Bean root rot complex; its management by microbial agents and plant resistance.
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%.