Horticulture Forum: June 2001, Plant Maintenance: Tree & Shrub Diseases

People, including lawn care operators, are too often guilty of improper plant treatment such as poor plant selection, overwatering and overfertilization, which can result in disease.

Although proper maintenance is the No. 1 way to avoid plant diseases, Chatfield said lawn care operators still should remain educated on a few specific tree and shrub diseases for successful diagnosis and control.

SCAB OF CRABAPPLE. This is a disease that occurs throughout the season, Chatfield said. This fungus appears as olive green to black spots with radiating edges on leaves. The leaves turn yellow and fall prematurely beginning in June or July. Small, round dark areas that become corky are found on the fruits. Trees with this disease can be 75 percent defoliated in six to eight weeks.

To control this disease, rake and destroy or haul away fallen leaves in the fall and apply fungicide sprays several times at 10- to 14-day intervals starting at early leaf development. If planting susceptible cultivars, be sure to obtain good coverage. Also, plant resistant varieties in highly susceptible areas.

ROSE BLACK SPOT. This disease is caused by a fungus that infects rose leaves only if there is moisture on these leaves for a certain number of hours. Yet, outbreaks of rose black spot in some large rose gardens have been seen after several weeks following a long, intense hot and dry spell. Most of the time, though, this disease is a result of overwatering.

Management tactics for this disease include avoiding overhead watering, promoting leaf drying and removing diseased leaves during the growing season. After a mass infection, a protectant fungicide can be used. However, fungicide treatments will not work well if the diseased leaves are not removed prior to application.

ANTHRACNOSE LEAF BLIGHT. This disease produces small, purple-bordered spots and irregular blotches on leaves. Flower bracts also can be affected. Twigs can be infected through leaf petioles or through small lesions in the bark of shoots. As this disease progresses, twig dieback occurs and, in time, larger branches will die. The leaf spot phase can be mistaken for other fungal leaf spot diseases. If dogwood anthracnose is suspected, send samples to a laboratory to make sure.

Flowering dogwood is very susceptible, although some cultivars may be less susceptible. Other dogwoods are more resistant. Kousa or Chinese dogwood is sometimes used as a replacement, although this plant flowers one month later. The disease can become more serious when many dogwoods are planted together in a large, shady area.

To manage Anthracnose leaf blight, avoid heavily shaded sites where young leaves are constantly in cool, moist conditions.

Fungicides can be used for control, but they need to be timed properly. If applied at bloom through one month post-bloom, they can be helpful. But late June is too late to spray fungicides because the infection has already occurred, the damage is done and old infections are impossible to stop or eliminate.

In June, the plant tissue becomes less susceptible to new infections.

DIPLODIA TIP BLIGHT OF PINE. This disease infects new growth annually on Scots, Austrian, red and mugo pines. As new growth develops, excess moisture is the most common cause of this disease, which occurs just before new candle growth in the spring. Infections are greatest on 15- to 25-year-old trees. Symptoms include dieback, stunting of new growth, lower branch dieback and possible tree death.

Good site and plant selection and proper care, including fertilization, pruning and preventive fungicide treatments can be used to ward off this disease.