Arboriculture & Urban Forestry 47(2): March 2021 91 Table 6. The influence of fungicide and liquid chitosan combinations applied as foliar sprays on symptom severity of pear scab (Venturia pirini) on Pyrus communis ‘Williams’ Bon Chrétien.’ Treatment Water (control) Liquid chitosan (LC) Boscalid + pyraclostrobin 0.9 g/L Boscalid + pyraclostrobin 0.9 g/L + LC Boscalid + pyraclostrobin 0.6 g/L + LC Boscalid + pyraclostrobin 0.3 g/L + LC z Chlorophyll content Leaf scab symptom severityz 28.4a 32.4ab 42.8c 46.1c 39.2bc 30.7a 4.0c 3.6b 0.5a 0.5a 2.5b 3.5b Fruit scab symptom severityz 2.2b 1.8b 0.5a 1.0a 1.0a 1.8b Fruit yield 9.5a 10.9abc 13.6c 13.0c 12.5bc 10.0ab All values mean of 80 leaves (10 leaves per tree) and 40 fruits (5 fruits per tree) from 8 randomized complete blocks with a single tree replication per block. Numbers within a column followed by a common letter are not significantly different according to the Kruskal-Wallis Multiple Comparison Z-test (P ≤ 0.05). However, a significant difference between boscalid + pyraclostrobin (labelled rate) with and without LC and boscalid + pyraclostrobin (2/3 labelled rate) + LC was recorded with respect to leaf scab symptomology (Table 6). DISCUSSION Aesthetic damage such as symptoms of leaf blotches and scabs caused by fungal pathogens is viewed as a high priority by professionals managing urban land- scapes, as aesthetics (leaf colour, silhouette, shape) is a major characteristic by which trees are selected and planted (Hersh et al. 2012; Hantsch et al. 2013, 2014). However, loss of leaf photosynthetic area caused by these pathogens can also have a detrimental influence on the short- and long-term survival of trees (Hersh et al. 2012). Excess photosynthetic carbohydrates are predominantly stored in the sapwood and the bark (Mangel et al. 2000), and it is important for trees to have a large carbohydrate reserve for future growth and tolerance to abiotic stressors such as drought, salinity, and waterlogging (Martínez-Trinidad et al. 2009). In addition, the amount of reserve carbohy- drates in young trees and their root systems is crucial for successful establishment following out-planting (Ritchie and Dunlop 1980; Struve 1990). The process of recovery following root severance is dependent on the ability of a tree to manufacture abundant photo- synthetic carbohydrates, such as sucrose (Lonsdale 2001; Lindqvist and Asp 2002). While it could be argued that mature trees will be in a position to with- stand repeated loss of leaf photosynthetic area due to their larger storage reserves within the tree trunk and root system (Pallardy 2008; Martínez-Trinidad et al. 2009), it is debatable whether smaller trees (≤ 2.5 m high) are capable of such long-term survival. Our results demonstrated that currently labelled fungicides are effective against pear scab (Venturia pirina) and Guignardia leaf blotch (Guignardia aesculi) under field conditions, provided adequate spray coverage and frequent sprays are applied. Under these conditions, symptom control ranged from 75% to 100%. Boscalid plus pyraclostrobin is a systemic fungicide with protective and curative properties that has been shown to possess efficacy against a broad range of foliar pathogens under in vitro, glasshouse, and field conditions (Percival et al. 2009; Burnett et al. 2010; LaMondia 2015). Consequently, boscalid plus pyraclostrobin has approval for the management of foliar leaf pathogens under glasshouse, orchard, and urban landscapes (BCPC 2019). In contrast, although it is now well documented that pre-treatment of plants with various chemical and biological agents (e.g., virulent or avirulent pathogens, non-pathogens, cell wall fragments, natural plant extracts, and syn- thetic chemicals) can induce resistance to subsequent pathogen attack, efficacy is more uncertain. Induced resistance rarely leads to complete pathogen control, more often resulting in, for example, a reduction in lesion numbers and size (Walters et al. 2005). Despite this, the prospect of broad-spectrum disease control using the plant’s own resistance mechanisms has led to increasing commercial interest in the development of available IR agents which mimic natural inducers of resistance (Walters et al. 2005; Eyles et al. 2010). Our study, like others, found that the IR agents harpin ©2021 International Society of Arboriculture
March 2021
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