ETIOLOGY OF LEAF RUST CAUSING FUNGI INFECTING SELECTED BRACHIARIA (Brachiaria mutica; Forssk) GENOTYPES IN KENYA

Abstract

These rust diseases causing pathogens are also of importance to many crops, hence the need to clarify the species delimitations in the natural lineages infecting brachiaria grass. The classification of yellow leaf rust fungi on brachiaria grass of the family Poaceae has experienced a long history of controversy and uncertainty due to the reduced morphological characteristics available for taxonomy. Therefore, this research was conducted to identify rust causing pathogens in brachiaria grass in selected regions of Kenya based on pathological, morphological and molecular techniques. Identification of rust causing pathogen was determined through pathogenicity on brachiaria genotypes and further sequenced. All brachiaria genotypes MG4, Piata, Xareas and Marandu were susceptible to the rust diseases in the field and also when inoculated under controlled environment. In the field the disease incidence was 100% during 2016 and 2017. The disease severity was higher in MG4 during 2016 (7.5) and 2017 (8.5) in the disease scale of 1-9 (least severe to highly severe respectively), but least severe in Piata (2.5) in 2016 and in Marandu during 2017. MG4 proved more susceptible with a disease score of 6.5 and the least was Piata (1.7) under glass house, but under growth chamber Xareas was the most affected at severity of 6.5. On pathological characterization the isolates from MG4 were more virulent on MG4 than isolates obtained from other brachiaria genotypes followed by isolates from Marandu with the disease severity of 6.8 and 5.6. When the isolates were subjected to different conditions it was established that relative humidity of 75% and temperature of between 20oC – 25oC was optimal and ideal for germ tube germination. Morphologically the isolates varied from spore size of 20µm to79µm and spore shape was oval and globose but all were yellow in colour. BLAST analysis identified distinct phylogenetic lineages within the complex of brachiaria leaf rust fungi. Uromyces japonicas (E-value 92.44% (574/580 base pairs) was predominant at 18% of isolates followed by Kweilingia divina (15%) and Puccinia graminis f. sp. tritici (14%) Cronartium coleosporioides (9%), Puccinia amari, Puccinia coronata var and Cronartium comandrae (6%) each, Pleurotus djamor strain, Kuehneola uredinis and Puccinia nakanishikii were 3% each, as the fungal community responsible for leaf rust disease in Kenya. The yellow leaf rust fungus (Uromyces japonicas) grouped alone as a strongly supported clade based on the phylogenetic affinities of five type specimens and aecial host associations. All brachiaria genotypes were susceptible with varying levels of susceptibility