Arboriculture & Urban Forestry 46(5): September 2020 can result in a slow tree decline and possibly death (Percival 2018). Strategies for scab management are difficult because currently there are no synthetic fun- gicides with systemic properties that directly affect the pathogen registered for use within most European urban landscapes (Lainsbury 2018). An added diffi- culty is the occurrence and spread of strains of Ven- turia with resistance to triazole-based fungicides due to overreliance in the production of “scab free” fruit- ing apples used for human consumption (Cuthbertson and Murchie 2003; Villalta et al. 2004). Consequently, the development of an effective treatment appears a necessary tool to slow apple and pear decline and to protect valuable trees from infection and death. Tree resistance against foliar and root diseases can be enhanced by exposing them to a range of natural and/or synthetic compounds, such as inorganic potas- sium and phosphate salts, low molecular weight pro- teins, and salicylic acid (Bécot et al. 2000; Fobert and Després 2005; Garbelotto et al. 2007; Percival et al. 2009). After exposure to these compounds, a suite of physiological, biochemical, and anatomical changes occur within leaf, wood, and root tissue. Changes include synthesis of low-molecular-weight phenols, terpenoids, and alkaloids that possess antimicrobial, antinutritive, and antidigestive activity, synthesis of protein-based oxidative and hydrolytic enzymes, increased protein- ase inhibitors, increased leaf lignification, enhanced resin production, and initiation of wound periderm. Inducement of these changes within plant tissue is termed induced resistance (IR). In essence, IR is a form of disease resistance caused by activation of the host plant’s own genetically programmed defence path- ways (Hammerschmidt 2007; Akinsanmi and Drenth 2013; Walters et al. 2013; Aćimović et al. 2016). IR has been studied in herbaceous plant species and, in recent years, woody plants as a potential eco- friendly concept for enhancing tree resistance to diminish the effects of foliar and root pathogen attack and severity (Walters et al. 2013; Percival 2018). Developments in plant protection technology have led to the formulation and commercialisation of a range of IR agents such as harpin protein (trade name Messenger), benzothiadiazole (trade name Bion), potas- sium phosphite (trade name Phusion), salicylic acid derivative (trade name Rigel-G), and probanazole (trade name Oryzemate® ). These products are regis- tered for commercial plant protection use, although their availability differs between countries (Percival 359 and Haynes 2008). Because of their non-direct chem- ical mode of action aimed at enhancing the defence mechanisms of treated host plants rather than at directly arresting or killing a disease agent, IR agents offer opportunities for the control of fungal patho- gens in ecosystems such as urban landscapes. As effects are mostly on the tree itself, there will be little, if any, consequence on the existing tree fungal com- munities. IR agents have low toxicity to invertebrates, aquatic organisms, or animals, including humans, an important factor when applying plant protection agents in densely populated urban areas (Fernandez-Escobar et al. 1999; Garbelotto et al. 2007). Research conducted using IR agents has focused mainly on their potential for disease control of eco- nomically important crop plants (wheat, rice, potato) with application primarily by foliar sprays. Few stud- ies exist evaluating IR agents as root drenches against diseases of urban landscape trees. Of those available, root drenches have been shown to offer potential against Phytophthora spp. and bacterial diseases, such as horse chestnut bleeding canker (Pseudomonas syringae pv aesculi)(Garbelotto et al. 2007; Percival and Banks 2014). This paper aims to build on existing research by generating new and novel data by answering the fol- lowing questions: (1) Do IR agents offer viable man- agement options for scab protection when applied as root drenches? (2) Is efficacy influenced by number of applications, i.e., how many soil drenches need to be applied to achieve scab protection? (3) What is the influence of preventative versus curative treatments, i.e., do they both offer viable options, or is one supe- rior to the other? (4) To study the influence of IR agents applied as root drenches for scab management, since this has not been investigated. MATERIALS AND METHODS Field Site and Experimental Trees The apple trial site consisted of a 0.75-ha block of apple (Malus cv. Crown Gold) interspersed with indi- vidual trees of Malus cv. Red Delicious and Gala as pollinators. The pear trial site consisted of a 0.90-ha block of Pyrus communis ‘Williams Bon Chrétien’ interspersed with individual trees of Pyrus communis Beth and Concorde. Planting distances were based on 2 × 2 m spacing. Trees were planted in 2003 and trained under the central-leader system to an average height ©2020 International Society of Arboriculture
September 2020
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