Disease problems dog trees around state

COLUMBIA, Mo. – Two diseases this spring are making trouble for some tree species around Missouri.

Fire blight, a bacterial infection, and the fungal disease anthracnose gained footholds in some trees, aided by a stint of cool, wet weather in April and May.

“This spring presented a good illustration of the disease triangle, in which you need a susceptible host, a pathogen and a conducive environmental condition,” said Chris Starbuck, a University of Missouri Extension state specialist in woody ornamentals. “In some years, the right conditions bring on raging problems with fire blight in ornamental pear trees, crab apples and apple orchards, where it can be a major headache.”

Dead, blackened branch tips and leaves are a telltale sign of the disease. “They call it fire blight because the tips appear to have been burned by a blow torch,” Starbuck said. “You’ll get wilting initially that leads leaves to curl over into a characteristic shepherd’s crook that’s black and shriveled.”

Fire blight is one of several pernicious diseases of apple and crab apple trees, he said. Cool, wet weather puts increased disease pressure even on so-called blight-resistant varieties. Because the bacteria prefer succulent plant growth, fertilizing trees can make them more vulnerable to infection.

Plants in the Rosaceae family, like hawthorn, spirea and cotoneaster, as well as trees like Bradford pear are most susceptible. Most reports this spring involved ornamental pear trees.

The disease typically begins when bees spread it during pollination. As they travel from flower to flower, the infection will take hold in the flower clusters then move through branch tips and into branches. In some cases, fire blight can cause lesions on leaves.

The main defense against fire blight involves pruning out infected branch tips.

“Fire blight can actually spread down into the main trunk,” Starbuck said. “If you can’t prune out the fire blight strikes, the infection may spread to other parts of the tree.”

Infections in larger apple orchards are treated with antibiotic spray, but Starbuck warns that homeowners can promote bacterial resistance to antibiotics if they are not careful with this approach.

Anthracnose is another threat to certain tree species in early spring. Sycamores, maples, ash and some oaks are most commonly affected. Although many trees dodged the bullet by leafing out early, some trees like ash and sycamore didn’t avoid the infection.

Anthracnose is actually a catch-all term for a number of fungal diseases that cause blotches on leaves where water collects in pockets between the veins, Starbuck said. “The spores collect there, germinate and get started in one spot. The lesions tend to look tan or black on sycamore, coalesce and cause the leaf to be distorted or fall off.”

There’s little you can do to combat anthracnose once it becomes apparent, but raking and disposing of dead leaves will limit its spread.

Homeowners should also use this year’s outbreak as a hint to treat their trees with fungicide early next spring.

“Anthracnose goes dormant when weather gets warmer, but you know the inoculum will be there next year,” Starbuck said. “Homeowners should be ready when conditions return next spring and spores are released.”

While it might be unsightly, anthracnose has more bark than bite. 

“Anthracnose tends to alarm people because lots of brown or black leaves on your tree make people wonder if their tree is going to die,” Starbuck said. “In general, it looks a lot worse than it is, and in the case of sycamore, most people by the end of the season will forget they had anthracnose.”

The MU Extension publication “Fire Blight” (G6020) is available for free download at extension.missouri.edu/G6020. Read more about anthracnose at soilplantlab.missouri.edu/plant/diseases/anthracnose.aspx.