| dc.description.abstract | The root rot problem which had been described as Fusarium yellows because
of yellowing of leaves, exhibited hypocotyl and root symptoms characteristic of those
caused by Fusarium root rot pathogen. No symptoms due to other root rot pathogens
were observed in all the 18 samples examined.
Laboratory isolation and identification of the root rot pathogen from
representative samples of hypocotyl, and lower roots showing characteristic root rot
symptoms and rhizosphere soil collected, confirmed Fusarium as the sole pathogen
causing the root rot problem. Five Fusarium isolates were isolated; three of them were
identified as Fusarium Oxysporum and two as Fusarium solani.
Pathogenicity tests of the five Fusarium isolates on large Rose coco (GLP - 2)
variety of beans using the root - clip and soil inoculation method revealed that all the
five isolates were pathogenic but different in virulence. The mean disease incidence
and severity values produced by all the isolates were higher for soil inoculation
method than the root clip method. The soil inoculation method was used in all
subsequent experiments.
Screening of antagonistic effects of two Trichoderma isolates ( Trichoderma
viride and Trichoderma koningii) against the Fusarium isolates in vitro gave positive
results. Trichoderma viride produced mean growth reduction of 100% using random
sprinkling and 98.2% using equidistant plating on Fusarium isolates whereas
Trichoderma koningii gave mean values of 92% and 56.4% using the two methods
respectively. Saprophytic growth and survival of the two Trichoderma isolates in
autoclaved half broken dehulled rice, sorghum and vermiculite used as food carriers
differed in their suitability for the isolates. There was a appreciable increase in the
number of conidia per gram of carrier for both Trichoderma isolates in rice and
sorghum during the first 14 days thereafter levelling off Growth of the two isolates
was very poor in vermiculite with Trichoderma koningii giving negligible growth after
18 days of incubation.
Greenhouse tests with the two Trichoderma isolates for the management of the
root rot of beans gave promising results. Trichoderma viride in half broken dehulled
, rice gave the best control of the disease under greenhouse conditions.
This was followed by Trichoderma koningii in sorghum, then Trichoderma viride in
vermiculite, then Trichoderma koningii in rice, then Trichoderma viride in sorghum
and finallyTrichoderma koningii in vermiculite. There was some correlation between
saprophytic growth of Trichoderma isolates in different carriers in the laboratory and
their antagonistic activity against the Fusarium pathogen in the greenhouse.
Trichoderma koningii in sorghum and Trichoderma viride in rice produced the best
growth in the laboratory and also the best management of the disease in the
greenhouse.
The two best biocontrol agents (Trichoderma viride in rice and Trichoderma
koningii in sorghum) were compared with a conventional chemical in the management
of the Fusarium root rot of beans in the greenhouse. A fungicide murtano containing
26% Thiram and 20% Lindane was used to dress Rose coco bean seeds at the
recommended rate of 109 per 0.5 kg of seeds.
The two antagonistic fungi were applied at the rate of 1:3, 1:6, and 1:9 (w/w)
of colonized carrier to pathogen infested soil. Trichoderma viride in rice food carrier
when applied at the rate of 1:3 gave the best results. Murtano was the least effective
in controlling the disease under greenhouse conditions. Disease level increased with
decreasing antagonist colonized carrier.
Five bean varieties and fifty-one bean accessions screened for resistance to the
disease showed that all the lines were susceptible with most of them having a disease
incidence of 100%. | en |
