Effects of antagonists, alum and ultraviolet irradiation on major fruit rots of banana (Musa sapientum (L.) Kuntze)

The biocontrol ability of Trichoderma spp. and bacterial antagonists, as well as the effect of alum and ultraviolet light were evaluated against Lasiodiplodia theobromae (Pat.) Griff. & Maubl. and Colletotrichum musae (Berg & Curt) Arx., the major causes of fruit rotting in bananas. T. harzianum exhibited the widest zone of inhibition against L. theobromae while T. viride was the most inhibitory against C. musae in dual culture tests. Bacterial antagonist designated as B050, B076, B081 and B082 showed the largest zones of inhibition against both pathogens in potato dextrose agar medium. All bacterial antagonists (except Bacillus subtilis) exceeded the inhibitory zones exhibited by benomyl at 500 ppm. The Trichoderma spp. and the bacteria significantly inhibited spore germination and germ tube elongation of both pathogens but not the germination of one-celled spores of L. theobromae. The mode of action of Trichoderma spp. was direct parasitism and production of metabolites harmful to the pathogens. All bacterial antagonist produced harmful metabolites. The culture filtrates of Trichoderma spp. inhibited germination, germ tube elongation and rotting in banana fruits caused by both pathogens. Cell-free culture filtrates of bacterial antagonists grown in potato dextrose broth adversely affected germ tube elongation of L. theobromae. Banana fruits treated with T. viride 4 hr before inoculation with L. theobromae provided better protection than simultaneous or treatment 4 hr after inoculation. Among the bacterial anatagonists, B. subtilis showed maximum disease reduction against both L. theobromae (30.91 percent) and C. musae (72.30 percent). Minimal spore germination of L. theobromae was observed in 15 percent alum. The shortest germ tubes of C. musae were noted in 15 percent alum after 24 hr. The mycelial growth of L. theobromae was enhanced in amended PDA medium with 0.1 percent alum but was completely suppressed in 0.6 percent alum whereas growth of C. musae was suppressed completely in 1.25 percent alum. Rotting in banana fruits due to L. theobromae was completely controlled by 15 percent alum while C. musae rot was completely controlled by 8 percent alum. Alum treatment of banana fruits up to 4 hr after inoculation provided better control against L. theobromae rot than treatment at 4 hr before inoculation or simultaneous with pathogen introduction. One-celled spores of L. theobromae exposed to ultraviolet (UV) light (254 nm) lost their viability at 11 min exposure while two-celled spores stimulated germination and germ tuber elongation even after 30 min exposure. The spores of C. musae lost their viability at 3 min exposure. Disease reduction was not correlated with the duration of exposure to UV light but the lowest Lasio diplodia rot was recorded in inoculated banana fruits exposed for 10 min.