Screening of fungicides for the management of wattle rust (Uromycladium acaciae) in Acacia mearnsii plantations,South Africa

South Africa has c. 110 000 ha planted to Acacia mearnsii (black wattle) with 85% of the revenue from the species obtained from the timber and 15% from the bark. From 2012/13 a wattle rust disease has spread throughout the black wattle plantation area in KwaZulu-Natal and from 2015 it was recorded in southern Mpumalanga. The pathogen affects trees of all age classes, causing a reduction in growth, as well as mortality with severe infection. No fungicides are currently registered in South Africa for the control of wattle rust. In October/November 2014, three black wattle trials were initiated in the KwaZulu-Natal Midlands and south-eastern Mpumulanga where fungicides were tested at varying rates for the control of wattle rust. The same trial design was used for all three trials and consisted of a 3 × 3 factorial with one additional control, replicated three times and laid out as a randomised complete block design. The factorial combination consisted of three fungicides (azoxystrobin + difenoconazole; azoxystrobin + cyproconazole; azoxystrobin) applied at three rates (×, 2× and ½ ×). The additional control was not treated with fungicides. Wattle rust had a significant and negative impact on tree growth, irrespective of site and/or previous infection. All fungicides tested, and at all rates applied, proved effective for the control of wattle rust. The optimum rate selected, timing and frequency of application will be based on a combination of prevention of resistance as well as optimisation of growth. For the most effective control of wattle rust, fungicides should be applied as a preventative, rather than corrective, measure. However, in cases of severe infestation, a corrective application could also be used to aid the management of wattle rust.