Effectiveness of Fosetyl-Aluminum and Streptomycin Alone and In Combination for Control of Blister Spot on ‘Mutsu’ Apples in Ohio and New York

Abstract

The efficacies of fosetyl-Al (Aliette 80 WDG) and streptomycin (Agri-mycin 17 WP) were evaluated alone and in combination for control of blister spot of ‘Mutsu’ apple in Ohio and New York. In both test orchards, streptomycinresistant strains of Pseudomonas syringae pv. papulans were present. The results of field tests conducted in two consecutive years in Ohio were very similar. Fosetyl-Al and streptomycin applied alone resulted in significantly fewer infected fruit and significantly lower disease severity than the nontreated control. In 1997 only, the combination treatment of fosetyl-Al and streptomycin resulted in significantly less infected fruit and significantly less disease severity than the other treatments. For field experiments conducted in two consecutive years in New York, streptomycin and fosetyl-Al plus streptomycin resulted in significantly fewer infected fruit and significantly less disease severity than the untreated control, with no significant differences between treatments. Fosetyl-Al decreased disease severity and increased the percentage of marketable fruit in 1995, but it did not in 1996. The results suggest that fosetyl-Al and streptomycin usually provide a significant level of blister spot control when applied alone. When applied in combination, they may have an additive effect that increases the level of disease control. However, the level of control under the heavy disease pressure that occurred in both years of testing in Ohio was still less than 50% as compared to the nontreated control. The level of control obtained in New York tests was also unsatisfactory. Results indicate that fosetyl-Al and streptomycin alone or in combination are not highly efficacious in orchards where streptomycin-resistant strains of P. syringae pv. papulans are present, and their use in such orchards is not economically feasible. The lack of effective chemical controls could increase the economic importance of blister spot in the U.S. Accepted for publication 28 November 2000. Published 4 December 2000.