SUPPRESSION OF HYPHAL GROWTH OF SOIL-BORNE FUNGI BY DUNE SOILS FROM VIGOROUS AND DECLINING STANDS OF AMMOPHILA-ARENARIA

A study was carried out to determine whether expansion of marram-grass stands (Ammophila arenaria (L.) Link) on acidic inner Dutch coastal d unes was caused by suppressiveness of soils from these stands against three potential pathogenic fungi of marram grass, namely Fusarium culm orum (W. G. Sm.) Sacc., Phoma exigua Desm. and a Ulocladium sp. The su ppressiveness of the acidic inner dune soils was compared with that of lime-rich dune soils from vigorous and declining marram-grass stands. Suppressiveness of the dune soils against the saprotrophic fungi Chae tomium globosum Kunze: Fr, Mucor hiemalis Wehmer and Trichoderma harzi anum Rifai was also determined. All fungi had been isolated from marra m-grass roots. Suppressiveness was determined by comparing the formati on of hyphae from potato-dextrose agar discs into (layer method) or on top of (surface method) dune soils with that of controls consisting o f sterile, acid-washed beach sand. The growth of the three root-infect ing fungi was strongly inhibited in all soils regardless of the method used. Hence, there were no indications that the potential pathogenic fungi were selectively suppressed by the acidic dune soils and, conseq uently, the results did not give any indication for the involvement of a fungal component in the decline of marram grass. Growth of the sapr otrophs C. globosum and M. hiemalis was much less inhibited than that of the root-infecting fungi. Growth of T. harzianum was strongly inhib ited in alkaline soils but not in the acid ones. The suppression of fu ngal growth could be partly or completely eliminated by a microwave tr eatment, indicating that biological components of the soil were essent ial to suppressiveness. The suppression of the fungi by colonies of du ne soil micro-organisms on water-agar differed considerably from soil alone. Yet, all methods indicated the occurrence of general suppressiv eness against fungi by dune soils, irrespective of the origin of the s oil samples. This suppressiveness was probably not due to direct compe tition with other soil micro-organisms for nutrients but to inhibiting compounds produced by the soil micro-organisms.