Homeowners are finding symptoms that resemble frost affecting the leaves on ash and maple trees. Although a few leaves may have actually been touched by frost (or in a few cases – winter injury) the probable cause of the symptoms is a fungal disease called anthracnose. Normally the disease affects the leaves when they are full blown, but this year it seems to be attacking the leaves as they emerge from the bud. Anthracnose is common and often present in the spring, but thrives in years in which there are extended periods of cool, wet weather. This year we had the cool weather, but until recently it was not wet. Severe leaf infection can cause extensive leaf drop, which is true of this spring. Many shade trees are affected by anthracnose. Ash, maple, oak and walnut are especially susceptible. The leaves develop brownish colored spots that are irregular in shape. Infections early in the season may result in blotching, distortion and large areas of dead tissue. The leaves often look as if they have been hit with frost. Severe infections may result in the loss of the infected leaves and even some of the newer leaves that show no symptoms. Anthracnose is most severe on the lower and inner portions of the tree where the humidity is higher and air movement is reduced. This helps distinguish it from wilt disease and root rot where symptoms first appear in the top of the tree. In most cases, anthracnose does not cause permanent damage to established trees. If leaf loss occurs in the spring, healthy trees will put forth new leaves in a few weeks. Oaks, particularly bur oak, typically retain the distorted, irregularly spotted leaves throughout the season. Trees stressed from root restrictions, drought, heavy insect infestations or other causes are less tolerant of anthracnose and may show decreased vigor after a season of severe infestation. Sufficient watering, especially during dry periods, is essential to help these trees survive the infection. Control measures should include cultural practices such as proper watering, fertilization and mulching to maintain tree vigor. Sanitation procedures like raking leaves in the fall and pruning out dead or dying branches will help reduce the number of new infections the following year. Fungicides can be applied if trees are severely stressed from defoliation for three years or more. Fungicide application should begin at bud break to protect young, succulent growth. During cool, wet weather, repeat applications according to the fungicide label. Read the label carefully and apply only as directed. Currently labeled fungicides include chlorothalonil (Daconil 2787) and thiophanante-methyl (Cleary’s 3336 or Bonomyl).
