Monday May 4, 2015; 10:08 am EDT
Your Fruit Trees Will Survive
But fireblight will leave them looking burnt
As we drove by a row of Bradford pear trees that had small clumps of black leaves clinging to the stems, my friend worried that the tree was dying.
Similar black leaves on any pear, apple, crabapple or hawthorn don’t forecast death. They do show that the tree has been infected by a bacteria that causes a disease known as fireblight. The disease gets its name from the charred appearance of leaves and stems.
This disease is caused by the bacterium Erwinia amylovora, which becomes active when the plant is in flower during cold, wet periods. The bacteria enter through the flowers and migrate down into the stem. In most pear, apple, hawthorn and crabapple trees, the disease can penetrate into the main stem of the plant and can kill whole branches. However, as soon as the weather warms and dries, the progression of the disease stops.
What is interesting about Bradford pear and all selections from that species is that the disease cannot proceed into the stem. Thus only small clusters of leaves and flowers are affected. Which is why this species was selected for breeding new varieties of pears and for rootstock-grafting old varieties such as Bartlett.
The only effective method of controlling fireblight on susceptible varieties is to spray with a bactericide such as Agrimycin when the flowers are fully open. This creates a problem in Maryland, where it is unlawful to spray trees in full flower. This law was established to protect the bees.
If you have a Bradford pear tree, you need not worry because the blackened clusters of leaves will drop to the ground. If you have apple, pear, crab apple and hawthorn trees that are affected, prune out all diseased branches, sterilizing your tools with rubbing alcohol between each cut. Carry away and burn all the pruned branches.
Despite your best precautions, the bacteria are easily spread when pruning in the spring and summer. But the disease cannot spread though winter pruning, so return to these plants then and prune the blighted areas again.
To avoid the problem in the future, select resistant varieties of pear, apple, hawthorn and crabapple trees. This information is generally provided in any good and reliable nursery catalog.
You’ll have to wait to buy Chesapeake Blue
Q Can you tell me where I can buy bags of Chesapeake Blue compost in the D.C. area?
–Barbara Charlton, by email
A Chesapeake Blue, composted of waste crab shells, is no longer available. However, it should become available within a year because Maryland Environmental Services will soon be composting it on the Eastern Shore.
Ask Dr. Gouin your questions at firstname.lastname@example.org. All questions will appear in Bay Weekly. Please include your name and address.