Inter- and intraspecific restriction fragment length polymorphisms in the genus Aphanomyces

Abstract

Aphanomyces cochlioides Drechs. causes sugar beet black root. The fungus induces an acute seedling blight and a chronic root rot. Severe tip root rot may occur. This tip root rot phase is more damaging in Texas than in other sugar beet-growing states, where the seedling blight phase is more important. A comparative analysis of fungal mtDNA was made to examine intraspecific variation of the fungus. Two isolates of the fungus from Texas, two isolates from Minnesota, and one isolate each from California, Michigan, Wisconsin and Sweden were compared based upon restriction fragment length polymorphisms (RFLPs). By using 1.3 kb and 2.8 kb cloned mtDNA probes prepared from the Swedish isolate, existence of RFLPs was observed, enabling differentiation of the Swedish isolate from the American isolates. A Texas isolate from mature sugar beet and a California isolate from sugar beet seedlings were distinguished from other American isolates, although both were not able to be differentiated. To examine host variation in A. euteiches Drechs., two pea isolates, one bean isolate, two alfalfa isolates and one red clover isolate were also compared by the same methods and DNA probes. RFLPs were detected within this fungal species, and were correlated with host specificity based on pathogenicity of the fungal isolates. A. cochlioides, A. euteiches, A. stellatus and A. astaci were also examined using the same methodology and probes. RFLPs were detected among the isolates of these four species, indicating that RFLPs offer a possible avenue to the taxonomy of Aphanomyces.