Management of fire blight with gibberellin inhibitors and SAR inducers

Apple trees sprayed with two inhibitors of gibberellin biosynthesis, prohexadione calcium (Apogee) and trinexapac-ethyl (Palisade), and a chemical that boosts plant systemic acquired resistance (SAR) to various pathogens, acibenzolar-S-methyl (Actigard/Bion; ASM), showed resistance to infection by Erwinia amylovora. Prohexadione calcium and trinexapac-ethyl reduced the terminal growth of apple trees measured 10 to 14 days after treatment and the inhibition continued for at least 30 days; shoots with suppressed growth were less susceptible to fire blight. ‘Jonathan’ trees sprayed once with 250 ppm prohexadione calcium or trinexapac-ethyl developed significantly less blossom and shoot blight following storms that occurred about 1 week after application. Two applications of 125 ppm prohexadione calcium reduced the incidence of fire blight in 1999 but not in 2000. No difference in shoot growth control was observed when 92 g of prohexadione calcium was applied in 2,800 L (250 ppm) and 700 L (1,000 ppm) of water per hectare, indicating that growth control with prohexadione calcium was rate dependant rather than concentration dependant. The best rate, application frequency, and timing of applications of ASM for disease control were established under field conditions. 'Jonathan' trees sprayed on a weekly schedule with ASM at 210 g/ha starting one week prior to bloom was more effective than a biweekly schedule and resulted in disease control similar to the standard treatment of streptomycin. On 'Fuji' trees sprayed with ASM, the severity of fire blight in inoculated shoots declined linearly as the rate of ASM was increased from 0 to 840 g/ha. Elevation of pathogenesis-related (PR) protein gene expression (PR-1, PR-2, and PR-8) was observed 7 days after ASM treatment of apple seedlings. This study shows that the effectiveness of ASM for fire blight control is less enduring than that reported for herbaceous plants and that high rates of application are more effective.