Fire blight: A Formidable Disease on the Rise

Written by: Tony Arnoldi, Board Certified Master Arborist WI-0102B

Posted: 2019 | Disease Treatment | Spring

Despite the name, arsonists are not responsible for this condition! Fire blight is a bacterial disease that only infects members of the Rose family (Apples, Crabapples, Pears, Mountain ash, Hawthorn, Serviceberry, and some others are the most commonly affected, but the host list is quite long).

In 2018, Fire blight was observed on a great number of Crabapples – on more trees than what is usually seen and the symptoms were much more severe and extensive.

Fire blight attacks blossoms and succulent branch or sprout tips. As the bacteria invade the tree tissue, blossoms, leaves, and young bark turn brown or black. It appears water-soaked at first, but soon dries. The ends of the young shoots may curl back upon themselves, forming a “shepherd’s crook.” The affected tissue appears to have been scorched by fire, hence the name “fire blight.”

In 2018, infections began to appear right after flowering, because bees often carry the bacteria to the flowers (from an infected tree) when they are pollinating them. Fire blight progresses fast and can kill entire limbs or the whole tree if weather favorable to the disease persists long enough. The warm, moist and humid conditions of the majority of 2018’s growing season were perfect for rapid development, quick distribution and spread. The more damaging infections progress to involve larger woody branches and sets up cankers in the bark. These cankers may last many years and allow the tree to infect itself in subsequent years when wet, warm and humid conditions return. They then exude a bacterial ooze that runs down the bark during rain events, infecting more bark along the way. Progression of the disease quickly slows when hot, dry conditions prevail.

Because of the build-up of the infection last year we expect symptoms to resume this spring, when rainy weather returns.

Control can be difficult but removing fallen leaves, twigs or fruit that has been infected is useful to reduce the infection sources. Pruning out infected branches is also recommended; disinfecting the cutting tool between cuts is necessary. The fungicide spray program for controlling Apple Scab, the common fungus disease of the leaves, unfortunately does not control Fire blight. Certain soil-applied copper treatments may provide management of the symptoms.

Although difficult to control we have provided care that has successfully managed Fire blight on a great many trees. Please contact your Wachtel Tree Science Certified Arborist resource to help you diagnose and manage this important issue.