Browsing by Author "Russouw, Andre"
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- ItemEtiology and management of Neofabraea lenticel decay (bull’s eye rot) of apples in the Western Cape of South Africa(Stellenbosch : Stellenbosch University, 2019-12) Russouw, Andre; Lennox, Cheryl L.; Meitz-Hopkins, Julia C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.ENGLISH ABSTRACT: Postharvest lenticel decay of apple and pear fruit caused by Neofabraea malicorticis, N.perennans. N. vagabunda and N. kienholzii is a disease more commonly known as Bull’seye rot. In South Africa, only N. vagabunda has been identified to cause this disease on apple fruit in Western Cape apple orchards, especially on the late harvested cultivar ‘Cripps Pink’. The pathogen infects the lenticels of fruit in the orchard and disease symptoms only become visible months after harvest. Symptoms include decay spreading outward from an infected lenticel as concentric dark and light brown discoloured rings. This disease does not spread in postharvest storage and preharvest infections thus ultimately determine disease incidence. Preharvest management strategies reduce infection levels by the pathogen, but the postharvest application of fungicides can reduce the decay incidence of already infected fruit. There are, however, no fungicides registered against bull’s eye rot in South Africa. To confirm the current causal pathogen of bull’s eye rot in South Africa, Neofabraea spp. were isolated from symptomatic fruit received from packhouses in the Western Cape. Neofabraea species were identified using a multiplex-PCR. A total of 91 isolates were all identified as N. vagabunda. Subsequently, N. vagabunda isolates from the Western Cape were tested on key apple cultivars Fuji, Cripps Pink and Golden Delicious to evaluate cultivar susceptibility. The isolates were equally pathogenic on tested cultivars with low variation between the isolates. ‘Fuji’ and ‘Cripps Pink’ were found highly susceptible to disease development, averaging lesion diameters of 8.36 mm and 8.15 mm respectively 14 days after inoculation. ‘Golden Delicious’ was significantly less susceptible averaging only 6.28 mm in lesion diameter after 14 days. Two fungicides registered for use on pome fruit in South Africa, that have reportedly been found to effectively control bull’s eye rot in other studies, are the phenyl pyrrole fludioxonil, and the anilinopyrimidine pyrimethanil. The curative ability of these fungicides was tested on N.vagabunda inoculated ‘Fuji’ and ‘Cripps Pink’ apple fruit. The fungicide efficacy wascompared as a dip, drench and thermo-fog application. Dip application with fludioxonil effectively controlled bull’s eye rot incidence on ‘Fuji’ by 83% and ‘Cripps Pink’ by 84% compared to the untreated control fruit. Pyrimethanil did not control bull’s eye rot incidence as a dip application. As a drench however, pyrimethanil could control incidence on ‘Fuji’ by 27%. Fludioxonil was less effective as a drench and controlled disease incidence on ‘Fuji’ by 73%, and on ‘Cripps Pink’ by 41%. Pyrimethanil was the most effective as a thermo-fog application, controlling incidence of bull’s eye rot on ‘Fuji’ by 59%. On ‘Cripps Pink’ however, pyrimethanil thermo-fogging only controlled bull’s eye rot incidence by 18%. As a thermo-fog treatment, fludioxonil had moderate efficacy, controlling bull’s eye rot on ‘Fuji’ by 47% and ‘Cripps Pink’ by 28%. To investigate pyrimethanil inefficacy in controlling bull’s eye rot, the sensitivity of different N.vagabunda isolates on inoculated fruit were evaluated towards pyrimethanil, as well as theeffect of incubation time before curative fungicide application. Neofabraea vagabunda isolates did not differ in their sensitivity towards pyrimethanil and reacted equally to a 500 mg/L and 1000 mg/L concentration fungicide treatment. Fludioxonil was effective regardless of the incubation time. Pyrimethanil was significantly more effective when incubation time was shortened to 6 hours before treating fruit with the fungicide. In conclusion, Neofabraea vagabunda is the causal organism of bull’s eye rot in the Western Cape province of South Africa, and the late harvest apple cultivars ‘Fuji’ and ‘Cripps Pink’ are highly susceptible to this pathogen. Fludioxonil can effectively reduce N. vagabunda bull’s eye rot disease incidence when applied postharvest. Pyrimethanil had variable efficacy towards the pathogen but should not be discarded as a postharvest treatment for bull’s eye rot in South Africa, as the inoculation method used in the trials did not truly simulate natural infection of fruit by the pathogen.