Reducing apple scab and frogeye or black rot infections with salicylic acid or its analogue on field-established apple trees

Abstract

Apple scab (Venturia inaequalis) (AS) and frogeye leaf spot or black rot (Botryosphaeria obtusa) (FLS or BR) are two diseases of apples that affect developing foliage and fruit during summer and cause significant losses in the field and during storage. In this 3-year study (2014–2016), salicylic acid (2-hydroxybenzoic acid) or its analogue (Benzothidiazole or Actigard®) were applied to field-established apple ‘Honeycrisp’ at the tight cluster stage of flower bud development and at late pink bloom as either trunk injection or foliar sprays and to ‘Cortland’ (injection only). AS and FLS leaf infections were assessed 1, 2, and 3 weeks after the second treatment while infections on fruit were assessed at harvest. Main treatment effects were significant (P = 0.05) and both salicylic acid and Actigard® showed less infections on foliage due to AS in 2014 and 2016, and only salicylic acid showed less BR infections on fruit in both years. The cultivar effects were also significant, showing consistently less AS infections on the foliage and fruit of ‘Honeycrisp’, and less FLS infections on ‘Cortland’ foliage. Injection and spray treatments of salicylic acid and Actigard® showed no differences in infections by AS and FLS or BR on foliage or fruit. The method of application × treatment interactions were only significant in 2014; both salicylic acid and Actigard®-treated ‘Cortland’ trees showed less FLS infections on foliage and less BR infections on fruit, and only salicylic acid-treated ‘Honeycrisp’ trees showed less percentage of AS and BR infected fruit. In 2015, both salicylic acid and Actigard® treatments showed less infections of FLS on foliage and lower BR-infected fruit on ‘Cortland’. In 2016, both these treatments reduced incidence of AS and FLS on ‘Cortland’ foliage, and only salicylic acid reduced AS infections on ‘Honeycrisp’ foliage. Further studies are needed to integrate salicylic acid or Actigard® treatments into an apple foliar disease management program with registered fungicides.