EVALUATION OF THE FORMA-SPECIALIS CONCEPT IN APHANOMYCES-EUTEICHES

One hundred and fifty-seven isolates of Aphanomyces euteiches were collected for comparison of morphology and pathogenicity. Pea, alfalfa or snapbean were used as baits for 140 isolates, 17 were collected using other crop and weed species. Isolates were tested for pathogenicity to seedlings of pea, alfalfa, snapbean, red clover and sugarbeet, and disease severity was rated with the aid of a five-class scale (1-5). Half of the isolates caused disease severity < 1.5 on pea, most of which were less pathogenic on pea than reference isolates of A. cochlioides. Ninety-five per cent of the isolates caused disease severity > 2.5 on alfalfa. Five per cent of the isolates were weakly virulent to both pea and alfalfa, but were morphologically similar to A. euteiches. Only one isolate was observed in the collection which matched the description of A. euteiches f.sp. phaseoli, and two isolates were highly virulent to red clover. Somatic segregation for virulence was also observed among cultures recovered from single zoospores of a broad host-range isolate. Such cultures differed from the parent isolate in virulence to every host except alfalfa. A relationship was found between the cropping history of agricultural soils and host specialization of isolates baited from such soils. Only 20% of the isolates baited from alfalfa root-rot soils were virulent to both pea and alfalfa, whereas 80-100% of the isolates from pea root-rot soils, and pea/alfalfa root-rot soils were moderately to highly virulent to both hosts.