GENETICS OF RUST FUNGI 



III. TELIA .AND TELEUTOSPORES 



Teleutospores are binucleate cells produced in groups (telia) follow 

 ing infection by aeciospores or uredospores . They are not released but 

 germinate while still attached to the host material or debris. In some 

 rusts germination follows overwintering; in others, such as Cronartium 

 pibioota} teliospores germinate in the season they are formed. 



IV. BASIDIOSPORES 



Before the teleutospore germinates, its two genetically different 

 haploid nuclei fuse. This diploid nucleus moves into the germ tube and 

 undergoes meiosis. Three cross walls form which cut off the four haploid 

 meiotic products. Each cell forms one basidiospore (sporidium) which is 

 discharged at maturity. The germ tube, or promycelium, is thus a 

 basidium. 



In the heteroecious rusts, such as C. ribiaola or Puaainia graminis , 

 pycnia and aecia are produced on one host, uredia and telia on another 

 quite unrelated host. In autoecious rusts, such as Mel amps or a lini , all 

 phases occur on one host species. In some short cycle rusts, such as F. 

 malvaaearum , only telial and basidiospore phases occur. Such rusts are 

 presumably homothallic and the pycnial-aecial mechanism promoting out- 

 breeding is dispensed with. Other so-called "imperfect rusts" are only 

 known in aecial and uredial stages and are referred to form-genera. 



Some special points concerning C. ribiaola on white pine should be 

 noted here. Pycnia appear on the bark of adjacent stems or branches some 

 24 to 26 months after needle infection. The aecia appear among the pycnial 

 scars one year later. Thus from stage to I takes at least 3 years. In 

 spite of these difficulties, Hirt (1964) carried out a detailed study of 

 pycnial and aecial development which indicates that transfer of nectar 

 between pycnia is not necessary for aecial development. Unfortunately 

 single spore inoculation could not be made to establish whether or not 

 aecia will arise from single basidiospore infections. Prior to aeciospore 

 production, cell fusions occur in the developing aecium. Dikaryotic cells 

 are formed which then bud off chains of aeciospores. Aecia frequently 

 arise between pycnia, suggesting that they could well be of hybrid or 

 mixed origin even if Cronartium is homothallic. 



GENETIC MARKERS 



In any genetic study heritable differences are needed. To date the 

 obligate parasitism of the rusts has meant that only characters revealed 

 during growth on the host plant can be used. However, the discovery that 

 certain strains of P. graminis tritiai can be cultured on artificial 

 media opens up other possibilities. 



VIRULENCE 



The most commonly used markers are those affecting virulence. We 

 may define virulence as the pathogen property of producing severe disease 

 symptoms on a host variety carrying major gene resistance. Virulence, and 

 its alternative avirulence, toward members of a set of differential host 

 varieties provides us with the means of classifying rust isolates into 



1 Authorities for Latin binomials are given in the subject index. 



