Impact of the gall-forming rust fungus Uromycladium tepperianum on the invasive tree Acacia saligna in South Africa: 15 years of monitoring

Acacia saligna was regarded as the most troublesome invasive alien weed in the Cape Floristic region. The gall-forming rust fungus Uromycladium tepperianum was introduced into South Africa in 1987, and it established throughout the range of the weed. Five populations of the weed were monitored annually from 1991 to 2005. Tree density and stem diameter were recorded in each of four permanent transects at each site. Tree density declined by between 87% and 98% for the period 1991–2005 at these five sites. Average annual mortality rate (±SE) of infected trees dying was 18% (±2%) during the same period. Canopy mass and age were determined by destructively sampling trees at 19 sites. Canopy mass at all sites was lower than data published before introduction of U. tepperianum, and the longer since the last major disturbance (fires or clearing operations), the greater the reduction. Average age of trees in these sites was between 2.2 and 6.1 years, despite the last major disturbance being at least 10–20 years previous at 11 of the sites. Pod and seed production were determined, and seed fall calculated, at each of four sites in 2004. These data were compared to the same variables gathered in 1989. Seed fall in 2004 was reduced compared to that recorded earlier. Results show that U. tepperianum remains a highly effective biological control agent against A. saligna in South Africa.