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.