The Influence of Inducing Agents Applied by Soil Drenches on Disease Severity of Apple and Pear Scab

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. However, 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.