Quantifying the impact of Uromycladium acaciae (wattle rust) on black wattle (Acacia mearnsii) growth and productivity

Nxumalo T, Morris A, Germishuizen I
Publication Type
Technical Note
Publication Year
TN 14/2017

Uromycladium acaciae is a pathogen-causing disease on black wattle plantations. First discovered in 2013 near Eston in the KwaZulu-Natal Midlands, the disease has since spread throughout the wattle growing regions of South Africa and Swaziland. Symptoms are characterised by leaf deformation, defoliation, stem infection and stunted tree growth. The disease affects trees of all ages, from seedling stage to end of rotation. Since its appearance, the disease has been a major concern to wattle growers; however, its impact on tree growth and stand productivity has not yet been quantified. This study was conducted to determine the impact of wattle rust on tree growth and stand productivity and to understand interactions between the disease and co-occurring insect pests. Three exclusion trials were established in October 2014 using a randomised complete block design of 4 x 4 factorial combination. The factorial combination consisted of fungicide mixture (azoxystrobin and difenoconazole) to exclude the pathogen, insecticide (cypermethrin) for insect exclusion and combination of the fungicide and insecticide applied as “piggy-back” treatment. Disease assessments were conducted using a modified Cobb scale, and tree height and diameter were measured to assess tree growth. Only two trials were measured to completion, and significant responses to pesticide application were only observed at one site, where fungicide application reduced disease severity by 82 % and improved tree growth by over 34%. Insecticide application also improved tree growth at this site, suggesting an interaction between insect pests and wattle rust. Further studies are required to confirm the potential effectiveness of pesticide for the control of the wattle rust in support of the development of a successful integrated pest management strategy.