to sample daily for spotted wing drosophila (SWD) in an unsprayed tart cherry block at the station. We check our traps for adult flies daily as well as sample 750 fruit for larvae. Our numbers continue to be lower than expected for this time in the season. We have also had optimal conditions for SWD population growth including warm weather (but not too hot) and relatively high humidity. We do not understand why we are not seeing more flies and/or larvae in ripe fruit.
However, despite the overall low numbers of SWD flies and larvae, the numbers are increasing this week. Yesterday, 19 July, we caught at total of 19 flies. Our past fly counts have only been one fly per trap, so this jump is quite significant. Additionally, we are starting to catch male flies. Up until last Friday, July 16, we had caught no male SWD. On Monday, we caught five male flies out of the total 19 flies. Fly counts have also increased at grower farms where we are trapping every Monday, Wednesday and Friday.
Today’s trap counts were down again, and we only caught one male and one female fly today. We did recover three larvae from our 750 fruit sample.