392 BROOKLYN BOTANIC GARDEN MEMOIRS 



Glyceria, Agropyron and Bromiis; most of the races of P. gliimarum 

 and P. dispersa. 



3. Specialized races occurring on two or more genera but belonging 

 to the same family. As examples, we may mention the specialized 

 races Avenae, Secalis and Tritici of Puccinia graminis: Lolii, Calama- 

 grostis and Phalaridis of P. coronata; Secalis of P. glumarum; Orchide- 

 arum-phalaridis of P. sessilis; the races of P. Ribesii-Caricis, P. 

 extensicola, P. silvatica, P. Bistortae, P. mammillata, Albugo Candida 

 and Claviceps purpurea. 



4. Races occurring on hosts belonging to different families as 

 Puccinia subnite7is, Uromyces Scirpi and Erysiphe dehor acearum. 



When we compare the specialization of a parasite in relation to a 

 particular host we also find the greatest differences. For example 

 Puccinia graminis avenae occurs, according to Eriksson (41), on twenty 

 species of grasses belonging to fourteen genera, Carleton (25) recording 

 it on nineteen species belonging to fifteen genera, Jaczewski (68) on 

 seven species belonging to six genera and Stakman and Piemeisel (149) 

 on thirty-three species belonging to twenty-one genera. All agree 

 that this rust occurs on a wide range of more or less unrelated hosts. 

 On the other hand, Puccinia coronata avenae is restricted to species 

 of Avena and possibly Arrhenatherum . The powdery mildew of oats, 

 Erysiphe graminis avenae, is also sharply restricted to Avena, infecting 

 to some extent Arrhenatherum. The crown rust and powdery mildew 

 are similar in infecting a large number of species and varieties of 

 Avena. 



A similar condition is found in the case of the parasites occurring 

 on Secale cereale: Claviceps purpurea secalis occurs on eighteen species 

 belonging to eleven genera; Puccinia graminis secalis, according to 

 Eriksson (41) on eleven species belonging to five genera and according 

 to Stakman and Piemeisel (149) on twenty-three species belonging to 

 nine genera ; Puccinia glumarum secalis on Secale cereale and Triticum 

 vulgare; Erysiphe graminis secalis on two species of Secale; and Puc- 

 cinia dispersa secalis on Secale cereale. 



Many of the specialized races, while in the main restricted to certain 

 hosts, yet are able to infect to a greater or less extent a number of 

 other plants. Generally these races are distinguished by their ability 

 to fully infect certain hosts while their development on others is 

 weak and limited. Very good illustrations of such races are found 

 among several of the rusts. The various races of Coleosporium 

 Campanulae are not sharply limited to definite hosts, but the hosts of 

 one race may also be attacked by other races. The same is true of 

 the races of Puccinia graminis, P. coronata, P. Ribesii-Caricis, Melamp- 

 sora Larici-epitea and Uromyces Poae. 



