In the Netherlands in 1961 three new field races were found of Puccinia striiformis, the causal fungus of yellow rust on wheat. Two of these, viz. the 'Falco' race and the 'Opal' race, together with the already known 'Etoile de Choisy' race, constitute the so-called Nord-group of races, which is now deafly distinguished from the so-called Rubis-group. Most dangerous and interesting is the third of the new races. This so-called 'Cleo' race proved to be the first to break both resistance to the Nord-group and resistance to the Rubis-group. On wheat seedlings in the greenhouse all three new races behaved like greenhouse race 'W(ageningen) 8' = 'B(raunschweig) 55'.
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