Remove Fire Blight Cankers in the Winter

Fire blight is a one of the most serious diseases of apples and pears worldwide. Over wintering cankers are the primary sources of fire blight bacteria for subsequent infections. During the spring under warm, humid conditions, the pathogenic bacteria multiply on the edge of cankers formed the previous year and are disseminated by insects, wind, and rain splash to susceptible tissue.

Removing cankers as well as severely infected trees and blighted limbs is the first step to reduce over wintering bacteria. Late winter, when temperatures are still cold but not cold enough to cause winter injury, is the best time to remove over wintering cankers. The bacteria are still active in the fall, so pruning too early can spread fire blight, rather than remove the disease. What you can do in the fall is take a walk through the orchard and mark obvious fire blight strikes and cankers with spray paint - that will make your winter pruning job easier.

It is particularly critical to prune out cankers formed the previous year on the larger susceptible trees. These cankers generally have smooth margins and are the ones most likely to become active in the spring. When removing cankers, it is important to cut well into healthy tissue so that all of the infected tissue is removed. The bacteria in prunings cut in the winter will probably die, but it is still a better strategy to remove all fire blight prunings from the orchard and burn them. Disinfecting pruning tools between cuts is normally recommended when pruning for fire blight but is probably not as critical when doing winter pruning because the bacteria is not active at that time.

Pruning out cankers on the main trunk or large limbs may not always be practical or pruning may significantly alter the structure of the tree. Often these types of cankers develop where a blighted spur or smaller branch meets the main trunk or large limb. If the canker does not surround more than half of a large branch or trunk, it may be removed by scraping. In this situation, it is best to cut off the blighted branch or spur flush with the larger branch or main trunk. Using a pruning knife, scrape away all of the bark in the canker area as well as the surrounding healthy bark at least to 2 cm (approximately 1 inch) from the margin of the canker. The bark should be scraped or cut perpendicular to the branch to encourage rapid healing and callus formation. When done properly, the scraped area will appear as an oval with the points orientated longitudinally with the axis of the large branch or limb. Swab the wound with either a 70% denatured alcohol or 10% bleach solution and cover with tree paint or a commercial wound dressing.

If fire blight was encountered in the orchard last season, you may have used what is known as the "ugly stub" method for pruning it out. This method required the fire blight strikes to be cut out when symptoms first appear last season and before extensive necrosis developed. The cuts should have been made about 12 inches or more below the visible symptom, into 2-year old wood or older, leaving at least a 5 inch naked or "ugly" stub. If you notice a small canker on the ugly stub, don't worry, the tree usually confines the disease to the ugly stub. Winter is also the time to remove all ugly stubs from the orchard when temperatures are too cold for the bacteria to be active.

Good fire blight management is a multi-phase and year long program that begins with the removal of all infectious tissues regardless of how insignificant they may appear. There is no better time than winter to start implementing the first phase- removal of over wintering cankers. For more information on fire blight management, see OMAF factsheet "Fire blight of apple and pear in Ontario".

Learn more:

• Fire Blight of Apple and Pear in Ontario, OMAFRA Factsheet Order No. 02-011
  

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