Competition alters temporal dynamics of sporulation in the wheat stem rustfungus

To investigate the effects of competition on the timing of pathogen reprodu ction, urediniospores of two strains of Puccinia graminis f.sp. tritici (SR 22 and SR41) were inoculated onto leaves of wheat seedlings singly and in 1 :1 mixture at three inoculum densities. On randomly sampled leaves, uredini a were counted 9 days after inoculation and urediniospores were collected a nd quantified every other day from the seventh to the 29th day after inocul ation. Increases in inoculum density resulted in progressively smaller incr eases in uredinial numbers. However, total urediniospore production per lea f was not significantly affected by inoculum, and hence uredinial, density over a range of approximately 10-300 uredinia on the leaf, Total urediniosp ore production per uredinium generally decreased with increasing inoculum o r uredinial density. At high densities, sporulation per uredinium peaked ea rlier in the sporulation period, had a less distinct peak, and dropped off earlier than for the lower densities. Logistic model fits to cumulative spo rulation curves over time revealed that strain SR41 had a greater epidemic rate parameter (r) than SR22 at low and intermediate inoculum or uredinial densities, while SR22 had a higher r-value than SR41 at high density. Both strains also exhibited greater r-values in the presence of the other strain than when alone. Results suggest that strains may have different ecologica l strategies in their timing of reproduction, and that both intra- and inte rstrain competition can have complex effects on the temporal dynamics of sp orulation in pathogen strains.