But now they are known to be there for a reason. And, says Cook, it is a general phenomenon. We're finding that roots do support a select group of organisms adapted to physical habitat provided by those roots. A major focus of Cook's research lab has been the wheat disease take-all, which is caused by the fungus Gaeumannomyces graminis var. We'll just call it the take-all fungus.
Roots turn black as they're eaten by the take-all fungus.
Take-all can be controlled through rotation, by planting at least one other crop between crops of wheat. When the residue from the old wheat roots rots away from microbial activity, the fungus loses its food base. But farmers have longed to plant wheat continuously, as it's more profitable. Gradually, the crop improves, and finally the take-all is gone. In 1988, plant pathologist David Weller and geneticist Linda Thomashow proved that take-all in these fields was being controlled by antibiotics produced by the bacteria Pseudomonas fluorescens .