THE GENETICS OF RUST FUNGI 



p. R. Day 

 The Connecticut Agricultural Experiment Station 

 New Haven, Connecticut, U.S.A. 



ABSTRACT 



Genetic studies of rusts depend on a thorough understanding and 

 manipulation of their life cycles. The two rusts we know most 

 about genetically, stem rust {Puccinia graminis tritici) and 

 flax rust (Melampsova lini) , differ principally in that the 

 former infects two host genera, wheat and barberry, while the 

 latter infects only flax. In both cases the economically 

 important phase is the dikaryon which is clonally multiplied 

 on an enormous scale by uredospores. In white pine blister 

 rust (Cronartium ribicola) perennial haploid homokaryotic 

 infections are important on the economic host, white pine, but 

 are not multiplied by a spore form. In the past control has 

 been based largely on eradication of the alternate host, Ribes , 

 which harbors the dikaryon. Breeding white pine for resistance 

 is thus directed against the haploid meiotic products of the 

 rust (basidiospores) , a situation closely analagous to breeding 

 apples for resistance to scab (Venturia) , We may expect that 

 genetic variation among white pine infections is likely to be 

 maximal unless reduction of the alternate host population is 

 severe enough to promote inbreeding of the rust. 



Until now the obligate parasitism of the rusts has limited 

 spontaneous and induced markers to those which affect host 

 range and morphology. With these markers sexual and parasexual 

 genetic systems have been established as well as cytoplasmic 

 inheritance. In addition to growth chamber, greenhouse and 

 field studies on the host, genetic analysis can make use of 

 electron microscopy, serology, recovery of intact mycelium 

 from the host plant, and more recently, culture on synthetic 

 media. 



There seems little reason to doubt that the chief features of 

 other genetically better known fungi apply also to the rusts. 

 At the present time this assumption offers little by way of 

 directly suggesting new control measures. What it does mean, 

 however, is that our understanding of the mechanisms of 

 inhibition, antagonism, enzyme regulation and repression and 

 other aspects of fungal metabolism may well give a "spin-off" 

 in the form of improved fungicides and more sophisticated ways 

 of evaluating host resistance. 



