A comparison of organic fungicides: alternatives for reducing scab on pecan

Authors: Bock, Clive; Hotchkiss, Michael - Mike; Shapiro Ilan, David; BROCK, JASON - University Of Georgia; BRENNEMAN, TIMOTH - University Of Georgia; WILKINS, BRYAN - Auburn University; WELLS, DANIEL - Auburn University; WELLS, LENNY - University Of Georgia; MIZELL, RUSS - University Of Florida

Submitted to: Organic Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/13/2018
Publication Date: 9/1/2019
Citation: Bock, C.H., Hotchkiss, M.W., Shapiro Ilan, D.I., Brock, J.H., Brenneman, T.B., Wilkins, B., Wells, D.E., Wells, L., Mizell, R.F. 2019. A comparison of organic fungicides: alternatives for reducing scab on pecan. Organic Agriculture. https://doi.org/10.1007/s13165-018-0229-7.
DOI: https://doi.org/10.1007/s13165-018-0229-7

Interpretive Summary: Scab is the most damaging disease of pecan in the southeastern U.S.A. Although scab can be controlled using conventional chemical methods, organic pecans that attract a premium price mandate the use of organic fungicides. Also, organic production is an environmentally sustainable method. However, where susceptible pecan cultivars are grown there are limited options for organic management of scab. We conducted experiments to compare organic fungicides to control scab on the susceptible cv. Desirable in 2011, 2012, 2014, 2015 and 2016. We compared Bordeaux mixture, compost tea, sodium bicarbonate, Bacillus subtilis (Serenade), sulfur, cuprous oxide (Nordox) and extract of the Giant Knotweed (Regalia). Extract of the Giant Knotweed as a fungicide was included in 2012, 2014, 2015 and 2016, and fruit on those trees had less severe scab in all years compared to that on fruit of the control trees. In three seasons (2012, 2015 and 2016), applications of Bordeaux mixture resulted in a reduction in scab severity. Compost tea, Sodium bicarbonate, B. subtilis, sulfur and Cuprous oxide all significantly reduced scab compared to the control in one or two seasons, but were not consistent among seasons, and were never more efficacious compared to the extract of the Giant Knotweed. Extract of the Giant Knotweed and Bordeaux mixture appear to offer the greatest potential as organic approaches for managing scab in pecan in organic systems.

Technical Abstract: In the southeastern U.S.A, the most widespread and damaging disease of Pecan scab, caused by Venturia effusa. Although scab can be controlled using conventional chemical methods, organic pecans that attract a premium price mandate the use of organic fungicides. Also, organic production is an environmentally sustainable method. However, where susceptible pecan cultivars are grown there are limited options for organic management of scab. We conducted experiments to compare organic fungicides to control scab on the susceptible cv. Desirable in 2011, 2012, 2014, 2015 and 2016. The alternatives compared included Bordeaux mixture, compost tea, sodium bicarbonate, Bacillus subtilis, sulfur, cuprous oxide and extract of the Giant Knotweed (Reynoutria sachalinensis). Rainfall and scab severity differed between seasons. There was consistently low severity on foliage, with little or no difference between treatments. Similarly at the time of the 1st fruit assessment, the severity was low and the differences in severity small and inconsistent between seasons and treatments. However, by the time of the 2nd fruit assessment severity of scab had increased and consistent differences among treatments existed (except in the drought year of 2011, when scab severities were very low and similar to the control). In all other years, the control treatment had significantly more severe scab compared to some (2012 and 2014) or all other treatments (2015 and 2016). Extract of the Giant Knotweed as a fungicide was included in 2012, 2014, 2015 and 2016, and fruit on those trees had less severe scab in all years compared to that on fruit of the control trees. In three seasons (2012, 2015 and 2016), applications of Bordeaux mixture resulted in a reduction in scab severity. Compost tea, Sodium bicarbonate, B. subtilis, Sulfur and Cuprous oxide all significantly reduced scab compared to the control in one or two seasons, but were not consistent among seasons, and were never more efficacious compared to the extract of the Giant Knotweed. Extract of the Giant Knotweed and Bordeaux mixture appear to offer the greatest potential as organic approaches for managing scab in pecan. However, wherever possible, planting of scab resistant cultivars should be considered as a first line of defense.