BIOLOGICAL CONTROL OF CACTUS PEAR PATHOGENS USING YEASTS - G. Tarekegn, Barbara K. Mashope & W. J. Swart

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

To identify yeasts with biocontrol activity against cactus pear pathogens, over 270 strains isolated from the surface of cactus pear fruit were screened in-vitro in a dual culture test. Many strains belonging to different genera of yeasts showed antagonistic activity against Fusarium proliferatum.  Ten strains were selected for further in-vitro evaluation on nutrient agar against six pathogens of cactus pear [Alternaria alternata, Fusarium oxysporum (two isolates), F. proliferatum, Lasiodiplodia theobromae and Phialocephela virens].  All yeast strains significantly reduced (P = 0.05) colony diameter of the six pathogens, except for L. theobromae, where some yeast strains did not significantly inhibit mycelial growth compared with the control.  Seven days after incubation, colonies of most of the pathogens exposed to Cryptococcus saitoi (CS25) did not grow more than 50 mm in diameter, while the colony diameter of yeast-free cultures were nearly 80 mm. Averaged over all pathogens, the highest inhibition of mycelial growth (35%) was obtained with C. saitoi (CS25) followed by C. saitoi (CS26) (28%).  The ten isolates which showed in-vitro antifungal activity were tested for their effect on fruit rot on naturally infected fruits.  After 10 days in storage, all the isolates resulted in significantly lower incidence and severity of fruit rot than the control.  Three yeast strains [CS25, CS26 and Cystofilobasidium feraegula (CF72)] which have relatively consistent antifungal activity were evaluated in-vivo against two more virulent pathogens.  These strains significantly reduced lesion size caused by F. oxysporum and L. theobromae.  These results suggest that using yeast strains for controlling post harvest diseases of cactus pear diseases could be a viable alternative to chemical control.