Question of the Week
I’ve inherited an orchard with four- to five-year-old apple and pear trees that have been suffering fire blight. What can I do?
Answer: First, break out infected limbs six to eight inches or more below the last visible sign of infection, and remove those infected parts from the orchard area. Do not fertilize any trees with fire blight, as the bacteria favor young, succulent growth. That's about the best you can do right now. But it’s important that you understand as much as possible before taking some action steps.
Fire blight, caused by the bacterium Erwinia amylovora, is the most serious and economically damaging disease of pears and one of the most important of apples. It is particularly troublesome in the humid eastern United States. Infection is triggered by heat and moisture, and can spread rapidly—even within a matter of hours. It can be transmitted by bees, aphids, psylla, or other insects, and can also be spread by blowing wind and rain. Pruning can be another source of infection. Affected branches wither and turn black or brownish black, as if scorched.
Most branch tips, once infected, wilt rapidly, taking on the characteristic shape of a "shepherd's crook." The bacteria gain entry to the tree through blossoms or lush, tender new growth and, once inside, begin to work toward the roots. If the disease spreads unchecked to the trunk and roots, it can kill the tree. Under the bark, the bacteria form a canker where they will survive the winter, only to infect more trees the next year.
Because fire blight development is greatly favored by the presence of young, succulent tissues, cultural practices that favor moderate growth of trees are recommended. These include using only half as much compost as for apples, never using fresh manure, and avoiding heavy populations of clovers and other legumes around the base of the tree. Likewise, heavy pruning can promote lush, vigorous growth, so such pruning should be avoided.
Choosing fire blight-resistant apple and pear cultivars is a good start to managing fire blight, but it is by no means a panacea. Relative resistance to the disease is sometimes informally measured by the extent of visible infection to the age of wood. For instance, the Kieffer pear cultivar is commonly referred to as resistant because the disease rarely proceeds beyond one-year-old wood; you might see a lot of blight on Kieffer in a bad infection year, but rarely will such infection move beyond young wood and threaten the tree’s life. Conversely, a highly-susceptible cultivar, like Bartlett pear or Gala apple, under disease-conducive conditions could incur an infection that could run from new growth to roots and kill a mature tree in a single year. Note that resistance does not equal immunity. "Resistant" cultivars can still get fire blight.
Preventing infection is the next key to fire blight control. Sprays of agricultural-grade antibiotics, applied at early bloom to prevent infection, have been the standard commercial control since the 1950s, but they are scheduled to be phased out for certified organic use in 2014 due to concerns about the build-up of antibiotic-resistant strains of bacteria. University researchers and the pear and apple industries have been diligently working to find viable substitutes to these antibiotics, and their diligence is paying off with the development of several biocontrol products.
In general, these biocontrols work similarly to one another. They are non-pathogenic competitors with E. amylovora, and as such do not directly kill propagules of E. amylovora; rather, they occupy the same
sites that E. amylovora would, provided they get there first. Therefore, in order to be effective, these should be applied to newly-opening flowers (multiple applications will probably be necessary). Since they do not directly challenge the metabolism of E. amylovora, there is little risk of the pathogen building resistance to them.
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—below the last point of visible infection, and burn them. After each cut, the shears can be sterilized in a strong bleach or Lysol® solution (one part household bleach or Lysol to four parts water) to help avoid transmitting the disease from one branch to another, although there is some disagreement about the effectiveness of disinfection. Lysol is less corrosive than bleach to the metal parts of the pruners. Some have found it more 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.
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.
For more information, consult the ATTRA publication Tree Fruits: Organic Production, available at https://attra.ncat.org/attra-pub/summaries/summary.php?pub=2.
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