EFFECT OF COPPER BACTERICIDES ON COPPER-RESISTANT AND COPPER-SENSITIVE STRAINS OF PSEUDOMONAS-SYRINGAE PV. SYRINGAE

Fourteen formulations of copper-based bactericides were evaluated for their efficacy in reducing populations of copper-resistant and -sensit ive strains of Pseudomonas syringae pv, syringae growing on tissue-cul tured lilac and of copper-sensitive strains of this pathogen on field- grown lilac. The amount of free cupric ions (Cu2+) in solution was the only predictor of formulation efficacy, but this variable could not b e estimated from the metallic copper content of the product. Relative to nontreated controls, all copper-based bactericides reduced the popu lation size of copper-sensitive strains by 50%, but only cupric hydrox ide mixed with mancozeb or ferric chloride reduced the population size of copper-resistant strains by an equivalent amount. Several noncoppe r bactericides, including streptomycin-sulfate, caused only small redu ctions in bacterial populations on tissue-cultured or field-grown lila cs. In the field, two applications of cupric hydroxide (wettable powde r) when plant growth stages were at dormant (mid-February) and delayed dormant (late February) provided better control than either one or no treatments.