©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(2): March 2023 65 to protect newly planted and established mature trees through their incorporation into the bulk soil sur- rounding existing root systems and the rhizosphere, using, for example, air excavation technology (Percival and Graham 2021). However, care should be taken when selecting chitin- or chitosan-based fertilisers for plant protection purposes as efficacy can differ mark- edly between formulations. Little information exists on the efficacy of soil- applied chitin and chitosan against pathogens of urban tree species. Consequently, this paper aims to generate original and novel data by answering the fol- lowing questions: 1) Do chitin, chitosan, and chitin/chitosan-based commercially available fertilisers offer viable management options for scab protection? 2) What is the optimal chitin/chitosan/fertiliser concentration that can be applied to plants for maximal efficacy? 3) Can chitin/chitosan/fertiliser be phytotoxic to trees? MATERIALS AND METHODS Container Trials (2018) Experiments used cell-grown bare-rooted stock of Malus sylvestris (crab apple, susceptible to apple scab [Venturia inaequalis]). Trees were approximately 90 cm high, ± 6 cm. Trees were purchased from a commercial supplier during December and January and planted upon delivery in February. Trees were potted into 10-L plastic pots filled with soil: loam tex- ture, 23% clay, 44% silt, 30% sand, 3% organic matter, pH 6.6, and supplemented with the controlled-release nitrogen-based NPK fertiliser Basacote® Plus 9M 16-8-12(+2+TE)(Compo Expert UK Ltd, Stourport- on-Severn, England, UK) at a rate of 5 g L-1 of soil. At this stage, soil was also amended with chitin or chi- tosan at zero (control), 0.25%, 0.5%, 1.0%, 1.5%, and 2% by volume. Malus sylvestris trees from the same cell-grown bare-rooted stock were planted into nonamended soil and sprayed with a conventional fungicide (penconazole at 0.75 mL L-1) at 4 growth stages identified as key application times for scab control under field conditions (Bevan and Knight 2001): namely, bud break (March 13), green cluster (April 12), 90% petal fall (May 19), and early fruitlet (June 12). Therefore, the container trials had 12 dif- ferent treatments: chitin and chitosan at 5 different concentrations, a penconazole fungicide industrial control within the UK urban landscape. Therefore, a soil-applied product with the capacity to reduce apple and pear scab disease severity would be valuable (Cuthbertson and Murchie 2003; Villalta et al. 2004). After cellulose, chitin is the second most abundant polysaccharide on the planet. Developments in fer- tiliser formulation have led to the manufacture of chi- tin and its partially deacylated derivative known as chitosan (Sharp 2013; Sharif et al. 2018). Both chitin and chitosan are found in, and can be sourced from, a variety of living organisms including the exoskele- tons of arthropods and crustaceans and the cell walls of fungi (El Hadrami et al. 2010). Use of these chitin, chitosan, and/or fertilisers containing these 2 com- pounds as active ingredients has repeatedly been shown to be beneficial in the management of fungal diseases such as Fusarium wilt (biotrophic pathogen) in tomato and Botrytis cinerea in strawberry (Benha- mou et al. 1994; Rabea et al. 2003). Similarly, control of the oomycete pathogens Phytophthora capsici on peppers (Xu et al. 2007) and P. infestans in potato (O’Herlihy et al. 2003) has been achieved following soil amendment with chitosan. Chitin and chitosan applied to plants in low concentrations as a soil amend- ment or foliar spray activates biochemical, genetic, and physical defence mechanisms which in turn have allowed plants to resist or tolerate a broad range of pathogenic fungi, bacteria, and viruses (Zhang et al. 2003; Radwan et al. 2012; Sharp 2013). This concept of using compounds to activate defence mechanisms is widely referred to as induced resistance. In addi- tion, chitin and chitosan have been found to stimulate the activity of plant symbiotic microbes and promote the colonisation of root tissue by arbuscular mycor- rhizal (AM) fungi and Rhizobium bacteria (Li et al. 2020; Volpe et al. 2020). This is a 2-fold benefit for the plant due to an increase in water and nutrients (notably nitrogen with Rhizobium symbiosis and phosphorus with AM symbiosis) and an alteration in the rhizosphere microbial equilibrium which disad- vantages soil-borne pathogens (Bell et al. 1998; Mur- phy et al. 2000; Sharif et al. 2018). Due to chitin and chitosan’s low toxicity and envi- ronmental compatibility, chitin- and chitosan-based soil amendments offer opportunities for the control of fungal pathogens in urban landscape ecosystems through their induced resistance mode of action (Zhang et al. 2003). Likewise, given the relative low cost, these products offer an economically feasible option
March 2023
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