86 Percival and Graham: Inducing Agents and Fungicide Combinations for Pathogen Management Application of specific biological and/or chemical agents to plants can lead to the induction of local and systemic resistance to subsequent pathogen attack (Walters et al. 2013). An alternative approach to fun- gicides is to make use of the plants’ own resistance mechanisms. Induced resistance (IR) is characterized by a restriction of pathogen growth and suppression of symptoms compared to non-induced plants infected with the same pathogen. IR onset is associated with an accumulation of salicylic acid (SA) at sites of infection and with the coordinated activation of a spe- cific set of genes encoding pathogenesis-related pro- teins. Developments in plant protection technology have led to the formulation and commercialization of a range of IR agents. These include SA, marketed in the UK under the trade name Rigel-G® tional analog (acibenzolar-S-methyl [ASM]), mar- keted in Europe under the trade name Bion® North America as Actigard® . Other examples of IR . Treatment of plants with these prod- ucts induces resistance and activates the same set of pathogenesis-related genes as SA (Spoel and Dong 2012; Pieterse et al. 2014). However, these IR agents are generally less effective than standard synthetic fungicides for total pathogen control (Agostini et al. 2003; Krokene et al. 2008; Percival and Haynes 2008; Percival et al. 2009). This has led to the sugges- tion that a more appropriate role for IR agents would be in combination with a reduced dose of synthetic fungicide to achieve control comparable or signifi- cantly higher than stand-alone applications of fungi- cides at full dose (Bécot et al. 2000; Van Loon et al. 2002; Ilhan et al. 2006). This in turn is likely to reduce potential environmental impacts and extend the working life of existing fungicide products. In this paper, we report the results of a number of agents are the harpin protein, marketed in the US as Messenger® as Oryzemate® , and probanazole, marketed in Japan field experiments conducted over 3 consecutive years to determine the potential of 3 commercially avail- able IR agents, namely, harpin protein, salicylic acid derivative, and liquid chitosan. All were applied sin- gly and in combination with a synthetic fungicide to control 2 foliar pathogens widely encountered in urban landscapes, namely, pear scab (Venturia pirina) against Pyrus communis ‘Williams’ Bon Chrétien’ and Guignardia leaf blotch (Guignardia aesculi) against horse chestnut (Aesculus hippocastanum). , or an SA func- and in MATERIALS AND METHODS Field Trials (2016 to 2018) The pear trial site consisted of a 0.90 ha block of Pyrus communis ‘Williams’ Bon Chrétien,’ identified as sensitive to Venturia pirina infection, interspersed with individual trees of Pyrus communis ‘Beth’ and ‘Concorde.’ Planting distances were based on 2 m × 2 m spacing. Trees were planted in 2003 and trained under a central-leader system to an average height of 2.5 m ± 0.25 m, with mean trunk diameters of 12 cm ± 1.4 cm at 45 cm above the soil level. The horse chestnut trial site consisted of a 2.0 ha block of horse chestnut (A. hippocastanum) located adjacent to the pear trial site. Planting distances were based on a 2 m × 2 m spac- ing. The trees were planted as large standards in 1999 and trained under a central-leader system to an aver- age height of 4.5 m ± 0.35 m with mean butt diame- ters of 50 cm ± 10 cm. Both pear and horse chestnut trial sites were located at the University of Reading Shinfield Experimental Site, University of Reading, Berkshire (51o 43’, −01o 08’). At both trial sites, the soil was a sandy loam containing 4% to 6% organic matter, pH 6.6, with available P, K, Mg, Na, and Ca nutrients at 60.0, 693.7, 190.1, 48.8, and 2542 mg/L, respectively. Weeds were controlled chemically using glyphosate (Roundup; Green-Tech, Sweethills Park, Nun Monkton, York, UK) throughout each growing season. No watering or fertilisation was applied to plants during any of the trials. Historically, both pear and horse chestnut trees suffered heavily from pear scab and Guignardia leaf blotch infection, respec- tively, on an annual basis. Prior to trials commencing, trees were inspected late in the growing season (Sep- tember) of the previous year, and only those trees visually rated with a high degree of fungal infection, i.e., > 50% of leaves infected, representing high foliar discolouration and scab/leaf blotch infection, were used the following year. Five independent experi- ments were performed once over three years, with each one or two experiments occurring in a different year. The treatments (1 water control, 1 IR agent [sal- icylic acid, harpin protein, liquid chitosan], 1 fungi- cide [boscalid + pyraclostrobin], 3 fungicide + IR combinations) were applied in 8 randomized com- plete blocks with a single tree as the experimental unit, giving a total of 48 observations per response variable. ©2021 International Society of Arboriculture
March 2021
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