Olive scab: a review1

Abstract

Olive scab or leaf spot, caused by the fungus Spilocaea oleagina, is widespread in the Mediterranean region. Losses arise mostly from defoliation of severely infected trees, with consequently reduced yield. Symptoms are mainly confined to leaves and appear as dark brown, circular, zonate spots surrounded by yellow haloes (‘peacocks eye’). S. oleagina shows a typical subcuticular growth, forming flat colonies within the cutinized layer of the thick epidermal cell wall. This habit has been associated with a defence reaction of the host involving mobilization and breakdown of the phenolic glucoside oleuropin and inhibition of pectolytic enzymes produced by the pathogen. The disease is particularly severe in densely planted groves of susceptible olive cultivars and in nurseries. Infections may occur throughout the year, except during hot and dry summers, when favourable temperatures (opt. 16–21°C) and rains occur. Conidia, formed at the apex of short ampulliform conidiophores, are usually carried by rain droplets, but recent data show that humid air currents and insects also contribute to limited aerial dissemination. Usually, the incubation period is about 2 weeks; however, if infection is followed by a hot season, it may last several weeks. Spots already formed in spring may stop growing in summer and resume their growth and sporulation in autumn. Chemical control schedules include fungicide (especially copper) treatments during the main infection seasons (spring and autumn).