by Michael Celetti, OMAFRA Plant Pathologist Horticulture Crops Program Lead
Fire blight is a serious disease of pear and apple. Last year in 2014, several apple orchards in Ontario, Quebec and the Maritime provinces where severely infected with this disease. It is this time of year that the symptoms of the shoot blight phase of fire blight begin to show up in infected orchards. Although symptoms are obvious now, the infection most likely occurred during bloom. This year, the Cougar Blight models predicted at least 3 periods during May when conditions were favourable or extremely favourable for the infection of open blossoms by this bacteria pathogen (see http://www.omafra.gov.on.ca/english/crops/facts/fireblight-2.htm or http://www.omafra.gov.on.ca/french/crops/facts/fireblight-2.htm). Orchards should be scouted for fire blight symptoms every 7 to 10 days until terminal bud set or the end of July regardless if streptomycin, Kasumin, or Blossom Protect was applied at bloom.
Growers, who find fire blight in their orchards should prune out and remove the infected shoots promptly, particularly if it is a young orchard between 1 to 3 years old and there are only a few strikes per tree. When removing infected shoots, be sure to cut at least 30 cm (12 inches) beyond the water soaked margin of the infected shoot, preferably into healthy 2nd year wood. Only prune out the fire blight strikes when there is a stretch of 2 to 3 consecutive days of dry weather forecasted with low humidity and temperatures below 25oC. It is best to disinfect the pruning tools between each cut by soaking them in a solution of 1 part bleach to 5 parts water or in a 65% to 70% alcohol solution. To be effective, the pruning tools should remain in the solution for about 2 minutes before reusing. It is a good idea to supply a couple of pruning tools per pruner so that while one tool is being disinfected the other can be used to prune out the trikes. The infected shoots can be thrown in the grassy pathways where they will dry up within a few days and not be infectious. The prunings in the pathway can then be mowed after they dry up to further break down the infected shoots.
If the trees have so many strikes that pruning at this time of year will stimulate the tree to send out more succulent shoots, it may be best to remove the entire tree. Trees with succulent shoots are more susceptible to fire blight infection. If tree removal is necessary, remove the entire tree, roots and all. If the tree is small enough, cut it up and place the infected tree pieces into a large garbage bag, seal the bag and leave it in the center row for a few days to allow it to heat up and kill the pathogen. Do not attempt to drag an uncovered infected tree through the orchard, since this may result in spreading the fire blight pathogen.