Anthracnose

Anthracnose is a disease caused by a host specific species of the fungus Gnomonia (asexual stage named Gleosporium). Fungal spores overwinter in infected twigs, branches or fallen leaves and are spread by wind and rain the following season. In ash trees, infection usually occurs in the foliage or twigs, while in oak, infection can occur in the leaves, twigs, shoots or buds. Ash anthracnose develops best at temperatures of 15-20°C and may be favoured by wet weather, while oak anthracnose develops when temperatures around 10 °C.

Ash leaves infected in spring become distorted usually with blotches of necrosis, while later infections produce necrotic leaf spots with a chlorotic ring. Severe infections can result in significant defoliation and larger twigs and branches may eventually be cankered and killed. Several seasons of infection can cause trees to decline or become susceptible to other pests, although stressed trees can be significantly affected after only one only season of defoliation.

In oak, there are generally three phases of the disease; twig blight, where 1-2 year old twigs are girdled and killed, shoot blight, where new shoots are killed during expansion, appearing scorched, and leaf blight, where leaves become distorted and necrotic at the tips or along veins. Repeated infections of established trees seldom cause permanent damage, but younger trees may require protection.

Control

Remove cankered branches and prune trees to increase air circulation. Rake and destroy fallen leaves where the fungus can overwinter. Fungicide containing mancozeb may provide some control. For controsl of Ash anthracnose, apply as per label instructions at 10-14 day intervals beginning prior to bud burst and continuing with wet weather in spring. For control of Oak anthracnose, apply as per label instructions at 7-14 day intervals beginning when dormant buds begin to swell.