Impact of trunk injections and foliar sprays of salicylic acid and Actigard^® on apple scab and frogeye or black rot infections and changes in leaf protein profiles

Venturia inaequalis (apple scab) and Botryosphaeria obtusa (frogeye leaf spot or black rot on fruit) are two polycyclic fungal pathogens of apples that infect both foliage and fruit, resulting in substantial economic losses to growers. In this 3-year study (2014-2016), salicylic acid and Actigard^® (Benzothidiazole or BTH) were applied to field-established ‘Honeycrisp’ as either trunk injections or foliar sprays and ‘Cortland’ trees (injection only) at the tight cluster stage of flower bud development and at late pink bloom. Incidence of leaf infections was assessed 1, 2, and 3 weeks after the second treatment while disease incidence on fruit was assessed at harvest. For injection treatments, leaves and fruit on the branch directly above the injection site were also assessed. In 2014, both salicylic acid and Actigard^® treatments resulted in less leaf infections of apple scab and frogeye on ‘Honeycrisp’ trees but only salicylic acid reduced incidences of apple scab and black rot on ‘Honeycrisp’ fruit. In 2015, trunk injections of both salicylic acid and Actigard^® reduced incidences of frogeye or black rot on ‘Cortland’ foliage and fruits, respectively. In 2016, salicylic acid and Actigard^® treatments reduced incidences of apple scab and frogeye on foliage of both cultivars, but there were no significant treatment effects on scab or black rot infections on harvested fruit. Quantitative proteomic investigations employing LC-MS analysis of leaf samples from 2014 and 2015 showed that both salicylic acid and Actigard^® treatments resulted in changes in several proteins involved in many metabolic and regulatory pathways. Further studies are needed to integrate salicylic acid or Actigard^® treatments into an apple foliar disease management program with registered fungicides.