Are you spraying your cherry trees enough?

In the Pacific Northwest, industry standards to apply the amount of fungicides are generally variable but do not exceed 200 gallons per acre. The application volume is usually tuned to specific training systems, tree height, canopy structure etc. The synthetic chemicals are often effective in controlling the disease, late season disease intensification is obvious in the upper tree canopy. In 2018 we investigated spray coverage on large cherry cvs. ‘Bing’ and ‘Sweetheart’ trees using Pak-Blast MB-100 sprayer (Rears Mfg. Co., OR). Pyranine (Sigma-Aldrich, USA), a water-soluble and traceable dye was used as a spraying material. The pyranine was applied to trees at the volume of 400 gallons per acre. The spray conditions included wind speed of 1-2 mph, the atmospheric temperature of 75 ⁰F, and relative humidity (RH) 55.3.

Following in-situ drying of the dye, cherries were labeled and harvested separately from each canopy levels and each side of the tree. Samples from were divided into three zones representing lower canopy (1-5’), middle canopy (5-8’) and upper canopy (8-12’) while the “face” samples were exposed to the sprayer facing the rows. The “side” samples did not directly receive spray because of their position parallel and away from the spray plume. The cherries were handled carefully with forceps, transported to laboratory and, two cherries from each sample were photographed under UV light (Figure 1). The illuminated area due to the presence of pyranine on the cherry surface was scored as spray coverage. The raw images were color extracted using online tools and the spray coverage percentage was extrapolated. The data was analyzed using pictures obtained from all sides, zones and faces from five independent trees.

Results indicated that coverage on cherries facing the air-blast sprayer was highest in all zones when compared to sides parallel to the spray plume. However, the samples from the lower canopy received the best coverage among all zones (Figure 2).

The cherries in middle canopy zone received 47-65% coverage whereas, upper canopy cherries were least covered i.e. 28-42% with spray. The experiment was conducted once in 2018 and requires repetition in 2019, but preliminary results indicate that the evaluated spray methods and volumes may not be adequate for better spray coverage leading to poor disease management in the upper portions of the tree. In fact, the mid- to late-summer vegetative growth in the upper canopy contains the disease reservoir that may not be treated effectively using current methods.

For effective disease control, alternative spray technologies that target upper canopy should be evaluated in the commercial cherry orchards.

– Prashant Swamy and Gary Grove, Washington State University