THE PATHOGENICITY OF FUNGI ISOLATED FROM CACTUS PEARS - M. T. Tesfaendrias, G. Tarekegn & W. J. Swart

Centre for Plant Health Management, University of the Free State, Bloemfontein 9300, South Africa

Several fungal pathogens associated with different disease symptoms of cactus pears (Opuntia ficus-indica) were isolated.  The objectives of this study were to determine the pathogenicity of the fungi and to evaluate the susceptibility of different cactus pear cultivars.  The fungi used in the assessments were Alternaria alternata, Lasiodiplodia theobromae, Fusarium oxysporum, Phialocephalla virens and an unidentified fungal pathogen.  Studies in the glasshouse on young potted plants confirmed the pathogenicity of all fungi with a mean lesion length ranging from 8.8 to 13.59 mm.  The fungus A. alternata had the longest mean lesion length (13.59 mm), followed by L. theobromae (10.66 mm) and the unidentified fungus (10.45 mm).  To evaluate the variation between varieties and pathogens, a field pathogenicity test was conducted on four-year-old plants.  Five fungal pathogens were tested against five cactus pear varieties.  The developing lesions were measured 20 days post inoculation.  There was significant (P = 0.05) difference between the pathogenicity of the fungi tested.  Of these L. theobromae, with a mean lesion length of 11.76 mm, was the most pathogenic, followed by F. oxysporum (8.09 mm) and A. alternata (6.38 mm) in all the cultivars tested.  Lesions did not develop on control cladodes.  Cultivars did not differ significantly (P = 0.05) in susceptibility to the different pathogens, with the exception of P. virens and the unidentified fungus.  From this survey, it is evident that numerous fungal pathogens have the potential to pose a threat to cactus pear cultivation in South Africa.  These results will be useful in the implementation of an IPM program for commercial cultivation of cactus pear in South Africa.