Pest: Bacterial leaf spot – Pseudomonas syringae
Symptoms: Often produces small tan spots with reddish-brown margins on leaves. Spots may be surrounded by a light green to yellow halo and sometimes the centre of the spots dry and fall out leaving a shot-hole appearance. Symptoms vary depending on the host plant. There are many strains that are specific to particular plant species, but collectively they have a very wide host range.
Transmission: The disease is mainly introduced via infested cuttings, contaminated seed, or when seed pods come into contact with bacteria in plant debris during harvest and handling. Bacteria overwinter in cankers on twigs and limbs. Twig infection often occurs through leaf scars in autumn. It can enter leaves through rust pustules and hail or wind-damaged areas, survive on undecomposed crop residues and possibly other host plants. Once the disease is established, wind, rain, overhead sprinkler irrigation and the movement of workers and machines through the wet crop assist disease spread.
Favoured by: The disease is favoured by cool, wet, windy weather in autumn and early winter before and during leaf fall. Damage to trees and limbs during this period increases risk of infection. Periods of intense rainfall are particularly conducive to disease development.
An initiative of the Nursery Levy funded National Nursery Industry Biosecurity Program (NY15004) and Building the resilience and on-farm biosecurity capacity of the Australian production nursery industry Project (NY15002).