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.

Click to enlarge picture

Figure 1. Botryosphaeria canker on the trunk of a crabapple tree.

Figure 2. Apple scab infection on crabapple; early symptoms on left and severely infected leaf on right.

Figure 3: Fire blight (caused by the bacterium Erwinia amylovora) can cause rapid dieback on susceptible crabapple varieties.

Figure 4: This canker appeared to have started with fire blight but was later infected by Botryosphaeria and attacked by borers, leading to more damage.