INTERACTIONS OF ASPARAGUS ROOT-TISSUE WITH SOIL-MICROORGANISMS AS A FACTOR IN EARLY DECLINE OF ASPARAGUS

Sterilized root residues of asparagus added at a rate of up to 20 g kg (-1) fresh soil did not influence severity of root and crown rot cause d by Fusarium oxysporum f.sp. asparagi (Foa). Root residues accumulate d in field soil during asparagus growing for 10 years did not influenc e disease severity either. Inoculation of this soil with laboratory-pr epared Foa after treatment at 65 degrees C (30 min), at which the indi genous pathogen was killed but toxic substances present in asparagus r oot residues were left undamaged, led to the same disease severity as inoculation of similarly-treated fresh soil. On soil extract agar, aqu eous root extracts of asparagus but not those of other crops retarded growth of 31 out of 112 fungal isolates from a range of taxa. Sensitiv e fungi included Gliocladium spp. and Trichoderma harzianum but not Fo a. Colonization of Foa-infested soil by Fusarium species was greatly e nhanced by addition of root material from asparagus, Brussels sprouts, and chicory, bur not by that from strawberry and perennial rye grass. As the fraction of Foa amongst the Fusarium population was small, it is concluded that competitive saprophytic ability of the pathogen is f ar less than that of the nonpathogenic Fusarium species. Fungistasis t o Foa was not or was only slightly reduced in soils amended with root residues. In contrast to data reported in the literature, the present results do not suggest an appreciable increase of Foa root rot, or of the Foa population in soils, due to substances present in root residue s.