ISOLATION, SCREENING AND IDENTIFICATION OF PROMISING YEAST ISOLATES USED FOR BIOLOGICAL CONTROL OF ORANGE GREEN MOULD

Ninety nine yeast isolates were isolated from surface of apple, grape, orange and tomato fruits. The isolates were tested in vivo in preliminary study for biocontrol potential against green mould of navel orange fruit. According to primary screening, twenty two isolates were selected to continue the secondary screening (phase one) using different concentrations of the washed yeast cells in water suspension to evaluate their biocontrol efficacy at 21±1°C. Among twelve yeast isolates were passed to the secondary screening (phase two) at 7±1°C, washed cells of yeast isolates CT 503, CT 507, CT 508, CT 512 and CT 550 at 1x109, 2x108 and 1x108 CFU/ml produced complete protection for 21 days to wounded navel orange fruits inoculated with spore suspension of Penicillium degitatum  (1x104 conidia /ml).  Meantime, no lesions developed on the navel orange fruits treated with the yeast isolates CT 503, CT 507 (Debaryomyces hansenii var. hansenii strain C) and CT 512 (Endomycopsella vivi) at 6.6x107CFU/ml, while the percentage of rot reduction of the isolate CT 550 (Candida edax) was 99.81%. Culture filtrate of twenty two different yeast isolates used in secondary screening (phase one) did not prevent decay of wounded navel orange fruits but had an inhibitory effect on rot development. The relative abilities of the promising yeast isolates (CT 503, CT 507, CT 512 and CT 550) to induce disease resistance against P. digitatum on navel orange fruits were studied. Inoculation of promising yeast isolates significantly triggered induction of resistance in navel orange fruits. The lesion diameters of green mould 66 hours later after inoculation by spore suspension of P. digitatum  in a neighbouring wound that was made approximately 6 mm away from the initial woundwhich inoculated withthe isolates CT 512, CT 550, CT 507 and CT 503 were reduced by 25.5%, 20.5%, 16.7% and 14.1%, respectively. In this respect, there were no significant differences among the three different isolates CT 503, CT 507 and CT 550.