The developing relationship between Stylosanthes and anthracnose after 14 years in a North Queensland pasture. 2. Diversity in the pathogen population

Broad diversity was indicated in fungal isolates of Colletotrichum gloeosporioides collected from a mixed accession Stylosanthes pasture during 1987-89. Isolate pathogenicity studies revealed 7 different pathogenic groups in 1987, while 16 different groups were separated in the 1989 collection. About 40% of isolates from both years were capable of producing symptoms on only 2 differential host lines while the more complex groups (pathogenic on 3-6 differentials) were proportionately less common. Fungal growth at 4 temperatures revealed significant variation in growth rates although most isolates grew optimally at 28�C. When 3 isolates representing different pathogenic groups were examined in a series of glasshouse inoculum composition studies, all contributed evenly to the number of lesions produced when inoculated together in equal proportions. There was no evidence of domination by the most virulent isolate used in these glasshouse studies and similarly there was no evidence of particular strain dominance in the field. Continual natural diluting of virulent field inoculum by the large diversity of other strains would probably prevent rapid severe epidemics developing in this pasture.