West central Michigan small fruit update – Aug. 20, 2019

Blueberry harvest continues with good yields and excellent fruit quality. Maintain a strict insecticide application program to keep spotted wing Drosophila under control.

Weather update

Weather conditions in west central Michigan passed through a short drought period that lasted for more than two week. However, there were several thunderstorms during the last seven days that produced 0.5 to 1.25 inches of rain in the region. Although a very welcome event, these precipitations are not enough to meet the hydric needs of blueberries. Thus, growers are applying supplemental irrigation to blueberries to compensate for the lack of precipitation.

On the other hand, daily temperatures during the last seven days have remained around the mid- to low 80s. On average, daily minimum temperatures fluctuated between 60-61 degrees Fahrenheit and the daily maximum between 81 and 84 F. Except for those days when rains occurred, lack of regular precipitations is affecting the environmental relative humidity (rH). It has fluctuated between 37 to 57%. The highest rH occurs during the morning and evening hours.

Crop update

Fall raspberries are in the green fruit stage with some leading berries already ripening.

Day-neutral strawberries are in harvest with good flavor and good yield. Lack of precipitation and low relative humidity are limiting the presence of fungal diseases.

Blueberry harvest is in full swing. The variety Bluecrop is almost finished and remaining berries are machine harvested. Legacy is in the last harvest for fresh packing and Jersey is in the middle of harvest. Jersey fields are with a full crop and some fields are producing between eight and nine thousand pounds per acre of good quality fruit. Elliott and Aurora are hand harvested at this time. Yields are high and fruit quality excellent.

Spotted wing Drosophila (SWD) update

SWD remains the major problem for blueberries, raspberries and strawberries. However, until the past two weeks, we observed a difference in the size of SWD populations affecting small fruits in southwest and west central Michigan. Traps placed in blueberries, raspberries and strawberries at Ionia, Kent and Ottawa counties yielded between seven and 13 flies per trap. In contrast, traps in some places at Allegan and Van Buren counties yielded more than 50 flies per trap per week. Last week, things start changing—some traps in Ottawa County started reporting up to 40 flies per trap. Still, these numbers are lower than those observed in southwest Michigan.

In southwest Michigan, an increased number of growers started reporting fruit loads rejected or downgraded by packers/shippers due to the presence of SWD larvae. SWD infestations and market problems are forcing some small growers (those with less than 40 acres) to stop harvesting. Unharvested fields may increase the SWD pressure over those farms still in full harvest.

Growers able to maintain a relatively low SWD pressure by maintaining a good fly monitoring program and tight spray schedule are not reporting fruit infestations. In addition, low relative humidity resulting from limited precipitations, regular pruning of blueberries and daily temperatures in the low to mid-80s are contributing to keeping SWD populations under control.

As we know, insecticides utilized for SWD control are more effective when daily temperatures are below the mid-80s. In addition, VanTimmeren et al. (2017) and Tochen et al. (2016) indicated that a decrease in the ambient rH (due to dry conditions and pruning) greatly contribute to reduce SWD survival and reproduction.

Some potential problems during insecticide applications may contribute, in part, to control failures. For example, growers with heavy fruit infestations reported spraying during the hottest hours of the day, from noon to 4 p.m. VanTimmeren showed that during these hours of the day, flies remain hidden inside the blush canopy protected from the insecticide application. Another situation reported by producers with large farms—more than 100 acres—is that they cannot stop spraying during those hours without risking fruit infestation. They need to finish the application in time.

Further complications may occur by spraying during the hottest hours of the day. One of those complications is the low rH at that time. Insecticide applications under low rH may result in a large portion of the insecticide not reaching the target. Evaporation at rH lower than 50% may result in 30% or more of the spray volume lost before reaching the target. That also explains some of the SWD control failures during the season.

The recommendation to optimize SWD control under high pressure is to: A) Spray at the time of the day when the rH is higher and temperatures are low, less than 80 F; and B) maintain a tight SWD monitoring program with short time periods between trap inspections. At this time, checking traps twice a week provides precise information on the SWD population for implementing a timely and swift response. Finally, check Michigan State University Enviroweather to match the insecticide with the best weather conditions of temperature and precipitation to optimize their effectiveness.

Under the current weather conditions, if you are harvesting for fresh markets, Pyrethroid insecticides—Mustang Maxx, Brigade and Danitol—are excellent products for SWD management. They have a 24-hour preharvest interval. Imidan and Lannate are also excellent for controlling SWD, but they have a three-day preharvest interval.

For more insecticide options, consult MSU Extension bulletin E154, “Michigan Fruit Management Guide.”

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