ERWINIA AMYLOVORA INFECTION OF FLOWERS AND SHOOTS IN APPLE TREES TREATED WITH PROHEXADIONE-CA

Resistance elicitors are excellent candidates for controlling fire blight caused by Erwinia amylovora. Two of these agents are prohexadione-Ca (ProCa), the active ingredient of Regalis®, and acibenzolar-S-methyl (ASM) in Bion® 50WG. Mature trees of various apple cultivars were treated before bloom (at pink bud stage) with Regalis® (180 ppm of prohexadione-Ca) and Bion® 50WG (50 ppm of ASM), respectively. Detached flowers were incubated and the next day inoculated with E. amylovora (strain Ea1). Three days after inoculation the population size of E. amylovora was estimated by semi-quantitative PCR. On the 5th day the flowers were scored for disease development, using the rating scale of Pusey (1999). No correlation was found between the population size and the severity of the disease symptoms. Nevertheless, the flowers of susceptible cultivars tended to harbor larger pathogen populations than the less susceptible ones, and both resistance inducers reduced the population size by one order of magnitude. The extent of necrosis in the receptacle (nectary and ovary) was reduced by ProCa treatment. One-year-old, container-grown trees of ‘Idared’/M.9 and ‘Freedom’/M.9 were treated with ProCa or ASM using doses and timing recommended for field conditions. Vigorously growing shoots were inoculated by cutting through the youngest leaflets with a scissors that had been immersed in a 10⁷ cfu/ml suspension of E. amylovora. ProCa was more effective in reducing the length of the developing fire blight lesion than streptomycin. Preliminary experiments with ASM suggested a similar effect.