Streptomycin and Copper Formulations for Control of Citrus Canker on Grapefruit

Control of citrus canker (caused by Xanthomonas citri subsp. citri) on grapefruit is required to reduce blemishes on fresh fruit. The objective was to evaluate formulations, rates, duration of residues for copper materials, combinations of copper materials and streptomycin, and application timing for effective canker control on young, fruiting grapefruit trees. Copper applications were made at 21-d intervals starting at spring flush in March until fruit were fully expanded in October. The period of fruit susceptibility to canker was established by applying sprays at 21-d intervals through the fruit growth period, with separate treatments ending spray application at each interval so that duration of protection varied and number of applications ranged from 3 to 11. The risk and predisposing factors for fruit phytotoxicity were also assessed. In addition, the residual activity of Kocide 3000 (copper hydroxide, Dupont) on fruit, as affected by the adjuvant polymer delivery system (PDS), was measured at 7, 14, and 21 d after application. Copper formulations containing copper hydroxide or basic copper sulfate (metallic rates of 0.67 to 1.12 kg/ha), varied from low to moderately effective for canker control depending on late-season, wind-blown rains. Copper pentahydrate, at a lower metallic copper rate/ha per application, provided equivalent control to other copper formulations. Streptomycin (FireWall, Agrosource, Inc.), alone or with a reduced rate of Kocide 3000, in July and early August gave equivalent control to Kocide 3000 alone. Copper phytotoxicity risk coincided with accelerated fruit growth after summer rains. Greater canker susceptibility of fruit in later season is likely because more prolonged opening of stomates in cooler weather may enhance bacterial entry, when coincident with more numerous windblown rain events. The only factor that led to significant reductions (P < 0.0001) in copper residue per fruit surface area over time was the Kocide 3000 concentration. PDS had no effect on the residue.