Fireblight in ‘Aristocrat’ Pear
July 26, 2010
Trees and People
Bill Pramuk
A Problem with a Flowering Pear
Hi Bill,
I am concerned about my flowering pear tree. I just bought the home two weeks ago and a key selling point for me was the tree. I didn’t notice the dead spots until I moved in. On several branches all the leaves are dead and still hanging on.
Is this something that can be controlled or not?
Thank you,
RR
I had the opportunity to go see this tree, growing in a front yard, in a nice planned community in another city. It is an ‘Aristocrat’ Callery pear (Pyrus calleryana ‘Aristocrat’) one of the most widely planted flowering shade trees in our region. This specimen has a trunk diameter of 18 inches, which is quite large for this type of tree. With its strong looking trunk and symmetrical canopy, the tree fits nicely in the front yard and makes a handsome compliment to the home, except for the dead branches scattered through the canopy.
Looking both ways along the street I saw the same tree variety with the same problem in several other front yards. It appears that all were planted about the same time when the development was built, about 25 years ago.
A close look at the tree and the dead branches revealed the typical symptoms of fireblight: brown dead leaves that remain on the tree, blackened stems and shriveled, dead branch tips. It is a bacterial disease that attacks many trees and plants in the rose family. Long term infection can kill pear trees. The problem is made worse by persistent, warm, rainy spring weather and a supply of inoculum in neglected trees in a neighborhood.
The key question was: Can the tree be treated and saved? The answer was yes, since this tree was not too far gone. But it will require a long term commitment to a care program, and it would help if the neighbors with the same problem made the same commitment.
The bacteria that cause the disease - Erwinia amylovora - are spread from cankers on infected limbs by various flying and crawling insects. Bees and splashing water can spread the bacteria from one flower to another. The disease progresses from the flowers, or other openings in the plant downward, killing the stems and causing lesions on the bark.
Treatment for the disease is a management plan, not a cure. Dead and dying stems must be pruned off about 8 to12 inches beyond any sign of stem discoloration, back to a healthy lateral branch, is possible. Since the bacteria swim, the pruning should be done when the plants are dry to reduce the risk of spreading the disease. Most references also recommend sanitizing pruning tools by keeping them wet with a disinfectant during the pruning process. A handy one is a spray can of Lysol. Interestingly, Dr. Robert Raabe, a plant pathologist at U.C. Berkeley reported years ago that he deliberately attempted to infect plants with fireblight with “infected” tools and was unable to cause any infections.
Some references mention treatments for bark lesions on the trunk and limbs such as scraping away diseased tissue or painting them with various mixtures of chemicals. I do not know of any professionals who currently uses those methods.
In addition to eradicative pruning to physically remove the bacteria from the tree, management must include a protective spray program. One reference mentions that a dormant spray to control aphids can help reduce the spread of the disease.
The most commonly recommended and used protective spray is a weak mixture of a fixed copper compound sprayed at intervals throughout the the blooming period. A tiny amount of copper inside an open flower is enough to stop the bacteria.
Unfortunately, ‘Aristocrat’ pear - one of the most widely planted varieties of Callery pear - is very susceptible to this disease. There is no “immunization” or “induced resistance”, but a consistent management program can save trees that hold high value to their owners.
(Bill Pramuk is a registered consulting arborist. Visit his website, www.billpramuk.com, email questions to info@billpramuk.com, or call him at 707-226-2884)