THE DEVELOPING RELATIONSHIP BETWEEN STYLOSANTHES AND ANTHRACNOSE AFTER 14 YEARS IN A NORTH QUEENSLAND PASTURE .2. DIVERSITY IN THE PATHOGENPOPULATION

Broad diversity was indicated in fungal isolates of Colletotrichum glo eosporioides collected from a mixed accession Stylosanthes pasture dur ing 1987-89. Isolate pathogenicity studies revealed 7 different pathog enic groups in 1987, while 16 different groups were separated in the 1 989 collection. About 40% of isolates from both years were capable of producing symptoms on only 2 differential host lines while the more co mplex groups (pathogenic on 3-6 differentials) were proportionately le ss common. Fungal growth at 4 temperatures revealed significant variat ion in growth rates although most isolates grew optimally at 28-degree s-C. When 3 isolates representing different pathogenic groups were exa mined in a series of glasshouse inoculum composition studies, all cont ributed evenly to the number of lesions produced when inoculated toget her in equal proportions. There was no evidence of domination by the m ost virulent isolate used in these glasshouse studies and similarly th ere was no evidence of particular strain dominance in the field. Conti nual natural diluting of virulent field inoculum by the large diversit y of other strains would probably prevent rapid severe epidemics devel oping in this pasture.