358 Percival: Induced Resistance and Scab Severity Arboriculture & Urban Forestry 2020. 46(5):358–370 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & The Influence of Inducing Agents Applied by Soil Drenches on Disease Severity of Apple and Pear Scab By Glynn C. Percival Abstract. Apple and pear scab are foliar diseases of ornamental and fruiting apple and pear trees. Unmanaged, yield and aesthetic losses can be severe. Overreliance on synthetic fungicides means novel means of disease management are required. Field trials were conducted using apple (Malus cv. Crown Gold) and pear (Pyrus communis ‘Williams Bon Chrétien’) to assess the efficacy of a range of commercially available inducing resistance (IR) agents (harpin protein, potassium phosphite, salicylic acid derivative, and chitosan) as root drenches against both scab diseases. A synthetic fungicide (penconazole) spray program used within the UK for apple and pear scab control was included for comparison. Each IR agent was applied four times, (i) before the visible appearance of scab (April through June, i.e., preventatively) or (ii) after symptoms of scab were visibly observed (June through August, i.e., curatively). Limited efficacy as scab protectants was demonstrated when IR agents were applied curatively. Likewise, limited efficacy was recorded when IR agents were applied once or twice as a preventative measure. How- ever, when IR agents were applied as root drenches greater or equal to three times, efficacy as scab protectants was confirmed (increased leaf chlorophyll content, increased fruit yield, reduced leaf and fruit scab severity). A synthetic fungicide penconazole spray program provided the greatest protection against apple and pear scab in all trials when sprayed preventatively rather than curatively. Results suggest application of at least three root drenches from April through June with an appropriate IR agent provides a useful addition to existing methods of apple and pear scab management under field conditions. Keywords. Fungicides; Integrated Disease Management; Orchard Management; Pathogen Control; Plant Health Care; Urban Landscapes; Venturia inaequalis; Venturia pirina. INTRODUCTION Ornamental apple (Malus spp.) and pear (Pyrus spp.) varieties/cultivars are a popular and attractive choice for both public and private urban landscapes through- out Europe. Trees are hardy, adaptable to varied soil conditions, and are available in a wide range of sizes and shapes. Species exist to suit almost any purpose, from small urban gardens, entry courtyards, and park- ing strips, to public parks and highway plantings. Both Malus and Pyrus species bloom profusely in eye-catching shades of pink, red, and white, accented by purple or bronze leaf colour. Their colouration makes them effective ornamental trees in the spring landscape. Because of these characteristics, many species from both the Malus and Pyrus genus are fre- quently planted within European urban landscapes (Percival 2018). During the growing season, how- ever, both ornamental and fruiting Malus and Pyrus species are highly susceptible to attack by the foliar ©2020 International Society of Arboriculture pathogens Venturia inaequalis (Cke) Wint. and V. pir- ina Aderhold, causes of apple and pear scab, respec- tively (Sabri et al. 1997; Cuthbertson and Murchie 2003; Villalta et al. 2004; Hailey and Percival 2014). Scab symptoms initially appear in the spring as small, olive green lesions on the lower leaf surface. As leaves age, the lesions become darker and more dis- tinct in outline. If heavily infected, the leaf becomes distorted and drops prematurely in the summer. Trees of highly susceptible varieties may be severely defo- liated by mid to late summer. Fruit symptoms are similar to those found on leaves. The margins of the spots, however, are more distinct on the fruit. The lesions darken with age and become black and scabby. Badly scabbed fruit becomes deformed and may fall before maturity (Percival and Boyle 2005; Aćimović et al. 2016). Within the United Kingdom, thousands of ornamental apple and pear trees exhibit symptoms of these two pathogens, which untreated over time
September 2020
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