LAUSR

To reduce the severity of shoot blight and prevent the resulting development of cankers on perennial apple wood, we evaluated eight fire blight post-infection spry programs of prohexadione-calcium (PCA) alone or with acibenzolar-S-methyl (ASM) over two years. On mature trees of cv. Royal Court, a single application of the high PCA rate (247 mg/L) at 2 to 3 days after inoculation resulted in 89.5 and 69.5% reduction of shoot blight severity after inoculation. Two applications of PCA 247mg/L, 12 or 14 days apart, with the first one applied 2 to 3 days after inoculation, resulted in 78.8 and 74.5% reduction of shoot blight severity in both years. A 100% control of canker incidence on perennial wood from infected shoots in both years was achieved with a single application of PCA (247mg/L) applied at 2 or 3 days after the inoculation, while three applications of PCA (125 mg/L) + ASM (25mg/L) 12 - 16 days apart, reduced canker incidence by 83.5 and 69% in the two years. The other programs with lower PCA rates and frequencies of application reduced shoot blight severity for 50.8 and 51.8% (PCA) and for 62.6 - 72% and 59.3% (PCA + ASM), over two years, respectively. Reduction of canker incidence on wood by other programs was 66.5% and 69 - 90.4% in the two years, respectively. As fire blight cankers lead to death of dwarf apple trees and serve as primary sources of inoculum, our effective PCA and PCA + ASM programs could serve as viable post-infection management options. These treatments can reduce or prevent canker development and thus significantly abate tree losses in high-density apple orchards after fire blight epidemics occur.