Screening of olive cultivars for tolerance to Fusicladium oleagineum in South Africa

Abstract

Olive leaf spot (OLS) caused by Fusicladium oleagineum, is one of the major fungal diseases affecting olive orchards in South Africa and worldwide. Fungicides (copper containing sprays) successfully control OLS, when disease risk is low, but heavy infestation impairs effectiveness of chemical control and could lead to severe yield loss. Regular chemical treatment may prevent disease build-up in the orchard, but with cost and chemical residue implications. This study was aimed at evaluating commercial olive cultivars (‘Mission’; ‘Manzanilla de Seville’; ‘Frantoio’; ‘Nandi’; ‘Nocellara del Belice’; ‘Coratina’; ‘Barouni’ and ‘Leccino’) for resistance against OLS. A growth chamber trial was conducted at Bien Donne experimental farm of the Agricultural Research Council. Only one temperature treatment (16°C) was applied during inoculation, while a relative humidity of above 80% was maintained. Plants were kept for 48 hours at 99% RH after inoculation and then moved to a shade net area for disease development at a temperature of 25±5°C. The results were used for categorising the eight olive cultivars evaluated in this study according to their OLS susceptibility. ‘Frantoio’ was categorised as highly tolerant, while ‘Nandi’ and ‘Leccino’ were found to be moderately tolerant and ‘Nocellara del Belice’ fairly tolerant. ‘Coratina’ was found to be the most susceptible cultivar. ‘Mission’ and ‘Manzanilla de Seville’ were found to be fairly susceptible. The findings of this study provide a basis for preliminary recommendations of OLS tolerant cultivars and selections for commercial olive production, as well as for further evaluation of OLS tolerance of olive cultivars and selections both in a controlled environment (glasshouse/ growth chamber), and under field conditions. As the results of this study were obtained over one season only (2011/12), it is recommended that the evaluation of OLS tolerance of the eight cultivars included in this study should be repeated both under controlled conditions, as well as under field conditions in the major olive production regions of South Africa, to verify the preliminary results.