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New Aggressive Colletotrichum gloeosporioides Strains on Stylosanthes scabra Detected by Virulence and DNA Analysis

Widespread use of Stylosanthes scabra cv. Seca to improve native pastures in northern Australia makes it necessary to monitor changes in the anthracnose pathogen, Colletotrichum gloeosporioides, because new damaging races have arisen to devastate many agronomically promising cultivars in the past. A total of 103 isolates collected during the past 20 years were analyzed by using virulence and molecular markers to determine whether aggressive strains have evolved in the field. Data on severity for eight host differentials were obtained from a greenhouse assay and analyzed by linear discriminant functions developed from existing data on 182 isolates of known races. A molecular analysis of a subset of 21 isolates by random amplified polymorphic DNA (RAPD) and electrophoretic karyotyping gave comparable results for both markers, although electrophoretic karyotyping detected a higher level of polymorphism. The majority of the 103 isolates were placed into three groups on the basis of RAPD analysis. Virulence analysis detected isolates that were highly aggressive toward Seca. During the past 20 years, the frequency of aggressive isolates has increased. Many of the aggressive isolates were collected during the 1990s, and one had a distinct RAPD genotype. The lack of a strong association between RAPD genotypes and pathogen races suggests that races have arisen independently within each genetic group. This work highlights the need for regular monitoring of the pathogen population.