Parasite: Host: Environment Specificity in the Cereal Rusts

At least seven discoveries in the study of resistance to cereal rusts are worthy of consideration as truly classic events: 1. Biffin's (4) discovery that reaction to Puccinia striiformis is inherited in a Mendelian fashion. 2. Stakman & PiemeiseI's (71) discovery of variation in the pathogenicity of P. graminis to host cultivars. 3. Waterhouse's (77) discovery that the expression of low infection type from the interaction of some wheat cultivars inoculated with some cultures of P. graminis is negated by high temperature. 4. The discovery by Gassner & Striab (30) that the expression of low infection type from the interaction of some wheat cultivars inoculated with some cultures of P. recondita is negated by low temperature. 5. Newton et aI's (56) discovery that the pathogenicity of P. graminis to specific cultivars of wheat is inherited in a Mendelian fashion. 6. Flor's (27) discovery that what we often consider resistance as a host character is in fact the phenotype of a parasite: host interaction. Flor (27) stated his discovery as: For every gene for resistance in the host there was a gene for pathogenicity in the parasite. 7. Loegering's (46) development of the concept of the aegricorpus as the living manifestation of the genetic interactions in and between parasite and host and his consideration of the aegricorpus as a third living entity within para­ site: host systems.