Last year, I visited a blueberry site with significant mummy berry pressure in one of our southern Georgia counties. Based on the producer spray records, the materials applied and application timings were on target; there initially appeared to be no good reason for the level of mummy berry observed – assuming all was accurate. At some point, the conversation turned to alternate-row-middle (ARM) application (spraying every other row with an airblast sprayer) and its use at this location. This raised quite a few questions, as we did not have information to support the use of ARM in rabbiteye blueberries.
Recently, a replicated test was conducted to determine the efficiency of ARM applications in a mature rabbiteye blueberry site in Alma, GA. We utilized both water-sensitive and dye cards for determining the degree of coverage observed at different points in the canopy. No leaves were present, and though the plants had not been pruned well, the plant architecture was relatively open (absolutely no leaves). We applied the dye-water combination at a rate of 62 GPA (a higher total spray volume than most producers would use at this time of year; many would use half this rate for early-season applications) at a ground speed of 3 miles per hour. Even without computer analysis, it was apparent that the insides of unsprayed rows did not receive sufficient coverage. We were actually pretty surprised by the results, but under the parameters we utilized, we simply did not achieve good coverage; exposed tissue could easily have been infected. Though producers may increase pressure, slow ground speed, or increase total spray volume, this initial trial calls into question the use of ARM in rabbiteye blueberries. It has also been reported that some producers are using ARM for control of SWD, and that really needs to be carefully scrutinized. ARM with full foliage is likely pretty dicey, even if one does substantially increase the total spray volume.
In addition to accurate calibrations, producers should utilize water-sensitive cards to determine spray coverage. Based on the replicated trial we conducted, one should be careful about the use of ARM. However, producers may be able to change parameters to make this work at their locations, especially if they have pruned well. They need to conduct their own trials if they plan to use the ARM approach for early-season applications. There is a report by VanEe et al. (2000) that indicates ARM can work at budbreak (but not full bloom) on northern highbush plants, so there might be limited utility. However, we could not recommend ARM as a general research-based method at this point. Even limited air movement (wind) can cause erratic coverage with ARM, so still conditions would be very important with ARM.
Bottom line – producers should conduct their own testing with water-sensitive cards under their conditions. Such tests should be conducted throughout the season as the canopy closes and conditions change. These cards are really useful, as they can help one modify sprayer setups, etc. for better coverage; without use of water-sensitive cards, it is very difficult for one to visually assess spray patterns with airblast or other sprayers in the various canopy layers.