Induced resistance against Sclerotinia sclerotiorum in kiwifruit leaves

Salicylic acid (SA), 1‐aminocyclopropane‐1‐carboxylic acid (ACC) and DL‐β‐amino‐n‐butyric acid (BABA) were screened for the ability to induce phenylalanine ammonia lyase (PAL) activity in kiwifruit (Actinidia deliciosa‘Hayward’) leaves. SA (2 mM) was the most effective and induced a 10‐fold rise in PAL activity after 2 days compared with a four‐fold rise 5 days after ACC (0.5 mM) treatment. BABA was not an effective elicitor of PAL. SA was further tested, alongside a chlorinated analogue 4‐chlorosalicylic acid (4CSA), for the ability to control Sclerotinia sclerotiorum on kiwifruit leaves. Pretreatment with SA and 4CSA caused a reduction in the size of lesions arising from subsequent S. sclerotiorum infection. 4CSA was the more effective and reduced disease levels, relative to controls, by up to 85% on leaf discs and 78% on leaves on the vine. This compares with a 48% reduction by SA on both. Resistance to infection was not affected by washing treated leaf discs prior to inoculation or by delaying inoculation for up to 4 days following 4CSA application. SA and 4CSA were rapidly absorbed and metabolized by kiwifruit leaves and had no apparent phytotoxic effects at the concentrations used for disease control studies. It is proposed that SA and 4CSA operate through the induction of host resistance mechanisms.