Diversity of the Pythium community infecting roots of the annual legume Kummerowia stipulacea

A combination of glasshouse and field surveys was conducted to characterize the diversity of root-colonizing Pythium spp. on Kummerowia stipulacea in one small (ca. 10 m2) restored prairie site, using naturally produced K. stipulacea plants or soil and seeds from the same site. In aggregate, 633 K. stipulacea root systems were completely extracted and examined, yielding 489 Pythium isolates. Of these, 381 isolates (78%) were identified to 15 Pythium species, only three of which were previously associated with K. stipulacea. The observed species richness was greater than that previously reported at spatial scales of 10–100 m2. In contrast to the total species richness, a maximum of three Pythium species was recovered from any single root system. Jaccardʹs similarity coefficient (SJ=0.33) indicated low similarity between the portions of the complete community detected in the glasshouse vs. the field surveys; 13 and 7 species were recovered in glasshouse and field surveys, respectively. The most common species recovered in both field and glasshouse surveys were Pythium dissotocum and Pythium irregulare. Pythium diclinum was also commonly recovered in the glasshouse surveys. These results strongly suggest the importance of environmental factors for detection of resident root-infecting Pythium species. Morphometric root classification was used to characterize the architectural root order from which each Pythium isolate was derived. P. irregulare was isolated more frequently from higher-order root segments (i.e. those which do not terminate in an apical meristem) than were either P. dissotocum or P. diclinum in glasshouse surveys, though all three species were also common in first-order roots. Two glasshouse infection experiments supported the null hypothesis that when P. dissotocum and P. irregulare are present at the same inoculum concentrations, infection by each is independent of the other. Further, infection by each species alone significantly shifted root system branching ratio (Rb) from a dichotomous branching pattern towards a simpler, monopodial pattern, whereas infection resulting from comparable concentrations of inoculum of both species together did not significantly affect Rb. The low isolation frequency of P. dissotocum and P. irregulare from apparently healthy K. stipulacea root systems at the conclusion of glasshouse infection experiments, despite the significant alteration of Rb by each Pythium species, emphasizes the ephemeral nature of both Pythium infections and infected roots.