Competition alters temporal dynamics of sporulation in the wheat stem rust fungus

To investigate the effects of competition on the timing of pathogen reproduction, urediniospores of two strains of Puccinia graminis f. sp. tritici (SR22 and SR41) were inoculated onto leaves of wheat seedlings singly and in 1: 1 mixture at three inoculum densities. On randomly sampled leaves, uredinia were counted 9 days after inoculation and urediniospores were collected and quantified every other day from the seventh to the 29th day after inoculation. Increases in inoculum density resulted in progressively smaller increases in uredinial numbers. However, total urediniospore production per leaf was not significantly affected by inoculum, and hence uredinial, density over a range of approximately 10-300 uredinia on the leaf. Total urediniospore production per uredinium generally decreased with increasing inoculum or uredinial density. At high densities, sporulation per uredinium peaked earlier in the sporulation period, had a less distinct peak, and dropped off earlier than for the lower densities. Logistic model fits to cumulative sporulation 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 ecological strategies in their timing of reproduction, and that both intra- and interstrain competition can have complex effects on the temporal dynamics of sporulation in pathogen strains.