Septoria leaf spot of birch

Overview

• Native to North America.
• This fungus causes brown spots with dark margins to develop on birch leaves.
• Periods of extended cool moist weather favours pathogen development.
• This disease does not kill trees or cause lasting damage, but it can cause premature leaf colour change and severe defoliation.

Host species

Septoria leaf spot affects all native birch (Betula) species and can also infect ornamental birch.

Species identification and life cycle

• The fungus overwinters on fallen leaves and tree branches.
• In the spring, spores are spread by wind and rain splash onto newly emerging leaves.
• The spores germinate during periods of cool, wet weather and penetrate the leaf surface.
• Initial leaf symptoms include chlorotic specks, which later expand into irregularly shaped necrotic lesions with dark brown margins.
• Small black fruiting bodies form on the undersides of the leaf spots.
• Infected leaves are capable of spreading the disease throughout the summer under ideal conditions.

Symptoms and damage

• Leaf spots are often irregular shaped and brown with a darker margin; they typically range from 1 to 16 mm in size and often coalesce into larger spots.
• Small black fruiting bodies are sometimes visible on the underside of the leaf spots.
• Infected leaves turn yellow earlier in the season.
• During severe infections leaves may fall prematurely.

Control measures

For ornamentals, raking up leaves around affected trees will reduce spread and infection the following year. In addition, pruning out shading vegetation promotes better sunlight penetration and air movement, which reduces the chance of infection. No control is necessary in forest stands.