DYNAMICS OF PACHYMETRA ROOT ROT SPORE POPULATIONS IN A SUSCEPTIBLE CROP: A COMPARISON OF TWO CONTRASTING ENVIRONMENTS

THIS paper addresses the dynamics of Pachymetra inoculum in soil, as influenced by initial spore density. Two experimental sites were selected, at Miriwinni and Mourilyan in northern Queensland. These sites provided a unique opportunity to assess population dynamics within a growing crop and illustrate how populations are influenced by site. The analyses showed that at a Pachymetra-conducive site, population increases were proportionally greater in plots with lower initial inoculum densities. There appeared to be a plateau in spore populations in all plots after a plant and first ratoon crop. This population was very high at the Miriwinni site (around 600 000 spores/kg), and at a level where very severe yield losses could be expected. Such a plateau is likely to vary according to the fertility of the cropping system. Data from the Mourilyan site illustrate that in some years and locations, populations are greatly limited by the environment. It is not clear from these data whether climatic factors, or soil suppressive factors, are the reason for the very limited increase in inoculum density.