Fire blight is very common on the Prairies. Some years are worse than others. In Calgary and Red Deer, in 2009, it was particularly severe. Epidemics occur during warm, humid weather, with relative humidity above 60%. The bacteria responsible for the disease can multiply at temperatures between 15-32C, but do so most readily between 27-29C. Outbreaks can follow severe storms, where broken branches leave entry wounds for the bacteria.
Symptoms of fire blight can affect blossoms, fruiting spurs and twigs. In Alberta, it is often too cold for fire blight to appear in the blossoms, and I usually see infected spur leaves (see picture) and cankers on twigs.
Plants grown in moist, well-drained soil, with adequate nutrition, can best resist the disease. Once infected, proper pruning will delay the spread of the disease. Infected branches should be removed to about 8″ below the last sign of bacterial canker. It is best to prune infected trees during the dormant season to limit the potential for disease spread. Avoid pruning blight infected trees in summer or when conditions favour the disease.
I encounter fire blight most commonly among crab apples, mountain ash, and cotoneaster, although it occurs on many other species of trees and shrubs. Severely infected hedges can be renovated (cut down to about 6″ above soil level) in late winter or early spring. Badly infected trees should be removed and replaced. Moderately infected trees should be pruned.
There are chemical controls for fireblight that are available from pest control professionals. Control measures consists of spraying repeatedly with copper oxychloride and streptomycin, about every 10-14 days during the growing season. Obviously, due to cost and environmental implications, I do not recommend chemical control measures, except on highly valuable landscape trees. Replace badly infected trees with fire blight resistant plant varieties.
(c) 2011 Shane LePage, Wild Rose Garden & Tree Service, Red Deer, AB