PPDL Picture of the Week
April 11, 2016
Botryosphaeria and Fire Blight on Crabapple
Tom Creswell, Plant Disease Diagnostician/Director, Purdue Plant & Pest Diagnostic Lab
As spring approaches and buds are beginning to swell many
people are taking more notice of trees and shrubs in their lawns. Crabapple trees are one of the best flowering
specimen trees for the Midwest yet they have their share of problems. Two diseases you might notice on bare
branches at this time of year are cankers and diebacks due to Botryosphaeria
and fire blight. Both have similar symptoms but very different causes.
Botryosphaeria dieback and canker (Fig. 1) is caused by any
of several closely related fungi that tend to invade wounded, stressed or weakened
trees. The fungus infects many different trees and shrubs, and spores are
spread by wind and wind-blown rain. Many older varieties of crabapple are
highly susceptible to apple scab, a different
fungal disease, (Fig. 2) and routinely defoliate during the summer. This
type of stress from premature defoliation can attract insect pests (borers)
making trees more susceptible to infection by Botryosphaeria. Improper pruning
practices, injuries and cankers from fire blight (a bacterial disease) can all
provide infection sites for the fungus. Drought stress will further reduce tree
defenses and allow cankers and dieback to advance more quickly.
Fire blight (caused by bacterium Erwinia amylovora) (Fig 3.) causes a dieback that can become a
canker when it moves down a small branch to infect a larger branch. As the tissue
on the larger branch dies and dries out it becomes cracked and sunken.
To make matters more confusing both pathogens
(Botryosphaeria and Erwinia amylovora) may be present together in canker tissue we
examine in the lab (Fig. 4).
Fungicides sprays are not effective against Botryosphaeria
because it is inside the wood and can’t be reached. Pruning out infected
branches several inches below any sign of discoloration can help if caught
early, but it is important to have a correct diagnosis of the cause, as fire
blight infections require more extensive pruning. Trees that get routine good care are more
resistant to infection so proper pruning, fertilization, mulch and irrigation
are your first line of defense.
Fire blight management is discussed in the publication
BP-30-W: Fire Blight on Fruit Trees in the Home Orchard (https://www.extension.purdue.edu/extmedia/BP/BP-30-W.pdf).
The best solution to preventing fire blight in crabapple is
to choose a resistant variety to plant in your yard. If you are managing a
susceptible variety pruning during dormancy is the best treatment. Note that
use of antibiotic sprays is NOT recommended for ornamental trees in the
landscape to help avoid development of antibiotic resistance in bacteria that
may compromise control in commercial orchards.