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Rhonda Ferree's ILRiverHort

Rhonda Ferree's Horticulture Blog

Tree Diseases Abundant This Year


This seems to be the year of tree diseases, but don't be alarmed. Rhonda Ferree, Horticulture Educator with University of Illinois Extension, says "I've seen a lot of diseases on trees this year, but happily most of them are not devastating."

The reason for the increase in tree disease this year is weather. Spring weather conditions this year were perfect for many tree diseases to develop. Many plant diseases prefer wet spring conditions, which we had. In addition, the odd weather patterns the last couple of years have left many trees stressed and less able to fight off pest infestations.

Some of the diseases Ferree saw this spring and early summer include anthracnose, black rose spot, fire blight, cedar-apple rust, and apple scab.

"Black spot on rose is an all too common problem, but is especially bad this year," says Ferree. Black spot form on mostly the upper leaf surface. Leaves turn yellow except for the black spots. This disease can cause significant damage and weaken plant growth. To control it use good sanitation to remove fallen leaves that have the fungus. "This disease favors wet and warm weather, so if the rains ever stop it should diminish." Most newer roses, such as knockout, have resistance to this disease.

"Anthracnose can attack many trees, but is most common on sycamore." This year it has also attacked many oaks leaving some tree owners thinking their trees are dying. Actually, Ferree says that the trees are just fine, and in fact most sent out new leaves already this summer.

"Cedar-apple rust and apple scab cause unsightly spots on the leaves of apples (including susceptible ornamental crabapples)." If severe "enough, the spots can entirely cover the leaves, causing the leaves to fall from the tree." The best way to control these diseases is to plant a resistant variety. Otherwise, use good sanitation and rake up all fallen leaves and debris to remove the fungus. Sprays are done early next spring, but timing is difficult.

Ferree reports that University of Illinois Extension Master Gardener Plant Detectives have fielded numerous fireblight calls this year. Fireblight is the most common and destructive bacterial disease of apples and pears (including ornamental pears). The disease is so named because infected leaves on very susceptible trees will suddenly turn brown or black, appearing as though they had been scorched by fire. The end of the branch may bend over, resembling a shepherd's crook or an upside down "J".

Fireblight is one of the most difficult diseases of apple and pear to control, and there is no one procedure that will give complete control. "We are long past the time when chemical sprays would help," says Ferree. "We recommend removing blighted branches as they occur." "Preferably, this should be done in dry weather and tissue removed from the site." "Remember to disinfect pruners before every cut and to make cuts 10 inches beyond the blighted area." "Do not fertilize infected plants now." "That would encourage succulent new growth, which is susceptible to infection."



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