Pest of the month: Alternaria leaf spot

P1701-alternariaest of the month: Alternaria leaf spot (Alternaria spp.)

Category name: Fungi

Importance: Major on susceptible hosts.

Symptoms:  Vary according to host, but often appear initially as small water-soaked flecks on lower leaves, enlarging into circular or irregularly-lobed lesions. Lesions typically become brown-black, and may or may not display concentric rings (the darker bands contain spores of the fungus). Lesions are sometimes surrounded by a chlorotic halo. Older leaf spots may become papery or brittle, leading to tearing and/or the development of ‘shot-hole’ symptoms. In some cases, spots coalesce to cover large areas, resulting in blighting of foliage. Alternaria spp. may also cause a range of other diseases including crown, stem and fruit rots.

Transmission:  Spores can be spread by wind, splashing water, infected plant material, contaminated machinery/equipment or workers.

Favoured by: Warm, humid conditions favour spore production. Infection requires several hours of leaf wetness, therefore repeated heavy dews or extended periods of wet weather favour disease development.

Host range: A very wide host range including many ornamental plant species.

Further industry pest information is available at

An initiative of the National Nursery Industry Biosecurity Program (NY15004) and the ‘Building the resilience and on-farm biosecurity capacity of the Australian production nursery industry’ Project (NY15002).