Investigations on Biosuppression of Wilt Disease in Tomato Using Cell-Free Culture Filtrate of Phytopathogenic Fusarium oxysporum f.sp. lycopersici

Abstract

Potency of cell-free culture filtrate of pathogenic Fusarium oxysporum f.sp. lycopersici as systemic defense inducer against wilt disease in tomato was tested by treating seedlings with the filtrate before pathogen challenge. Infective conidia and the cell-free culture filtrate were prepared from F. oxysporum (Sensu lato) previously isolated from wilt diseased affected plants. Growth relations of the isolates were characterized under ambient temperature (25 ± 2°C) and viability of the infective conidia of the two isolates were evaluated. Treatments were done as follows: (a) treatment of seedlings with infective conidia only (Treatment-A) (b) culture filtrate treatment only (Treatment-B) (c) treatment with culture filtrate followed by inoculation with infective conidia (Treatment-C) and (d) the control which consisted of plants sprayed with sterile distilled water only (Treatment-D). Effects of fungal conidia on tomato seed germination as well as some agronomic characters of the tomato plants and disease incidence under treatments A, B, C and D were evaluated. The growth and sporulation rates of the F. oxysporum F-isolate-1 were 12.1 mm day-1 and 6.5 x 104 conidia cm-2 colony area respectively and the values for the second isolate, F-Isolate 2 were 6.1 mm day-1 and 5.4 x 104 conidia cm-2 colony area. The germination rates of the infective conidia of F-isolate 1 and F-Isolate 2 after 24 hours incubation period at ambient temperature were 70% and 85% respectively. Treatment of tomato seeds with conidia suspension containing 1.0 x 106 had no statistically significant effects on seed germination [F(2,12)=0.148, P=0.64]; the mean percentage germination of the seeds treated with F-Isolate 1, F-Isolate 2 conidia and the control were 52%,46% and 52% respectively after five days. In the Treatment-A (Infective conidia only), F-Isolate 1 and F-isolate 2 caused 100% and 80% wilt of the plant populations at 7 weeks post-inoculation, when the tomato plants were 9 weeks old and no wilt was recorded in the control. There was no wilt recorded under Treatment-B and Treatment-C also, but there was reduced flowering and the mean percentage number of leaves showing chlorosis were significantly higher than the control [F (4, 192) =6.641, P=0.001]. When the plants were 10 weeks old, withered branches at the lower vegetal strata and the senescent leaves in the control were comparable with that recorded under Treatment-B and Treatment-C (F (4, 159) =3.563, P=0.08].