Microbial communities ofPyrenophora-infested wheat straw as examined by multivariate analysis.

Winter wheat straw naturally infested withPyrenophora triticirepentis andSeptoria nodorum was incubated in the field in one of three placements: buried in the soil, placed directly on the soil surface, or placed 2 cm above the soil within a straw layer. Periodically between June and April, straws from each treatment were collected and assayed by dilution plating to determine microbial populations in the following categories: total yeasts, total actinomycetes, total other bacteria, and genera or species of fungi. The data were analyzed by the multivariate procedures of ordination and classification, using detrended correspondence analysis (DCA) and cluster analysis, respectively. The major DCA axis displayed a gradient starting with pioneer colonizer communities of above-soil straw (dominated byPyrenophora, Septoria, Alternaria, andCladosporium), through on-soil straws (colonized by secondary saprophytes such asAcremonium strictum, Diplodia, Fusarium, and bacteria) to buried straws (dominated by actinomycetes, bacteria, and soil-inhabiting fungi). The second DCA axis showed a gradient separating different types of late secondary communities. In vitro tests for degradative abilities of straw-inhabiting organisms showed that low-temperature cellulolysis is characteristic of pioneer and early secondary colonizers, whereas chitin degradation is limited to secondary colonizers, particularly soil inhabitants. The results suggest the vulnerability ofPyrenophora andSeptoria to displacement by secondary colonists in an active microbial community, and indicate a number of organisms that may be useful in biological control of these plant parasites through exploitative or interference competition in plant residues.