Northwest Michigan fruit update – July 16, 2019
Hot and humid weather has dramatically increased spotted wing Drosophila counts; these weather conditions continue to challenge disease management programs.
Daytime temperatures have been hot in the 80s and reaching into the low 90s; we did have an exception of one cooler day in the upper 60s last Thursday, July 11. In contrast to multiple days with rain two weeks ago, there were only two days of rain this week with yesterday’s showers resulting in an inch of rain at the Northwest Michigan Horticulture Research Center. We received reports that some areas received as much as 1.75 inch of rain yesterday.
In the week ahead, there is a chance for rain/thunderstorms every day, and temperatures are predicted to remain hot. We have accumulated 1,480 growing degree days (GDD) base 42 and 881 GDD base 50, and our 29-year average is 1,749 GDD base 42 and 1,082 GDD base 50. According to our records, our degree day accumulations this season are most similar to accumulations in 2009.
Sweet cherry harvest is underway and some growers are making preparations to start tart cherry harvest at the end of this week or early next week. Sweet cherries have sized well this season. However, with an inch or more rain recently and more in the forecast, cracking in sweet cherries is a concern. Birds have been more of an issue in ripening sweet cherries this season.
Tart cherries are also ripening and gaining size, from 14 millimeters last week to 21 millimeters this week. In areas with severe cherry leaf spot pressure, ripening fruit is a concern as well as leaf spot marks on fruit.
Apples have made a jump in growth from 30 to 42 millimeters in Honeycrisp and 27 to 39 millimeters in Gala at the station.
We have had two days with measurable rainfall since our report last week, and growers are challenged with finding a good window to recover orchards following substantial amounts of precipitation across the region yesterday. Juggling pre-harvest intervals (PHIs), retreatment intervals and maximum allowable amounts of materials has also been a challenge in this season’s wet weather.
Recent rains have been a concern for American brown rot and cherry leaf spot. We have received more reports of leaf spot on both tart cherry and sweet cherry fruit this season. We cannot recall a season that has been more challenging for leaf spot, and the leaf spot on fruit is worse than we have ever observed in the past. Because we have so little experience with leaf spot symptoms on fruit, we are unsure how fruit with leaf spot will develop or whether the spot will be visible on a ripe fruit or if it will cause malformation of the fruit. To make matters more challenging for leaf spot control, products for disease management are becoming limited due to frequent rain showers/PHIs/maximum allowable amounts.
American brown rot has been observed in dark sweet cherries at the station and in commercial blocks, especially where the crop is heavy. The recent moisture, humidity and warm temperatures are very concerning for brown rot to spread, especially if cherries are damaged or cracked following the recent rain. We are collecting brown rot samples for fungicide sensitivity screening and growers are encouraged to contact us at firstname.lastname@example.org or email@example.com if you would like to send in samples for testing.
If you are using Indar, remember to use 12 ounces per acre of Indar in accordance with the Special Local Need (SLN) label and that the high end of the labeled rate for SDHIs should be used to ensure the best control of this disease. If you are using Merivon, remember to read the label carefully as there are some restrictions on mixing this product with other materials to prevent plant or fruit injury.
Additionally, some growers may have used Flint Extra this season to target cherry leaf spot. According to the Flint Extra label, a maximum of four applications of Flint Extra or other QoI fungicides are allowed per year. The QoI fungicides are quinone outside inhibitor (QoI) fungicides in FRAC group 11. This group includes Luna Sensation, Merivon, Flint Extra, etc. and due to high disease pressure this season, some growers may be close to this allotted amount. Again, read labels before making applications as there are also other restrictions for the SDHIs and other chemistries.
Spotted wing Drosophila (SWD) numbers are rising quickly in the region. We have checked traps from the station and all 40 traps are catching this pest; we found a total of 546 SWD. We also found larvae in sweet cherries that were covered with an insecticide for SWD and unsprayed tart cherries at the research station yesterday. Cherries are susceptible to SWD at this time and keeping good coverage will be necessary to protect fruit from infestation.
We have not detected cherry fruit fly at this time.
Obliquebanded leafroller moths are still active in both apples and cherries at the station. Since biofix (June 30), we have accumulated 398 GDD base 42 and an insecticide targeting small larvae would be well timed for this week.
San Jose scale crawler activity is ongoing and male flight ended last week. Crawler activity will likely end later this week.
Codling moth flight is ongoing at the station. Since biofix (June 8), we have accumulated about 600 GDD base 50 and egg hatch is ongoing. Larvae that escaped treatment could be entering fruit; we have observed codling moth larvae in crab apples.