How should I prune a pear tree infected with fire blight?
Answer: Once fire blight infection has occurred, there is no spray or other treatment, beyond quickly cutting or breaking out newly infected limbs, that will minimize damage. However, infection has almost certainly extended beyond what the grower sees; therefore, it is all too easy to spread the disease by trying to prune it out during the growing season. If you do cut during the growing season, remove all blighted twigs, branches, and cankers at least eight inches — some sources recommend 12 inches — below the last point of visible infection, and burn them. After each cut, the shears can be sterilized in alcohol or Lysol solution (one part household bleach or Lysol to four parts water) to help avoid transmitting the disease from one branch to another. Lysol is less corrosive than bleach to the metal parts of the pruners. If you have more than a few branches to cut out, it can be convenient to use a spray can of Lysol disinfectant carried in an apron, rather than a plastic holster or glass jar with a liquid solution. Some research has indicated that pruner sterilization is pointless, because infection has so often proceeded beyond the point where it is visible. Another option, then, to eliminate the sterilization step is to simply break out the fire blight strike with your hands where possible.
During the winter, when the temperature renders the bacteria inactive, pruning out fire blight infected wood can proceed without sterilization of pruning tools and need not extend as far below the visible canker, though it is still recommended to go six to eight inches below the last visible sign of infection before you cut.
The truly tricky aspect of removing fire blighted wood is knowing when enough is enough. Remember, anything that invigorates the tree can result in soft, rapidly growing shoots that are especially vulnerable to blight infection. And pruning, in general, invigorates the tree. If blight is severe, you could do more harm by removing too much wood, especially early in the growing season. At some point, if there is a lot of blight in your planting and conditions are favorable for blight (growth is vigorous, lots of rain, warm), it could actually be the best decision to simply wait until mid-summer or, better yet, dormancy to remove the affected wood. Such a decision is as much art as science and the pruner must weigh several factors — variety, the weather forecast, vigor or lack of it in the tree, expected fruit load (a heavy crop will slow down growth), history of blight in any given tree, etc. — before beginning.
All in all, though, research indicates that getting that blighted wood out of the orchard is important, so if in doubt, cut it out!
The same factors that the pruner must consider with regard to pruning trees actually showing fire blight must also be considered when pruning almost any apple or pear tree, with the possible exception of those in orchards that have never shown any fire blight. Otherwise, always keep in mind that over-pruning can predispose apple and pear trees to infection by the fire blight pathogen.
The new ATTRA publication Pruning for Organic Management of Fruit Tree Diseases will be of interest to you. It explains why pruning is an especially important disease-management tool for organic tree fruit growers, how to remove diseased wood and prune branches, and how doing so can control spread of disease and help prevent hosting of pathogens that cause rot. Pruning strategies for apples and pears are contrasted with techniques used for other stone fruits, such as plums, peaches, and cherries.
While you’re on the website, check out the resources in our Horticultural Crops: Fruits section, including videos, publications, and more.