Refinement of Peach Cover Spray Programs for Management of Brown Rot at Harvest

Peach cover spray applications of the protectant fungicide captan were previously shown to significantly reduce brown rot caused by Monilinia fructicola during the preharvest fruit ripening periods in the 2012 through 2015 growing seasons. The protectants sulfur, ziram, and thiram failed to yield this benefit. Percentage disease control with captan ranged from 50 to 69%. Results of a bioassay indicated that the mechanism for this control was the creation of an effective, persistent fungicide residue on the fruit surface. Given these findings, the current 2017 to 2018 study was initiated to further refine the cover spray program. Cover spray applications of captan were made at lower rates and fewer timings with the goal of minimizing fungicide usage while maintaining control. High concentrations of the protectants sulfur and ziram were also examined in cover spray programs to determine whether greater concentrations could improve control. Results of the captan treatments from both years showed that the concentration could be reduced 17%, from 3.36 to 2.80 kg/ha active ingredient, without a significant increase in rot at harvest. Disease control at this medium rate was 69% in 2017 and 51% in 2018. The late season timing treatment, which consisted of the final two cover sprays at fifth and sixth cover, significantly reduced brown rot at harvest and provided control equivalent to the full cover spray program consisting of seven applications. Thus, a buildup of residue from many cover sprays is not needed to achieve control. As hypothesized, the midseason treatments, which consisted of two sprays at third and fourth cover, did not provide control of brown rot at harvest. The bioassay confirmed that insufficient residue remained on fruit for adequate control. However, the early season treatment, which consisted of sprays at shuck split, first cover, and second cover, provided 40% control, even though the bioassay showed that an effective residue was not present during the preharvest period. Brown rot management for this treatment was probably caused by inhibition of quiescent or latent infections on young green fruit. If confirmed, this novel finding indicates that high levels of latent infections are possible in eastern U.S. peach growing regions. Finally, higher rates of sulfur and ziram cover sprays were still ineffective for providing brown rot control at harvest. Comparison of half maximal effective concentration values calculated from the dose-response models confirmed that the sulfur and ziram intrinsic efficacies were too low for adequate control, even at the highest registered rates. These findings demonstrated that late season captan cover sprays can contribute significantly to control of brown rot at harvest, thereby augmenting the efficacy of preharvest fungicide programs. The year-to-year consistency of control should also be improved because heavy rainfall during the preharvest period did not reduce control by the captan residue. Furthermore, any reduction of the M. fructicola population by the captan cover sprays should reduce selection pressure against the site-specific fungicides commonly used during the preharvest period. The development of resistance to captan, a multisite protectant fungicide, is not likely, so this resistance management strategy should be sustainable.