Arboriculture & Urban Forestry 47(2): March 2021 Arboriculture & Urban Forestry 2021. 47(2):85–95 https://doi.org/10.48044/jauf.2021.008 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & Evaluation of Inducing Agents and Synthetic Fungicide Combinations for Management of Foliar Pathogens of Urban Trees By Glynn C. Percival and Sean Graham Abstract. Unmanaged, foliar pathogens of urban trees can be detrimental to tree health and aesthetics. Overreliance on synthetic fungicides increasingly means alternative means of pathogen management are now required. The purpose of these studies was to investigate the efficacy of 3 commercially available agents, harpin protein, salicylic acid derivative, and liquid chitosan, which can initiate induced resistance (IR) in plants. IR agents were applied independently and in combination with a synthetic fungicide (boscalid + pyraclostrobin) against 2 foliar patho- gens (Venturia pirina and Guignardia aesculi) under field conditions with Pyrus communis ‘Williams’ Bon Chrétien’ and horse chestnut (Aes- culus hippocastanum) acting as tree hosts. These agents were tested over 3 consecutive years. In 4 of 5 field studies, the use of an IR agent alone reduced pathogen symptom severity, increased fruit/seed yield, and enhanced leaf chlorophyll content. In virtually all studies, application of boscalid + pyraclostrobin at 2/3 strength plus an IR agent provided the same degree of pathogen control as boscalid + pyraclostrobin at full strength. Application of boscalid + pyraclostrobin at 1/3 strength plus an IR agent provided a reasonable degree of foliar pathogen control. Results showed that a combined mix of an IR agent with a 1/3 reduced dose of boscalid + pyraclostrobin was as effective at reducing symptom severity of 2 foliar pathogens as boscalid + pyraclostrobin applied at full strength, provided at least 4 sprays were applied during a growing season. Keywords. Fungicides; Guignardia; Orchard Management; Pathogen Control; Plant Health Care; Synergism; Urban Landscapes; Venturia. INTRODUCTION Throughout the world, foliar diseases such as anthrac- nose, mildews, rusts, and sooty moulds occur on most species of trees and shrubs planted in urban landscapes. Symptoms of these diseases include scabs, spots, blotches, blights, needle casts, blisters, and curls (Hersh et al. 2012; Hantsch et al. 2013, 2014). Heavy patho- gen symptomology can result in a reduction in leaf photosynthetic productivity, loss of carbohydrates, premature leaf drop, decreased tree growth and seed yield, increased rate of crown dieback, and a reduc- tion in carbon allocation to fine roots, trunk, and twigs (Villalta et al. 2004; Tubby and Webber 2010; Hersh et al. 2012; Hantsch et al. 2013; Oliva et al. 2014). Repeat annual defoliations and subsequent loss of photosynthetic productivity can ultimately result in tree decline and sometimes even death (Percival 2018). To prevent foliage loss and ornamental disfig- uration, repeat fungicide sprays are usually undertaken 85 (Carisse and Dewdney 2002; Hailey and Percival 2014). However, public and government demands to reduce fungicide use, stimulated by greater awareness of environmental consequences (e.g., spray drift, soil contamination), human health issues (contamination of the food chain), and collateral damage to nontarget organisms, has placed a greater emphasis on the devel- opment of reduced fungicide management strategies (Gozzo 2003; Fobert and Després 2005; Ilhan et al. 2006; Hailey and Percival 2014). Increased legisla- tive restrictions on the registration, use, and applica- tion of fungicides has also resulted in more fungicides in Europe being withdrawn on an annual basis than are released onto the commercial market (BCPC 2019). This in turn means an overreliance on fewer fungicides, which increases the selection pressure for fungicide resistance in surviving pathogen popula- tions (Ishii 2006). ©2021 International Society of Arboriculture
March 2021
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