Manifestations of Anthracnose
In early June anthracnose leaf diseases can be very common across southern Minnesota and occur on many hardwood species. The symptoms vary greatly with the host species, the weather conditions and the stage of leaf development at the time of infection.
Manifestations of the symptoms tend to follow one of three patterns:
- 1. Rapidly developing blight of leaves and shoots, characterized by browning and shriveling of young leaves during the period of leaf expansion. Occurs very infrequently and could be easily confused with frost.
- 2. Large irregular dead areas on distorted leaves that otherwise remain green. Usually evident in late summer on bur oak, but during the June, can cause outbreaks on white oaks.
- 3. Small necrotic spots on leaves that have reached mature size. The usual foliar symptoms on black oak are brown spots up to 3/4 of an inch in diameter. Last occurred on black oak in June of 1991.
Outbreaks of anthracnose are promoted by wet weather and moderately cool temperatures. The leaf disease becomes prominent on the lower branches then spread upwards into foliage in the mid and upper crown. Enlarging lesions on the on leaves tend to follow the veins or midrib and to be bounded by them often killing all the leaf tissue on the side of a midrib or major vein and thus causing distortion. After drying, lesions are papery in texture and turn tan to nearly white before weathering to a grayish white. At the edge of the lesions there is an abrupt transition from brown to the normal green tissue. With all types of anthracnose on oaks the fruiting bodies of the fungus become visible to the unaided eye as raised brown flecks on the lower surfaces of the foliar lesions.
Anthracnose outbreaks usually subside before midsummer with the exception of those occurring on bur oaks which can develop as late as in August.
Usually, defoliation from anthracnose does not have an impact on the long term health of the trees. Thus, control measures are not warranted.