S. CHAKRABORTY^1,2, R. PERROTT², M.J. D'A. CHARCHAR³, C.D. FERNANDES⁴ and S. KELEMU⁵

¹CSIRO, Tropical Agriculture, St Lucia, Queensland, Australia
²Co-operative Research Centre for Tropical Plant Pathology, University of Queensland, St Lucia, Queensland, Australia
³EMBRAPA-CPAC, Planaltina, Brasilia, Brazil
⁴EMBRAPA-CNPGC, Campo Grande, Brazil S Tropical Forages Program, CIAT, Cali, Colombia

Abstract

This work provides information on genetic and pathogenic variation in isolates of Colletotrichum gloeosporioides which cause anthracnose disease in many species of the tropical pasture legume, Stylosanthes. Isolates collected from species other than S. guianensis mainly from the centre of host-pathogen diversity in South America were studied using Random Amplified Polymorphic DNA (RAPD) markers and differential virulence on a set of host differentials. A phenetic analysis of 90 isolates using RAPD markers showed a wide genetic diversity in the pathogen population. The genetic variation in the Australian population was very limited compared with that at the centre of diversity. A similar situation was detected for pathogenic variation in 69 isolates from Brazil, many of which showed higher severity on Australian cultivars and accessions. These include accessions and cultivars which are highly resistant to the Australian races of the pathogen. However, pathogenic diversity in the Brazilian population of C. gloeosporioides was only partly characterised due to the limited number of host differentials used in this study. Attention needs to focus on the development and use of a more extensive range of host differentials. There was no strong relationship between genetic and virulence markers. Consequently, pathogenicity tests need to be conducted to answer the more practical questions such as the risk to current commercial cultivars from damaging races. Use of molecular markers will be useful to understand better the movement of pathogen populations between geographically isolated regions. Isolates used in this work have come mainly from regions where S. capitata and S. guianensis are endemic. Further isolate sampling from regions representing the native range of commercial species such as S. scabra and S. hamata will be necessary for a more comprehensive understanding of this diversity.