A dry spring may indeed reduce some common tree diseases such as aspen leaf spot, anthracnose and fire blight. So far so good, but who knows what May will bring.
Aspen leaf spot is one of the most common problems with aspens, says Jerry Goodspeed, Utah State University Extension horticulturist. This fungal disease which is spread by wind and rain attacks the leaves as they begin to emerge in the spring.
Aspen leaf spot causes small dark spots on the leaves, he explains. In late spring and early summer, these spots are less noticeable, so most aspen owners forget to give them proper treatment or care. As the summer progresses, these spots enlarge and seem to envelope all the leaves and the entire tree.
"Severe infestations can defoliate the tree, weakening it and leaving it more vulnerable to other diseases and insects," Goodspeed says. "Regardless of the severity, it is always aesthetically unappealing to the tree owner. Unfortunately, by the time the damage is noticed and the owner goes into a mad panic, nothing can be done but start planning the funeral."
The timing for treatment is now, when the disease is spreading and the amount of damage is small, he says. Apply a registered fungicide, such as Daconil, that has aspen and leaf spot listed on the label. Be sure to follow all label directions.
Anthracnose is another disease that spreads in the spring, but usually goes unnoticed until summer, after the time of treatment has passed, he says. It can attack maples, sycamores, oaks and ash trees. Again, the problem is worse when spring weather is wet and cool. It is similar to leaf spot because once it is really noticeable, it is too late to do anything.
Anthracnose looks like small spots on leaves that appear to be water-soaked, Goodspeed explains. These spots, which are usually found along the veins, enlarge and eventually turn red, brown or black. It spreads to the petiole of the leaf, and can eventually infect small twigs and branches. Severe cases defoliate the tree and cause some twigs and smaller branches to die.
"Treatment is similar to that for controlling leaf spot," he says. "In addition to applying a fungicide now, the leaves should be raked up and destroyed in the fall. If possible, keep sprinklers away from the foliage of these trees to keep the leaves as dry as possible. Some sycamores are just too big to spray unless you hire a helicopter to do the job. Normally anthracnose and leaf spot are not fatal diseases, even if left untreated."
Fire blight, however, can be fatal, Goodspeed warns. This is not a fungal disease like the other two, but a bacteria that infects pears, apples, cotoneasters, hawthorns, pyracantha and mountain ash. Trees and shrubs infected with this disease have leaves that wilt and dry. The leaves and twigs turn brown and black, and appear to be scorched -- hence the name fire blight. Close examination of the affected area will reveal a white, cream or reddish-colored ooze coming from wounds. Often the twigs will take on a shepherd's crook shape as they wilt.
Fire blight is spread during warm, wet springs, he adds. It is generally carried by insects from flower to flower. It then spreads down the twigs from the infected flowers. Severe cases of fire blight can kill a pear tree in one season. Normally, however, this disease does not move that fast. If fire blight is noticed later in the summer after bloom, the only treatment is to remove the infected wood. Cut the twig or branch 8 to 12 inches below the infection and remove the wood from the landscape or orchard.
"Control measures for most of these diseases are taken when the trees are in bloom," Goodspeed says. "If it doesn't rain or the trees don't receive moisture during the bloom time, treatment generally isn't necessary. However, if the spring weather is warm and wet while the trees are in bloom, apply a fire blight spray (containing streptomycin) or a fixed copper spray according to label instructions."
Watch the weather this spring, and if it decides to get really wet and nasty, be ready to treat any trees that have had problems in the past, he says. If the weather stays dry, well, enjoy the spring and save yourself some money.
By Dennis Hinkamp - Utah State University Extension