Brachiaria grass is one of the most important grasses distributed throughout the tropics especially in Africa. The quality of forage yields and biomass of brachiaria is negatively affected by diseases among which leaf spot, leaf blight and leaf rust are the most common. This research was conducted to characterize the rust pathogen of bracharia grass infecting selected cultivars in Kenya based on pathological and morphological characteristics. Leaves with leaf rust pustules were collected from improved cultivars and natural population from brachiaria proÂ¬duction and demonstration plots at the International Livestock Research Institute, Nairobi, Kenya. Pathogenicity of rust isolates were determined on the brachiaria cultivars in glass house. Further, the rust pustules were infected to the most susceptible (MG4) cultivar for studies on aggressiveness of these Uredospores . The growth conditions of rust pustules were determined by exposing into different regimes of relative humidity, temperature and time to germtube development were established under rust ideal environmental conditions. All brachiaria cultivars tested (MG4, Piata, Xareas and Marandu) were susceptible to rust diseases in the field as well as in controlled environment. In the field the Per cent disease index.of 100% was recorded while disease severity was higher in MG4 (7.5 and 8.5) in 2016 and 2017 respectively, in a disease severity scale of 0 to 9. The disease was least severe in Piata (2.5) in 2016 but in 2017 Marandu was least infected. Under artificial inoculation and controlled growth conditions MG4 proved more susceptible (6.5) while, Piata (1.7) was least infected in glass house but under growth chamber Xareas was most infected with diseases score of 6.5. The test of virulence revealed that isolate from MG4 was more virulent on MG4 than isolates obtained from other brachiaria cultivars followed by isolates from Xareas which caused disease severity of (3.5) on MG4. When the isolates were subjected to different environmental conditions it was established that optimum relaÂ¬tive humidity of 75% and temperature of between 20-250C was ideal for germ tube germination. Germ tube germination started after 8th hour but stopped extending at 24th hour. Morphologically the isolates varied from spore size of 20Âµm to79Âµm and spore shape was oval and globose but all were yellow in colour. These findings indicated high occurrence of leaf rust disease in brachiaria cultivars with varying tolerance levels which can provide a potential source of resistance genes. Similarly the rust pustules showed morphological and pathological variation. Further it is advisable to institute control measures against leaf rust whenever relative humidity is above 75% and temperature of 20-250C prevails.
Keywords: Brachiaria, Leaf rust, Uredospores , characterization, disease severity, growth conditions