Impact of the Gall-Forming Rust FungusUromycladium tepperianumon the Invasive TreeAcacia salignain South Africa

The impact of an introduced, gall-forming rust fungus,Uromycladium tepperianum,on an invasive treeAcacia salignawas evaluated in the Western Cape Province. The number of infected trees, rust galls per tree, state of trees (dead or alive), stem diameter of trees, and number of seeds in the soil were recorded along transects at yearly intervals from 1991 to 1996 at eight sites where the fungus had been inoculated during 1988 or 1989. The levels of disease increased rapidly after 1992 at all sites. By 1995, the mean number of galls per tree varied from 5 to approximately 1500 per tree depending on tree size. Tree density decreased by at least 80% at all sites over the period, although young seedlings started to grow at some sites. The seed number in the soil seed bank stabilized after 1992 at most sites, except where fires occurred which reduced the number of seeds.U. tepperianumtherefore is proving to be a highly effective biological control agent, as shown by greatly reduced population densities ofA. salignain South Africa.