Expanding the host range of Rhynchosporium alismatis, a potential biological control agent for Alismataceae weeds in Australian rice fields

Whilst intraspecific variation in R. alismatis was minimal, isolates of the fungus clustered with Plectosphaerella cucumerina in a monophyletic group with 100% bootstrap support and were clearly not related to the forme species R. secalis nor R. orthosporum, which formed a separate group also with high bootstrap support. The genetic structures of populations of R. alismatis also displayed minimal variation with low to moderate (0.1924) levels of gene diversity across three geographically isolated populations. The average genetic distance (D = 0.0265) and overall population differentiation (Gst = 0.1001) were also small. However, measures of both genotypic and allelic diversity were significant (P = 0.100) between populations. Populations structures appeared clonal supplemented with intermittent rounds of recombination or gene flow over considerable distances. Migration via infected seed and/or plant material transported by irrigation channels and watercourses is probable. However, little evidence for gene exchange was apparent and evolution of pathogenic ability was minimal. Further investigation of the infection process of the fungus on target weeds indicated that the rates of conidial germination and appressorium formation on these species were almost identical to that of A. plantago“aquatica, a known host. Germ tube elongation and appressorium formation occurred randomly over the leaf surface with no apparent stimulus and the fungus entered the host by way of direct penetration facilitated by the production of a penetration peg. Holes left by penetration pegs, 0.25“0.5 µm in diameter, were observed by scanning electron microscopy on both A. plantago“aquatica and S. graminea following the removal appressoria.