COCHLIOBOLUS SATIVUS: VII. NUTRITIONAL CONTROL OF THE PATHOGENICITY OF SOME AUXOTROPHS TO WHEAT SEEDLINGS

The pathogenicity of some strains of Cochliobolus sativus to wheat seedlings was determined. The strains comprised prototrophic and auxotrophic mutants from a common parent, heterokaryons synthesized from auxotrophs, diploids obtained from heterokaryons, and recombinants selected from a diploid.Prototrophic mutants and auxotrophs requiring thiamine, arginine, and pyridoxine were as pathogenic as the parent. Methionineless and lysineless mutants were non-pathogenic. Evidence that the nutritional requirements of the latter mutants directly controlled their pathogenicity was: (1) two methionineless and two lysineless auxotrophs were uniformly non-pathogenic; (2) heterokaryons synthesized from methionineless and lysineless strains and from two methionineless strains were pathogenic; (3) diploids heterozygous for the requirements were pathogenic; (4) prototrophic recombinants from a diploid were pathogenic but auxotrophic ones were non-pathogenic; (5) a prototrophic mutant of a lysineless auxotroph was pathogenic; and (6) the pathogenicity of the auxotrophs was temporarily restored when the appropriate growth requirements were provided as external supplements to the host. The results are discussed.