Comparative studies of Dothiorella on avocado

Abstract

A market survey was conducted to determine the incidence of stem-end rot (SE) on avocado fruit obtained from the Pretoria Fresh Produce Market representing the Tzaneen production area. Dothiorella aromatica isolates collected from this survey were compared in terms of physiological characteristics i.e. growth and temperature, carbon and nitrogen utilization and pH response as well as genetic relatedness using random amplified polymorphic DNA (RAPD's). The incidence of SE was found to be as high as 31% and anthracnose 18%. Symptom development was more apparent when fruit was evaluated at the overripe than eating ripe stage. Of the 12 identified fungi isolated from SE lesions, D. aromatica was by far the most frequently isolated fungus. All D. aromatica isolates tested were found to be pathogenic using the fruit inoculation technique. Based on lesion size, isolates were separated into two groups of virulent and less virulent. Most isolates grouped within the one cluster, with only one isolate falling in the second group being less virulent. Although similar groupings were found between physiological tests, a lack of consistency as to which isolate belonged to which group was found. The optimum temperature for growth was 25°C and an initial pH of 6. The mean colony growth rate was 5 mm day-1. Isolates grew at a minimum of eight to a maximum of 27 mm within 24 hours. Isolates grew best on pectin and poorly on sorbitol when used as a carbon source. Urea supported growth best and poor growth was found on casein-amended sources. At a molecular level, the RAPD technique could be used successfully to seperate isolates into three groups based on cluster analyses. OPC02 was the most discrimatory primer and was therefore used in this study. Isolates produced DNA fragments ranging from 1500 bp to 450 bp. The results obtained from RAPDs could not be correlated with the pathogenicity and physiological tests. Future studies should focus on comparing isolates from different avocado production areas and testing different primers for the ability to distinguish between isolates of D. aromatica.