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Oak anthracnose

Anthracnoses are often the most conspicuous diseases but are among the least damaging of hardwood diseases. Anthracnose symptoms range from innocuous leaf spots through blotches and blights of leaves and shoots to cankers and diebacks of twigs and branches. Mature, full-sized leaves are quite resistant to anthracnose which accounts for its minor manifestation as only small, necrotic spots on oak leaves in mid-summer.

In the Lake States, anthracnoses are generally more severe than elsewhere due to the unique climatic conditions that favor disease development. Severity is usually dependent on the susceptibility of the individual tree and the occurrence of wet spring weather. It is not unusual in late summer to see heavily infected trees adjacent to unaffected trees due to the inherent differences in susceptibility of the trees. In a year with conducive weather, disease cycles are repeated during the growing season and the damage increases. As a general rule, symptoms are most severe in the lower crown, in the spring, and, as the season progresses, symptoms spread upwards. In most years anthracnose diseases are finished by mid-summer since mature leaves and shoots have formed and they are resistant to further infection.

The pathogenic fungi that cause anthracnoses overwinter in infected shoots, twigs and fallen leaves. During wet, rainy, spring weather fungal spores cause new infections on succulent leaves and shoots. During leaf expansion and shoot elongation, new infections cause rapid wilting and death of infected leaves and shoots. Early season anthracnose damage can often be confused with frost damage. After shoot elongation, infections progressively kill leaf tissue along the midrib or vein. Numerous infections on a leaf can cause leaf abscission and as more and more leaves are lost, trees become defoliated. Later in the season, as leaves mature and become more resistant, infections cause discrete, non-expanding leafspots. If anthracnose infection is severe enough to cause refoliation, the new succulent leaves and shoots are susceptible to anthracnose infection until they too are mature. Damage also results when the expansion of existing twig infections and cankers girdles and kills the shoots.

Apiognomonia quercina is generally given credit for causing oak anthracnose although several fungi can cause the disease. White oaks are most susceptible. There are three recognized patterns; browning and shriveling of young leaves during the period of leaf expansion, large irregular dead areas on distorted leaves that otherwise remain green, and small necrotic spots on mature leaves. Symptoms become prominent on low branches then spread upward. Enlarging blotches on leaves tend to follow the veins or midrib and be bounded by them. Fruiting bodies of the fungus become visible to the unaided eye as raised brown flecks on the lower leaf surfaces. Red oaks have similar symptoms except that leaf blight is rare.

Anthracnose diseases are difficult to control but they seldom result in any serious long term impact to the above hosts.