For the past two weeks, weather conditions in the fruit growing region have been characterized by lack of rain and temperatures in the mid- to upper 80s. These temperatures are conducive for outbreaks of spotted wing Drosophila (SWD) in both blueberries and raspberries. On the other hand, the lack of precipitation in the area – as of Aug. 27, only traces of rain are reported by Enviro-weather – is compelling growers to supplement their crops with irrigation. However, irrigation at this time when SWD is at its peak may pose a risk for SWD contamination at harvest if the irrigation occurs right after the insecticide applications.

So far, both blueberry and raspberry growers have been able to maintain a good SWD control through intensive IPM scouting and the use of chemicals recommended for its control. However, this is the time when every year there is a short drought period that requires growers to supplement their crops with irrigation. This is critical to maintain a healthy crop and fruit quality in both raspberries and blueberries. Unfortunately, the presence of SWD in both crops makes things more difficult because overhead irrigation partially removes the insecticide from the plant, which in turn reduces the effectiveness of SWD control.

In blueberries, according to the last Michigan Fruit Inventory (2011-12), around 50 percent of all cultivated area is irrigated using overhead sprinkle irrigation. For those fields, irrigation at the time when SWD is at its peak may result in lack of control and SWD contamination at harvest time if the irrigation is applied right after the insecticide applications.

Growers need to synchronize their irrigation periods with their insecticide applications so that all required insecticides are applied after the irrigation period to ensure that the insecticide is not washed off from the plant. In any case, the residual effect of all recommended insecticides for SWD will be affected by the application of overhead irrigation and reapplications of insecticide might be required after irrigation to ensure that no SWD infestations will show up in harvested fruit. For information on rain fastness of recommended insecticides for SWD control in both blueberries and raspberries, please see the Michigan State University Extension article by John Wise “Rainfast characteristics of fruit crop insecticides.”

We are putting significant attention on the control of insect pests, but in addition to SWD, there are other issues related to plant diseases that require growers’ attention. In blueberries we are observing symptoms of leaf rust. According to Annemiek Schilder, small fruit plant pathologist at MSU, small yellow spots appear on leaves between July and September and then turn brown with darker borders. On older leaves, lesions may be surrounded by red or purplish discoloration or, when leaves are about to senesce, rust spots may actually be surrounded by green areas, known as the “green island effect.”

Leaf rust can rapidly increase towards the end of the season under warm, wet conditions. It generally has little impact on current season yield, but may cause premature defoliation. Severe defoliation has the potential to reduce fruitfulness of new buds and winter hardiness of the canes. Cabrio (pyraclostrobin) and Pristine (pyraclostrobin + boscalid) are labeled for rust “suppression” and are not recommended for rust control except during the harvest season when other products cannot be used. For more information on the biology and management of leaf rust, see “Leaf rust spotted in Michigan blueberry fields.”