Effects of some Fungicides on Growth Inhibition of <I>Colletotrichum gloeosporioides</I>, Causal Agent of Onion Twister Disease

Abstract

Leaf twister diseased of shallot and onion were collected from 5 plantation areas in Chiang Mai and Lamphun provinces. The causative pathogen was isolated by tissue transplanting technique. Five isolates of Colletotrichum gloeosporioides exhibited white to gray colony color and light orange color of spore mass  were found as concentric ring in the middle of the colonies after cultured on potato dextrose agar (PDA) for 7-9 days. Slide cultures of the fungal morphological studies revealed hyaline-septate hyphae and conidia were cylindrical with both rounded apices and single cell. The pathogenicity tests conducted on shallot seedlings showed severe symptoms of more lesions and twisted leaves numbers after inoculation with the isolates 2, 3 and 4 when comparing with the control trials. Six fungicides including carbendazim, azoxystrobin, difenoconazole, azoxystrobin mixed with difenoconazole, maneb and mancozeb were used in testing of fungicide efficacy. Each fungicide was mixed into PDA medium at different concentrations, including the concentration of the recommended rate, 1/2 of recommended rate, 1/4 of recommended rate and 1/8 of recommended rate. The results showed that carbendazim, at all concentrations, exhibited the most effective in inhibiting the growth of fungi at 100% inhibition. The inhibition was statistically significant different in all concentrations when compared to mancozeb, with the lowest efficiency in inhibiting the growth of the fungus at inhibition percentages of 19.33, 18.56, 14.33 and 12.67, respectively. In addition, the result of the soaking C. gloeosporioides culture disc in each fungicides at different concentrations showed that all concentrations of carbendazim inhibited the growth of the fungus with the inhibition percentages of 100.00, 100.00, 100.00 and 90.00, respectively, and the differences were statistically significant when compared to maneb, the fungicide with the lowest efficiency in inhibiting the growth of fungi at inhibition percentages of 2.78, 1.39, 1.39 and 0.00, respectively.