Effect of phosphorous acid and pruning height on renovated 'Woodard' rabbiteye blueberry

Phosphorous acid is a fungicide that may have other plant health-inducing properties. A previous study showed greater vigor in blueberry plants treated with phosphorous acid. In the current study, old, low-vigor ‘Woodard’ rabbiteye blueberries were selected for renovation in 2017. The bushes had not been pruned or otherwise managed in several years. To test the hypothesis that adding phosphorous acid treatments (Agri-Fos) to the renovation process would increase plant vigor, three treatments were used: no addition of phosphorous acid, phosphorous acid applied as a drench at renovation pruning plus in the following spring, and drench plus monthly foliar applications of phosphorous acid after renovation pruning and again the following year. Renovation pruning was done at two height treatments on 14 July 2017: bushes were pruned at ground level and at 50 cm resulting in a 2 x 3 factorial. End of season shoot number was significantly different between the 50 cm cut and the ground level cut treatments (35.6 shoots/bush vs. 21.3 shoots/bush, respectively). However, shoot length was significantly greater for the ground cut (80.5 cm) vs. 50 cm cut (72.1 cm). Shoot diameters were not significantly different. Other factors such as first bud push after pruning and crop load rating were not significant. Yields were higher for the 50 cm cut treatment when compared to the ground level prune (318.7 g vs. 58.1 g, respectively) with similar results for average berry weight (1.78 g vs. 1.49 g, respectively). Interestingly, phenological development recordings of flower stage did not differ at stages 2 to 4, but were different in stages 5 and 6. Flower development on bushes in the 50 cm treatment was earlier by nine days at stage 5 and nine days at stage 6 compared to those in the ground cut treatment. There were no significant differences due to phosphorous acid treatments in any data evaluated, nor any significant interactions between phosphorous acid treatment and pruning height treatment. More time may be needed to ascertain any differences among phosphorous acid treatments as its effect may be induced rather than direct.