Inoculum potential of ericoid endophytes of western Australian heaths (Epacridaceae)

The nature and location of infective inocula of ericoid endophytes were examined in habitat soil sampled dry during midsummer from different depths down the rooting profile of an Epacridaceae-rich mediterranean-type ecosystem in south-western Australia. Efficacy of soil inocula was tested using aseptically germinated seedlings of Lysinema ciliatum R. Br. grown in a glasshouse. Hair roots, the principal structures colonized by endophytes, constituted c. 70% of the total length of root systems formed under glasshouse culture. Colonization of these hair roots by endophytes was greatest (17%) when seedlings had been inoculated with habitat soil from the upper 24 cm zone and declined to less than 1% following inoculation with soil obtained from a depth of 36 cm and beyond. By contrast, sampling of a naturally growing epacrid (Leucopogon conostephioides D.C.) at the same study site during the wet midwinter growing season showed uniformly high (34-43%) infection of hair roots down to maximum rooting depth at 70 cm. The infectivity of sieved soil fractions from bulk habitat soil was considerably greater (mean of 17% hair root infection) for glasshouse-cultured Lysinema ciliatum inoculated with the 251-305 mu m particle size fraction than for that inoculated with coarser (306-5000 mu m) and finer (39-250 mu m) sieve fractions (0-6% infection). Dilution of bulk soil with sterilized soil indicated maximum infection at 1:16 dilution, declining to negligible infectivity at a dilution of 1:100. The data support the hypothesis that inoculum persists through summer principally in relatively coarse fractions of organic matter near the soil surface. With the onset of autumn rains, downward mycelial growth emanating from this upper zone rapidly recolonizes the soil profile and eventually promotes extensive colonization of hair roots produced that season by the epacrid root system.