Use of fungicides for the management of Uromycladium acaciae in Acacia mearnsii plantations, South Africa

Abstract

South Africa has ca. 110 000 ha planted to Acacia mearnsii with 85% of the revenue from the species obtained from the timber, and 15% from the bark. Since its detection in 2013, wattle rust (recently identified as Uromycladium acaciae) 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. Research has been initiated to determine a number of strategies for the management of the pathogen. These strategies include understanding wattle rust biology and epidemiology, planting tolerant or resistant black wattle, the testing and use of fungicide for management, and remote sensing and process based modelling to assess black wattle loss and high risk areas. These, with the outcomes from this research, will be combined into an overall Integrated Pest Management plan. Of the various strategies, the management of wattle rust with the use of fungicides is important, not only as it will have the potential to reduce the negative impacts of wattle rust, but it will also provide an interim solution until the other research areas provide alternative solutions. To address the current lack of fungicides available (and knowledge around their application) for the management of wattle rust, a series of trials were implemented to screen fungicides for their potential use, extend periods between the re-application of fungicide (if possible), the linking of symptoms to Disease Expression to aid with the timing of application, and the cost:benefits associated with fungicide use. Prior to the initiation of research into managing wattle rust, no fungicides were registered in South Africa for the control of wattle rust. In October / November 2014, three A. mearnsii trials were initiated in the KwaZulu-Natal Midlands and SE Mpumulanga where fungicides were tested at varying rates for the control of wattle rust. Wattle rust had a significant and negative impact on tree growth, irrespective of site and/or previous infection. All fungicides tested and at all the rates applied, proved effective for control. For the most effective control of wattle rust, fungicides should be applied as a preventative, rather than corrective measure. In October 2015 a trial was initiated in southern KwaZulu-Natal to determine the effectiveness of varied application schedules and adjuvants of fungicides for the management of wattle rust. Two trials had initially been initiated but one had to be abandoned due to browsing damage. Wattle rust had a significant impact upon Groundline Diameter and Biomass Index but not Height. All of the adjuvants used and application schedules were effective in managing wattle rust. The most effective fungicide application used will therefore be based upon cost and in a manner that will reduce the likelihood of acquired resistance developing in wattle rust populations. The timing of fungicide application is necessary for optimal use of these fungicides. Fungicide applications could potentially be linked to the emergence of different wattle rust symptoms to optimize fungicide use. Wattle rust symptoms were analysed from the untreated control plots of two trials, one in the KwaZulu-Natal midlands and one in southern KwaZulu-Natal, to determine whether wattle rust Disease expression could be linked to black wattle tree growth. Regression trees were used for the analysis, as linear and multiple regression techniques would be unsuitable for the data. Regression trees were overfitted and attempts at testing the robustness of the model by cross-validation were unsuccessful. No individual symptom emerged as a significant predictor of tree growth, indicating that fungicide application should take place with the onset of any of the wattle rust symptoms tested. The results from six trials testing the use of fungicides for managing wattle rust were compared to assess costs associated with fungicide use. Relative growth for Biomass Index was compared to untreated controls to obtain comparisons within and between sites. Costs versus benefit were compared using a two-way table to determine the most optimum treatment. The largest portion of treatment costs was attributed to the cost of fungicide. No single treatment was found to be optimal for the recommended rate of application. The use of adjuvants increased the cost of treatment, without additional benefit in growth. Control of wattle rust is beneficial, although costly if over-applied. Rotation-end data is required to determine whether fungicide use is economical for managing wattle rust over an extended period of time. As a limited number of fungicides, from a limited number of fungicide groups were screened, the screening of additional fungicides from different fungicide groups will provide an additional selection of fungicides. If these are used in combination or alternation, the likelihood of acquired resistance developing among wattle rust populations will be reduced. Linking fungicide applications with wattle rust epidemiological and climatic data will aid in optimal use of fungicides, by timing applications to coincide with epidemiological and climatic cues. Rotation end research comparing final yield on fungicide treated versus untreated black wattle is needed to fully understand the economics of fungicide use. This will also aid in the understanding of the impact of wattle rust on tree age