UNIVERSITY PARK, Pa. -- Community trees help keep us healthy, grace us with their beauty and provide clean water and air. But if not cared for, trees can get potentially fatal diseases that may make them ugly and dangerous, says an expert in Penn State's College of Agricultural Sciences.
"The best way to prevent disease is to plant trees that are suited to the site," says Gary Moorman, professor of plant pathology. "Then, follow up with good care and maintenance by a qualified arborist, and protect trees from construction and other damage."
Early disease detection also is important, especially in ornamental trees, Moorman says. "If you suspect your tree is diseased, call your county Penn State Cooperative Extension office. They'll explain how to take samples from the tree and send them to Penn State's plant disease clinic for a free diagnosis."
Seven of the most troublesome -- and usually fatal -- tree diseases in Pennsylvania are:
Dutch Elm Disease. "This fungal disease of elms is carried by elm bark beetles, or spreads through root grafts between neighboring elms," Moorman says. "The fungus clogs the tree's water-conducting tissues. First you'll see one or two branches with wilted, yellow leaves. Then, the disease spreads rapidly, killing the tree in two to three years."
Moorman recommends calling an arborist to remove diseased trees promptly. "The arborist should debark, bury or burn all wood greater than 1/2 inch in diameter, as well as the stumps of removed trees," he says. "Where elms grow within 50 feet of each other, he or she also should dig a two-foot-deep trench around infected trees to cut potential root grafts."
To slow the course of the disease, arborists can spray insecticide during spring. They may hang sticky pheromone traps on nearby poles or non-elm trees to catch elm bark beetles and determine exactly when spraying should be done. Beetles also can be lured to infected elms with the herbicide cacodylic acid before destroying the trees.
Elms sometimes can be saved if less than five percent of the crown has been affected, Moorman says. The arborist should prune infected branches about 10 feet below the point of infection, then inject Alamo or Arbortect fungicide into the tissues at the base of the tree.
But the best tactic is to plant disease-resistant trees. "Promising cultivars include Princeton, New Harmony and Valley Forge," Moorman says. "But until they've been thoroughly tested, you should plant only a few in any area."
Oak Wilt. The oak wilt fungus kills red oaks by clogging their water-conducting tissues. "Suspect oak wilt if you see red oaks -- in late spring or summer -- shed a tremendous number of dull green leaves," Moorman says.
The fungus spreads short distances by beetles, or long distances when the infected trees are cut for firewood and the wood is transported elsewhere. The fungus also spreads through root grafts. Call an arborist to remove diseased trees immediately, and burn or bury them. The arborist can protect neighboring oaks by injecting Alamo fungicide into the base of the trunk, and by digging a trench similar to that suggested for dutch elm disease. If you prune, do so in the winter, when insects aren't active.
Anthracnose. The anthracnose fungi target sycamore, black walnut, ash, oak and maple. "Usually, this disease is only unsightly, killing a tree's twigs and buds, and making it drop a lot of leaves," Moorman says. "But native dogwoods can die if attacked for several consecutive years."
Moorman says homeowners can spray valuable trees with fungicide in the spring, as the buds open. Repeat applications during wet, humid periods, when new buds and twigs are growing.
Resistant cultivars include the Bloodgood, Columbia and Liberty clones of the London plane tree (a sycamore). The hybrids of kousa and native dogwoods -- including Celestial, Milky Way and Stellar Pink -- also are very resistant to anthracnose and the disease, powdery mildew.
Maple Decline. Sugar maple decline results from a mix of biological and site factors, Moorman says. "Maples next to roads and sidewalks are susceptible, because they're weakened by heat, poor and compacted soils, drought, restricted roots and road salt. Diseased trees show early fall color and major branch dieback. After several years, the tree dies."
Moorman suggests not planting sugar maples where they'll be exposed to harsh conditions. "Replace affected trees with better-adapted species, such as honey locust or zelkova."
Verticillium Wilt. Verticillium wilt resembles maple decline, but targets maples, redbud, catalpa, goldenrain and other street trees. "The fungus enters the water-conducting tissues through wounds in the roots and lower trunk," Moorman says. "Avoid wounding trees with lawnmowers, rotary weed trimmers or excavating equipment."
Suspect Verticillium wilt if -- in late summer -- leaves wilt, dry and fall on one side of the tree. Beneath the bark of branches one inch wide or larger, you'll see black streaks. "Verticillium isn't controlled easily, so your best bet is to extend the tree's life by promoting its vigor," Moorman says. "Water during dry periods, prune all dead wood and don't use a lot of fertilizer."
Verticillium survives for years in the soil, so susceptible trees should not be replanted in the same area. Resistant trees include yews, conifers, mountain-ash, birch, honey locust, sweetgum, crabapple, dogwood, sycamore, ginkgo, beech, white and bur oaks, pear and larch.
Fire Blight. Fire blight bacteria disfigure and kill many members of the rose family, such as crabapples, apples, mountain-ash, hawthorn and ornamental pears but not roses. "Infected trees will have cankers, which are brownish black areas on twigs, branches and trunks," Moorman says. "You'll also see dead blossoms, fruits and shoots as young flowers, leaves and twigs wilt suddenly and turn brownish black. The tree may look scorched."
The bacteria spread by water, flower-visiting insects and pruning tools. "Homeowners can prune diseased shoots, twigs and branches during dry, cold weather," Moorman says. "Then, disinfect tools with bleach or commercial disinfectant, and avoid using fertilizers that are high in nitrogen content."
Resistant cultivars of crabapples include Centurion, Harvest Gold, Prairefire and Robinson. Resistant varieties of pear include Cleveland Select and Whitehouse.
Root and Butt Rots. These commonly are caused by parasitic or wood-decaying fungi that attack a variety of trees, often invisibly weakening their structure and making them hazardous. Infected trees may topple before ever showing symptoms, Moorman says. "Symptoms include sparse, dull green foliage and dead main branches. Each year, shelf-like fungal fruiting bodies (conks) appear on exposed roots or the lower trunk. Some conks start off yellow to yellow-brown, later turning rough and blackish. Other conks may be gray, reddish-brown or cream colored."
Your best bet is to protect soils and trees from compaction, root damage, construction and other damage, Moorman says. "As soon as you see conks, hire an arborist to remove the infected trees."
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EDITORS: For more information, contact Gary Moorman at 814-863-7401.
Contacts: Kim Dionis KDionis@psu.edu 814-863-2703 814-865-1068 fax