Although they generally do not kill their host, leaf spots put stress on the plant, are unattractive and may cause leaves to fall off prematurely. A few of the most common types of leaf spots you are likely to see in your landscape are black spot, Entomosporium leaf spot and Cercospora leaf spot.
Black spot is by far the most common, widespread and damaging disease to roses. Rose leaves and canes often show symptoms of this disease as brown to black circular spots about ¼ to ½ inch in diameter. As symptoms develop, a yellow margin or halo develops around each spot. Several spots may grow together to form large blotches. Large yellow blotches often appear on heavily spotted leaves just before they fall from the plant.
Defoliation caused by blackspot is perhaps the most common symptom of this disease. Typically, leaf shed begins at the base of the canes and gradually spreads upward until all but the youngest leaves are lost. Heavy leaf shed reduces flower production and plant vigor. If severe drought or cold weather conditions further stress roses damaged by black spot, the plants may die.
Entomosporium leaf spot
Entomosporium leaf spot is a widespread and destructive disease of woody ornamentals. Red tip photinia, along with Indian hawthorn and a few others plants, are commonly damaged by Entomosporium leaf spot. Classic symptoms are tiny, circular, bright red spots on both the upper and lower surfaces of young expanding leaves. Large purple to maroon to bright red blotches may be found on heavily diseased young leaves. Tiny black specks, the fruiting bodies of the leaf spot fungus, are often found in the center of each leaf spot. Light infections usually cause little more than cosmetic damage, while severe infections often result in early and heavy leaf drop. Severe defoliation not only slows growth but also increases plant sensitivity to environmental and cultural stresses.
Disease development is more seasonal in home landscapes. Leaf spot symptoms appear mainly during the spring growth flush on younger leaves. The wetter the season, the more severe the spotting and shedding of leaves.
Cercospora Leaf Spot
Cercospora leaf spot is a common disease in landscape plantings of hydrangea, crapemyrtle, azalea, roses and dogwoods. It first appears as small, circular brown or purple spots with no yellow halo scattered on leaves near the base of the plant. Some diseased leaves may be twisted or distorted. As the spots become more numerous and increase in size, many of the diseased leaves turn yellow and fall to the ground.
While Cercospora leaf spot rarely, if ever, kills the target plant, heavy spotting of the leaves and premature leaf shed is unsightly and may reduce plant vigor and bud set. This disease is often most noticeable on low-maintenance landscape plantings.
Controlling leaf spot
Many of the fungal spores for leaf spots survive the winter on diseased fallen leaves. If possible, collect and discard all fallen leaves from around plants. In early spring, fungal spores are carried by wind-blown rain or irrigation water to the young, expanding leaves. Slow the spread of leaf spots by spacing plants to improve air movement and speed evaporation of moisture from the foliage. Surface or drip irrigation is preferred to watering with overhead sprinklers, as fungal spores are also spread to the healthy lower leaves by splashing water.
In addition, an effective fungicide will help control many of leaf spots. Begin applications when spotting of the leaves is first seen and continue applying treatment as needed, usually every 10-14 days. Some common multi-purpose fungicides are Chlorothalonil (Daconil, Bravo), myclobutanil (Immunox), mancozeb and thiophanate-methyl. Tebuconazole and triforine are two others used to control black spot and Entomosporium leaf spot. Always read the label on insecticides for specific details on listed plants, application procedures and dosage amounts.
For help with other home and garden questions, contact your local county Extension office or visit us online at www.aces.edu.
Shane Harris is an Extension Agent for the Alabama Cooperative Extension System.