BIODIVERSITY, EPIDEMIOLOGY AND VIRULENCE OF COLLETOTRICHUM-GLOEOSPORIOIDES - II - GENETIC AND PATHOGENIC DIVERSITY IN ISOLATES OF COLLETOTRICHUM-GLOEOSPORIOIDES FROM 8 SPECIES OF STYLOSANTHES

This work provides information on genetic and pathogenic variation in isolates of Colletotrichum gloeosporioides which cause anthracnose dis ease in many species of the tropical pasture legume, Stylosanthes. Iso lates 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 virul ence on a set of host differentials. A phenetic analysis of 90 isolate s using RAPD markers showed a wide genetic diversity in the pathogen p opulation. The genetic variation in the Australian population was very limited compared with that at the centre of diversity. A similar situ ation was detected for pathogenic variation in 69 isolates from Brazil , many of which showed higher severity on Australian cultivars and acc essions. These include accessions and cultivars which are highly resis tant to the Australian races of the pathogen. However, pathogenic dive rsity in the Brazilian population of C. gloeosporioides was only partl y characterised due to the limited number of host differentials used i n this study. Attention needs to focus on the development and use of a more extensive range of host differentials. There was no strong relat ionship between genetic and virulence markers. Consequently, pathogeni city 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 move ment of pathogen populations between geographically isolated regions. Isolates used in this work have come mainly from regions where S. capi tata and S. guianensis are endemic. Further isolate sampling from regi ons representing the native range of commercial species such as S. sca bra and S. hamata will be necessary for a more comprehensive understan ding of this diversity.