Link to Picture: (A "Short-cut" apple limb). Young pears must be cut more aggressively than older apples. However, this range of situations depends upon the age, the apple or pear cultivar's relative wood susceptibility, the susceptibility of the rootstock and vigor of the tree in trouble. On most older pears and apples, the running strike usually stops when it reaches older (3-4 year old) wood. On young pears or apples, the trunk may be fatally damaged.

Due to the youth and vigor of the wood, the bacteria move rapidly to, and down the trunk to the root stock, often killing the tree.(Fire Blight & Young tree). Recent research has determined that, by the time you discover fire blight symptoms on a young apple tree, the fire blight bacteria probably have already spread internally down the tree, without causing visible symptoms, and the blight may then kill some susceptible rootstocks, even after you remove the strike. (Dead Rootstock Picture).

Some older trees, such as Bosc (Alexander) pears, can also sustain considerable damage, especially if they are growing vigorously. On highly sensitive varieties such as Bosc and Red Sensation pears, numerous strikes near the trunk may lead to death of the tree, unless the limbs are quickly removed. (Bosc Orchard Being Removed) Older Bartletts (Williams) usually tolerate blight, unless the number of strikes per tree is extreme, such as what might happen with primary blossom infection.

You may read that the "rule of thumb" distance you must cut "below" the strike is about 18 inches. The actual distance you must cut to remove most (never all) of the bacteria from the tree is affected by the following factors:

1. The growing vigor of the affected portion of the tree. Young, rapidly growing portions must have more wood removed than older, low vigor parts of a tree.

2. The angle of the growth. Upright wood should be cut

a greater distance below the strike than low angle or pendant wood.

3. The age of the tree. Young trees are easily killed by cankers on the trunk or by sensitive rootstock death (Ottawa 3, M9, M26 and M106 are considered highly susceptible, Bud 9 and CG 41 are resistant).

It is easier to regrow parts of an established tree than to grow

a replacement tree. You should probably remove the entire limb (to a "Dutch cut") on a first or second leaf apple or pear tree. Be brutal. Older apple trees tolerate quite a few strikes, and, IF the infections are too numerous to practically remove, you might as well walk away. Try to clean the trees up the next winter, and hope that most of them live. If you try this on younger apples or pears, expect rootstock/collar blight and tree death for the next two seasons. (Don't try the "walk away" control method on orchards that (1.) has a relatively low number of strikes, so can be cut, or (2.) are less than 5 years old.) You will remove less wood from an older pear tree growing at normal high vigor if you cut blight when it appears, rather than waiting until winter.

4. The cultivar of the tree. As mentioned, Bosc trees are a special case. Wood sensitivity varies by variety and growing vigor. Many growers remove most of a "struck" Bosc limb, hoping to save the life of the tree. Ginger Gold and Cripp's Pink seem to be especially sensitive apple varieties. At the other extreme, the wood (but not necessarily the blossoms) of Red Delicious seems especially resistant to blight attack. Each cultivar has it's own level of wood resistance, which is poorly documented, especially for new varieties.

5. The time of season. Once the blight has stopped running and has formed defined cankers, usually July in the Northern hemisphere, you might as well just cut at the first "horticultural" cut below the strike, as long as that cut is about six inches to a foot (30 cm) below the canker.

6. Length of time since infection. Cutting blight in an orchard is something like a race. You against the blight. The earlier you remove the strike, the less total damage will be done.

When cutting actively growing portions of the tree, you are usually told to sanitize your cutting tools to prevent spreading the bacteria to newly cut surfaces. If you are cutting older wood in dry weather, there is very little experience that shows this disinfection to be important. It is easy to infect shoot tips and young fruit if you roughly remove a strike through the foliage, so it is probably more important to take care to lift the cut strike out of the tree, rather than letting it fall through the foliage below.

Chlorine (sodium hypochlorite) products are commonly recommended for cleaning cutting tools. If you use these, remember that the 1 part in 10 solution must stay clean and fresh and must wet the tool for about 5 minutes to be fully effective. A 1 part in 5 solution is effective in about 1 minute of wetting. These products are rough on your tools, not to mention your clothing.

There is NO benefit gained from spraying cut surfaces on the tree with chlorine-containing products. The chlorine binds with the wood and becomes inactive almost immediately. Cut the tree in a manner that does not contaminate your tools.

There are other effective sanitizers sold for cleaning bacteria from restaurant, hospital, and food processor surfaces. These products are less damaging to your cutting equipment. Some of these "Quaternary Amine" cleaning solutions may be available from agricultural suppliers in the near future.

Some growers have used trigger-ignited propane hand torches to lightly heat their tools during cutting. Remember, the bacteria cannot tollerate 160 degrees F, which is well below the boiling point of water, so it doesn't take much heat to sterilize the tool. Too much heat might take the temper out of the metal.