THE ORIGIN OF ABNORMAL RUST CHARACTERISTICS THROUGH THE INBREEDING OF PHYSIOLOGIC RACES OF PUCCINIA GRAMINIS TRITICI

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Abstract
The inbreeding of physiologic races of Puccinia graminis Tritici by means of the selfing of certain selected strains for several successive generations has given rise to rust strains with various abnormal characteristics manifested not only in the uredial and telial stages but also in the pycnial and aecial stages. These abnormalities include: (i) Abnormal uredial color—grayish-brown, orange, white. (ii) A decrease in the vigor of sporulation, that is, a tendency to form uredia that fail to rupture the epidermis of the wheat plant. (iii) A decrease of pathogenic vigor in certain strains, as shown, for example, by a tendency towards an "x" type of infection in cultures descending from strains that appeared homozygous for the more vigorous "4" type of infection. Certain strains originating through selfing have also shown a greater sensitivity to high temperatures than strains collected in the field. Wheat varieties susceptible to such strains at ordinary greenhouse temperatures develop resistance towards them at temperatures above 80° F. whereas their reaction to cultures collected in the field remains almost unaffected. (iv) Loss of ability to produce aecia on the barberry. (v) The development of uredia and telia on the barberry by some strains that have, partially or entirely, lost the capacity to produce aecia.The development of abnormal strains of rust is not an inevitable consequence of inbreeding, as many inbred strains show no abnormal characteristics. It is suggested that the abnormal characteristics are, in most cases, the result of recessive mutations that have taken place in the past history of the rust, the part played by the selfing being that of segregating and recombining the mutant factors in a homozygous state under which condition their effects are manifested in various types of abnormalities.

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