Influence of weed hosts on the ecology of Verticillium dahliae in newly cultivated areas of the Namoi Valley, New South Wales

SUMMARY The evidence suggests that Verticillium dahliae is not indigenous to the Namoi Valley, but was introduced, probably in weed seed, and then increased on the natural dicotylendonous weeds, from which it spread to introduced crops such as cotton. V. dahliae could have been introduced and disseminated as microsclerotia, which are plentiful in the woody fruits of noogoora burr ( Xanthium pungens ) and Bathurst burr ( Xanthium spinosuni ) and in achenes of saffron thistle ( Carthamus lanatus). V. dahliae was isolated from the stem and tap root of twenty‐six of fifty‐four species of weeds growing in naturally infested soils. It was not isolated from four species recorded as hosts elsewhere. Fifteen native or introduced weed species are newly recorded hosts. V. dahliae also colonized the roots of many wilt‐immune species, but died on the roots of most plants without producing microsclerotia. The only wilt‐immune plants in which microsclerotia formed in the roots were fat hen ( Chenopodium album ), red root ( Amaranihus macrocarpus ) and Canadian flea‐bane ( Erigeron canadensis ) but these are reported as hosts elsewhere.