Growth of Verticillium dahliae Kleb. hyphae and of bacteria along the roots of rape (Brassica napus L.) seedlings

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Abstract
Bacterial density and the hyphal density of Verticillium dahliae were determined along the roots of sand-grown rape seedlings. Verticillium dahliae microsclerotia were trapped in nylon nets which were placed in front of the growing root tip. The growing hyphae on the nets were stained with fluorescein diacetate and rhodamine B isothiocyanate. Bacteria on the petri dish lids adjacent to the nets were stained with acridine orange. Both fungal and bacterial densities increased exponentially over a distance of 15-20 mm from the root tip. Further back along the root, the densities of microorganisms stabilized, but in some cases the fluorescein diacetate active hyphal density increased. The hyphal densities stopped increasing before bacterial densities, suggesting an increasing bacterial competition for root exudates as their population grew. The hyphal activity, measured as the proportion of fluorescein diacetate to rhodamine B isothioicyanate stained hyphae, increased during the exponential growth phase of the bacteria, then decreased, and finally increased again as the lateral roots appeared.