REGIONAL AND LOCAL PATTERNS IN THE SPATIAL-DISTRIBUTION OF THE FLOWER-INFECTING SMUT FUNGUS SPORISORIUM-AMPHILOPHIS IN NATURAL-POPULATIONS OF ITS HOST BOTHRIOCHLOA-MACRA

Regional variation in the incidence of the systemic floral-smut fungus Sporisorium amphilophis (Syd.) Langdon & Fullerton on the perennial g rass Bothriochloa macra (Steud.) S. T. Blake was investigated through three surveys over a 12 yr period (1981-93). In all three surveys a ma rked north-south trend in percentage of infection was detected with a greater proportion of plants in northerly populations being infected t han in populations located to the south. The incidence of disease in p opulations was negatively correlated with the frequency of days with t emperatures < 0 degrees C in winter. Detailed exploration of local var iation in a subset of five populations showed that the incidence of S. amphilophis was density-dependent and was greater in the edge areas o f host populations than in the less disturbed core areas. Smut prevent ed seed production and negatively affected different aspects of the mo rphology of the plants, such as the height and basal diameter. Infecti on also significantly increased the number of inflorescences per plant . These field results were complemented by glasshouse-based competitio n experiments which indicated that, despite causing reductions in size , S. amphilophis only affected the competitive ability of infected pla nts grown under sub-optimal conditions. The results of this study indi cate that an interplay between physical and biotic aspects of the envi ronment determines regional and local levels of disease.