Managemt of Apple Blister Spot Caused by Streptomycin-Resistant Populations of Pseudomonas Syringae pv. papulans

In 1992, 34% of the Pseudomonas syringae pv. papulans (PSP) were resistant to streptomycin in a Mutsu apple orchard where streptomycin was regularly sprayed. The resistance determinant in PSP was detected on plasmids of either 94 or 132 kb which transferred among PSP at very iow frequencies or not at all In contrast, prominent resistance plasmids of 83 or 93 kb were carried by a common foliar and fruit epiphyte (a non fluorescent yellow Pseudomonas sp., closely related to P. syringae) and were transferred to PSP at very high frequencies (> 10-1(H per recipient). Applications of streptomycin in the orchard reduced the total number of epiphytic PSP and increased the proportion of resistant strains up to 99.3%. The proportion significantly declined after overwintering in both streptomycin-treated and untreated plots indicating that resistant strains have reduced fitness. Treatments were tested for control of PSP in the orchard four years after the last use of streptomycin (12.8% of PSP resistant). Aiiette plus streptomycin and streptomycin plus terramycin gave the best control. Aliette plus potassium bicarbonate and streptomycin alone also provided satisfactory control. Aliette and Quest, a water conditioning agent, demonstrated strong bactericidal effect on P. syringae pv. papulans in vitro. Buffers and commercial spray compounds were tested for their effect on the bactericidal activity of streptomycin. Activity was greatly reduced when streptomycin was dissolved in phosphate (0.02 M) containing liquids or when mixed with Cor-Ciear (34.5% Ca), Quest, or Nutra-Phos 24 (a foliar fertilizer).