How much captan is required for wound protection of <i>Neonectria ditissima </i>conidial infection in apple?

Abstract

Captan (a trichloromethyl sulfenyl fungicide) is commonly used for control of Neonectria ditissima in apple. In New Zealand, picking and leaf scars are the main source of new infections. The captan concentrations required for wound protection of leaf scars, picking and rasp wounds was determined in planta. Fresh wounds, inoculated with N. ditissima, were sprayed with captan using a motorised knapsack (leaf scars) or hand-held spray bottles targeting the wound area (picking and rasp wounds). Captan concentrations ranged from 0 to 400% field rate (100% = 2 kg captan/ha). Wounds were sampled pre- and post-captan application for residue analyses and the remainder assessed regularly in the field for disease expression. Disease development decreased as captan concentrations increased. Approximately 2 μg captan/scar (100% field rate) was required to protect leaf scars. Four times the field rate of captan was needed for a 55 and 70% disease reduction on picking and rasp wounds, respectively. Label-rate applications of captan controlled low incidence of N. ditissima leaf-scar infections in the field in autumn, but disease control of picking scars and other large wounds might be difficult to achieve.