A MOLECULAR APPROACH TO DISTINGUISH THE BEAN RUST AND SIRATRO RUST PATHOGENS

Rust disease on the tropical pasture legume Macroptilium atropurpureum (siratro) is caused by Uromyces appendiculatus var. crassitunicatus. This pathogen was believed to be closely related to the bean (Phaseolus vulgaris) rust pathogen Uromyces appendiculatus var. appendiculatus. The genetic relationship between these two fungi was investigated. Total DNA hybridisations indicated that little homology exists between the high copy genomic DNA of these two rust fungi. Random genomic probes cloned from the bean rust fungus detected extensive Polymorphisms between the two, with only one probe from 17 being monomorphic. The ribosomal DNA repeat unit was also distinguished by RFLPs. It was calculated from the RFLP data that the bean rust fungus and the siratro rust fungus share only 8-14% sequence homology. The results indicate that the two fungi, although morphologically very similar, are not closely related genetically.