Uromycladium tepperianum (a gall-forming rust fungus) causes a sustained epidemic on the weed Acacia saligna in South Africa

Abstract

Uromycladium tepperianum, a gall inducing rust fungus native to Australasia, was introduced into South Africa as a biological control agent in 1987. Incidence and severity of infection has been monitored from 1991 to 2010 at permanent sites. After release, disease incidence increased rapidly and it has stayed at (or just under) 100% on plants ≥5 cm stem diameter since. Incidence on smaller plants has decreased then increased rapidly following years of fires with mass regeneration from the soil-stored seed bank, or following years of high recruitment in the absence of fire. The maximum severity on mature trees is approximately 2000 galls per tree, with severity increasing exponentially with increasing tree size. Severity of infection on all tree size classes has shown a decreasing trend over the last 10 years, coinciding with a decrease in host tree density. It is concluded that infection is host density dependent.