Arboriculture & Urban Forestry 44(5): September 2018 mented with Si fertilizers constituted a physical barrier to impede fungal penetration and colo- nization (Kim et al. 2002; Hayasaka et al. 2008). Other studies, suggest that Si plays a biochemi- cal role (i.e., Si-induced accumulation of phenolic compounds, phytoalexins, lignin, and peroxi- dase) in mediating plant resistance to pathogens (Rodrigues et al. 2003; Liang et al. 2005; Sun et al. 2010). In the current study, a significant in- crease in leaf and fruit Si content was recorded in both Malus cv. Golden Delicious and Pyrus communis ‘Williams’ Bon Chrétien’ following application of any of the Si fertilizers. Such a re- sponse is consistent with other findings following application of Si fertilizers to peach, grape, kiwi, as well as several economically important grain and grass crops (Catherine Keller and Meunier 2012). Enhanced Si content in leaves and fruit suggests that Si-enhanced scab resistance may be partially caused by the role of Si as a physi- cal barrier (Sun et al. 2010), coupled with pro- motion of inherent biochemical plant defense systems in apple and pear not investigated here. Penconazole applied at bud break, green clus- ter, petal fall, and early fruitlet formation was highly effective in reducing apple and pear scab severity of fruit and leaves. The effectiveness of penconazole against apple and pear scab under laboratory and field conditions has been con- firmed elsewhere (Schnabel and Parisi 1997; Percival et al. 2009). However, due to the devel- opment of triazole-insensitive strains of scab, greater emphasis on alternative scab manage- ment strategies is now warranted (Gozzo 2003; Vallad and Goodman 2004; Fobert and Després 2005; Witzell and Martin 2008). The phenom- enon of inducing resistance in plants by bio- logical, natural, and/or inorganic compounds potentially offers an alternate approach to scab management (Hammerschmidt 2003; Percival and Haynes 2008; Percival et al. 2009; Hailey and Percival 2014). Consequently, developments in plant protection technology have focused on the production of commercially available IR agents. In this study, the IR agent salicylaldehyde reduced leaf and fruit scab severity by ca. 37.0% (Malus cv. Golden Delicious) and 30% (Pyrus commu- nis ‘Williams’ Bon Chrétien’). Salicylic acid (SA) and SA derivatives are phenolic-based com- 211 pounds found in a wide range of plant species that directly or indirectly influence plant metabolism. Application of SA to plants induces synthesis of low-molecular-weight (LMW) compounds (phe- nolics, terpenoids, alkaloids) that possess toxic, antimicrobial, anti-nutritive, and anti-digestive activity (Facchini 2001; Vallad and Goodman 2004; Fauteux et al. 2005; Keeling and Bohlmann 2006; Bernards and Bastrup-Spohr 2008; Witzell and Martin 2008). Consequently, stimulation and elevated concentrations of these compounds within plant tissue can significantly contribute to reduced severity of fungal, bacterial, and viral pathogens (Bokshi et al. 2003; Sparla et al. 2004). Results of this study demonstrate that the com- mercially available Si fertilizer Rigel (a.i. poly- ether modified trisiloxane + salicylaldehyde) was as effective a scab-protectant product as penconazole. In all cases, reductions in scab leaf and fruit severity and enhancement of leaf color and fruit yield were statistically comparable to penconazole when applied four times from bud break to early fruitlet formation. In comparison to all other Si fertilizers evaluated in this study and the IR agent salicylaldehyde alone, a com- bination of Si + IR proved superior to Si or IR applied individually. Consequently, this indicates additive and/or synergistic effects when combin- ing an Si fertilizer with an IR agent. From a com- mercial aspect, producers, suppliers, and vendors of apples generally adopt a low-tolerance policy toward apple and pear scab on fruit (Butt et al. 1990). Combining a Si + IR agent reduced scab levels to those achieved using penconazole, a synthetic fungicide. Such a result would prove of interest to professionals involved in the produc- tion, supply, and/or consumption of apple and pear especially where fruit produce is sold under an organic or naturally produced label, as scab severity levels tend to be less stringent (Bevan and Knight 2001) for ornamental apples planted for aesthetic reasons within town and city land- scape industry, lower scab levels are also accept- able (Percival et al. 2009). Because of their mode of action, aimed at enhancing the physical and biochemical defense mechanisms of treated host plants rather than at directly arresting or killing a fungal disease agent, an Si fertilizer + IR agent potentially offers opportunities for the control of ©2018 International Society of Arboriculture
September 2018
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