©2023 International Society of Arboriculture 70 Although greater degrees of scab control were achieved by the fungicide treatment in the container pot trials, in the case of the field trial the degree of scab control recorded via a single chitin or chitosan soil amendment was statistically equivalent to that of the fungicide treatment. These findings, combined with the relative low cost of chitin or chitosan com- pared to synthetic fungicides, mean the chitin- and chitosan-based products hold promise commercially to protect trees in large-scale urban forestry planting projects. Future research projects can be directed to further evaluate these products on larger trees and other species. A difference in efficacy between the commercially available chitin/chitosan fertilisers used in this study was recorded. Application of liquid chitosan for example reduced leaf and fruit scab severity by 31% and 28% respectively, while the chitin/chitosan fer- tilisers crab meal and Hortifeeds (Lincoln, England, UK) had limited effect on scab severity. Differences in the degree of pathogen protection between com- mercially available chitin/chitosan fertilisers are con- sistent with other researchers (Ozbay and Newman 2004; El Hadrami et al. 2010). The degree of resis- tance induced has been shown to be heavily influ- enced by the type of chitosan (pure or artificially modified), its degree of polymerisation, the host, the chemical and/or nutrient composition of the sub- strates, and environmental conditions (Walker et al. 2004; Sharp 2013). For example, in some studies, oligomeric chitosans (pentamers and heptamers) have been reported to exhibit a better antifungal activity than larger units (Rabea et al. 2003). In others, the antimicrobial activity increased with the increase in chitosan molecular weight (Kulikov et al. 2006). Results of this study show that care should be taken when selecting a chitin- or chitosan-based fertiliser for plant protection purposes as efficacy can differ mark- edly between formulations. However, the amendment of soil with chitin/chitosan- based fertilisers presents a number of desirable options for individuals involved in disease management within urban forestry landscapes. Both products are degraded enzymatically and are non-toxic to the ben- eficial rhizosphere biota at low concentrations, and both induce the symbiotic exchange between plant and microbes (Escudero et al. 2017). In addition, chi- tosan is a polysaccharide-based biopolymer, which stimulates the activity of plant symbiotic microbes, f.sp. radicis lycopersici in tomato was suppressed using chitosan amendments (Lafontaine and Benhamou 1996). Similarly, chitosan utilised as a soil amend- ment was shown to control Fusarium wilts in many plant species (Rabea et al. 2003). Applied at an opti- mal concentration, chitosan delayed disease develop- ment, leading to a reduced plant wilting in tomato (Benhamou et al. 1994). Similar results were reported in forest nurseries suffering from F. acuminatum and Cylindrocladium floridanum infections which were reduced following the use of chitosan as a soil amend- ment (Laflamme et al. 2000). In 3 separate field trials, foliar sprays of chitosan were shown to provide a higher degree of control of Pseudomonas syringae pv. actinidiae on avocado compared to commercially available copper hydrochloride and copper oxychlo- ride (Scortichini 2014). Foliar applications of chi- tosan were also shown to induce resistance against pitch canker (F. circinatum) and Diplodia tip blight (Sphaeropsis sapinea) in Pinus radiata with the authors concluding that chitosan offers potential for induced resistance in forest nursery disease management (Reglinski et al. 2004). The control of oomycete pathogens has also been achieved with chitosan treat- ment, with Phytophthora capsici controlled on pep- pers (Xu et al. 2007) and P. infestans in potato (O’Herlihy et al. 2003). It has been shown that chitin and chitosan induce host defence responses in both monocotyledons and dicotyledons. These responses include enhanced lignification of leaves, phytoalexin biosynthesis, generation of reactive oxygen species, expression of early responsive and defence-related genes, callose formation, and synthesis of proteinase inhibitors (Kuchitsu et al. 1993; Vasyukova et al. 2001; Wojdyla 2004; Sharp 2013). Although the mechanis- tic bases of each chitin/chitosan/fertiliser evaluated in these studies was not investigated, results published here are the first to show that applications of chitin and chitosan possess useful scab protectant properties when applied as soil amendments or foliar sprays. In line with other research findings, chitin applied at 0.5% and 1.0%, and chitosan applied at 1.0%, pro- vided a useful degree of efficacy as a scab protectant compound under containerised and field conditions, i.e., reduced leaf and fruit (field trials only) scab devel- opment: the main proxy of scab success or aggres- siveness. In terms of practical disease control, the frequency of application is a crucial consideration in terms of labour, chemical usage, and equipment. Percival et al: Chitin-Based Soil Amendments on Severity of Apple and Pear Scab
March 2023
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