N. I. Vavilov 57 



Only in Tr. vulgare and partly in Tr. coriipactum there are a few 

 relatively immune races — exceptions to the general characteristic of 

 these species, as susceptible to brown rust and mildew. One of the 

 extreme exceptions is the above-mentioned " Persian Wheat " ; several 

 of the other more or less immune races, without any doubt, represent 

 products of artificial crossing in recent times (16, p. 96). 



Also, briefly speaking, in the group of wheats we meet with a case 

 of specific peculiarities of whole species in their fungal reactions, not- 

 withstanding the great polymorphism of these species. 



The genetic significance of these data I shall shortly touch upon. 

 The practical importance of this generalization for the selection of 

 immune sorts is evident, for it simplifies considerably the work of the 

 plant-breeder. 



Characteristics of the Species of Oats in relation to Rusts. 



As in Russia, so in England and other countries, oats are attacked 

 very severely by two species of rust : crown or leaf rust P. coronifera 

 Kleb., and black or stem rust P. grariiinis Pers. 



Observations in Moscow showed that the majority of cultivated and 

 wild oats are very suscej)tible to crown rust. 



Of 323 sorts of Avena sativa L. (.4. diffusa Aschr. and Gr., and 

 A. orientalis Schreb.) examined, 297 belonging to the majority of 

 known botanical varieties of cultivated oats (8) proved to be very 

 susceptible ; 21 less susceptible, and 5 races (belonging to the varieties 

 var. cinerea Kcke., var. brunnea Kcke., and var. grisea Kcke.) proved to 

 be relatively very immune to crown rust. The great majority of these 



conditions of greenhouses are very favourable to mildew of cereals ; the fungus in 

 the conidial stage lives, for example, in greenhouses much longer than in the open 

 air, and in general the plants are always more attacked by mildew in greenhouses than 

 in fields. And even more or less resistant races of wheat, for instance different repre- 

 sentatives of Tr. durum, polonicum, maybe severely attacked in the greenhouse, as also 

 under the bell-jar, by Erysiphe graminis. Immune races do not " lose" their immunity 

 in greenhouses. The difference in susceptibility may be observed during the first days of 

 infection, but the fungus develops better under these conditions. The most important 

 fact is that even under these conditions such races as "Persian Wheat" or several races 

 of Tr. dicoccum remain uninfected. 



By what is said above is removed the controversy relating to the characteristics of 

 Tr. durum and Tr. polonicum in our work and that of Prof. Reed in America (Pliyto- 

 pathology. Vol. ii, No. 2, 1912), who defined the degree of susceptibility of 78 sorts 

 of wheat by the aid of artificial infection of seedlings under bell-jars. Tlie data of the 

 characteristics of other species of wheat, in Russia and America, in general coincide. 



