178 Percival and Graham: Calcium and Silicon Fertiliser and Fungicide for Pathogen Management al. 2007; Beresford et al. 2012; Hrabĕtová et al. 2017). Results of this study show that the synthetic triazole fungicide penconazole, when applied 6 times during the growing season, was highly effective in reducing apple scab and leaf blotch severity, where in most instances, 100% control was achieved. The effectiveness of penconazole against fungal patho- gens under laboratory and field conditions has been confirmed elsewhere (Percival et al. 2009; Derbalah 2011; Beresford et al. 2012). However, over-reliance on synthetic fungicides has led to decreased pathogen sensitivity, coupled with public demands to reduce fungicide use based on a greater awareness of envi- ronmental and health issues. This has placed a greater emphasis on the development of non- or reduced fun- gicide pathogen control strategies (Gozzo 2003; Vil- lalta et al. 2004; Hailey and Percival 2014). Three methods of achieving this objective are by (i) reducing the number of sprays per growing season, (ii) reduc- ing fungicide dosage, and (iii) adding non-fungicide additives such as fertilisers to enhance efficacy while not compromising plant quality and aesthetics. Results of this study show that a half-strength appli- cation of penconazole applied 3 times over a growing season had little influence on reducing leaf blotch and apple scab severity compared to water-treated con- trols. Consequently, this treatment cannot be recom- mended. Application of penconazole 3 times over a growing season at full strength, however, reduced leaf blotch and apple scab severity by 40% to 50%. Such a reduction may be acceptable for the produc- tion, supply, and/or consumption of apple fruit sold under an organic or naturally produced label, as scab severity levels tend to be less stringent (Bevan and Knight 2001). Similarly, with ornamental Aesculus spp. planted for aesthetic reasons within the town and city landscape industry, lower leaf blotch levels may be acceptable (Percival et al. 2009). A key finding of this study, however, was that addition of Ca- and Si-fertilisers, especially Ca chloride and tetra ethyl silicate, to penconazole at full or half strength enhanced efficacy compared to each product applied alone. Not only was leaf blotch and scab severity reduced, but leaf chlorophyll content, fruit yield, and crown volume increased. In most instances, these increases were additive, but in other cases a synergistic response occurred, especially when penconazole was combined with Ca chloride. Synergism related to plant protection agents such as fungicides has been ©2021 International Society of Arboriculture defined as “the simultaneous action of two or more compounds in which the total response of an organ- ism to the pesticide combination is greater than the sum of the individual components” (Burpee and Latin 2008). While many investigations addressing synergy in fungicide mixtures exist, most have been limited to laboratory studies using cultured fungal pathogens (Huang and Chen 2008). Few studies demonstrate synergistic interactions between fungicides at the field level (Burpee and Latin 2008). Our field trial results show that while the degree of control recorded was not as great as the conventional approach to leaf blotch and scab management, i.e., 6 repeat fungicide sprays where 100% control was achieved, in most cases, addition of Ca chloride or tetra ethyl silicate to penconazole at full strength reduced leaf blotch and scab severity by 75% to 85%. Addition of Ca chloride or tetra ethyl silicate to penconazole at half strength reduced leaf blotch and scab severity by 45% to 65%. A search of the literature indicates that there is clearly a lack of reports of fungicide synergism in trees. Our findings confirm that foliar Ca and Si applications alone or in combination with a full or reduced dose of synthetic fungicide offer a means of reducing the number of fungicide sprays currently used for patho- gen management that can be implemented with exist- ing spray technologies and may have applicability for the control of other foliar pathogens of ornamental and urban trees not explored within this study. Results of this study, however, do suggest that there is a high probability for individuals involved with the manage- ment of fungal pathogens in urban landscapes, as well as workers in other tree-related industries (horti- culture, forestry, nursery tree production), to take advantage of fungicide synergism to control apple scab and Guignardia leaf blotch. LITERATURE CITED Bain RA, Millard P, Perombelon MCM. 1996. The resistance of potato plants to Erwinia carotovora subsp. atroseptica in relation to their calcium and magnesium content. Potato Research. 39:185-193. https://doi.org/10.1007/BF02358218 Beresford RM, Wright PJ, Wood PN, Park NM. 2012. Sensitivity of Venturia inaequalis to myclobutanil, penconazole and dodine in relation to fungicide use in Hawke’s Bay apple orchards. New Zealand Plant Protection. 65:106-113. https:// doi.org/10.30843/nzpp.2012.65.5396 Berrie AM, Xu XM. 2003. Managing apple scab (Venturia inaequalis) and powdery mildew (Podosphaera leucotricha) using AdemTM . International Journal of Pest Management. 49(3):243-249. https://doi.org/10.1080/0967087031000101089
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