Streptomycin controls citrus canker on sweet orange in Brazil and reduces risk of copper burn on grapefruit in Florida

Reduced rates and frequency of copper as a bactericide to control citrus canker, caused by Xanthomonas citri spp. citri, could minimize risk of phytotoxicity (burn) to the fruit rind, development of bacterial resistance to copper, and reduce copper accumulation in the soil environment. Field trials in Brazil demonstrated that rates of 0.125x, 0.25x, and 0.5x copper hydroxide (CH), when applied at a 14-day interval, were as effective as the full rate (1.0x) of CH for controlling canker on foliage and fruit. Lower rates of CH also reduced canker-induced fruit drop of moderately susceptible sweet orange cultivars. Streptomycin sulphate (SS, Agrimicina, 15% streptomycin + 1.5% oxytetracycline) was as effective as intermediate rates of CH, whereas oxytetracycline (OT, Mycoshield) was ineffective. At 21-day interval of application, SS combined with reduced rates of CH reduced fruit disease incidence and SS combined or alternated with CH significantly reduced fruit drop. In Florida on white grapefruit, CH and copper sulphate (CS) applied at 14- or 21-day intervals controlled canker on fruit, but caused considerable copper burn to the fruit rind. Alternation of SS (Firewall, 17% streptomycin) with a reduced rate of CS provided a similar reduction in diseased fruit with less copper burn than the full rate of CH or CS. In the 2006 and 2007 Florida trials, copper burn on grapefruit occurred in July. These incidents of phytotoxicity were preceded by below average rainfall from January to June leading to drought conditions by mid-summer.