68 CEREAL RUSTS OF THE UNITED STATES. 



the greenhouse. In case there is an error, therefore, it is on the safe 

 side, and the number of possible host plants is gTaduall\' more and 

 more limited, until finally, by reverse infections, the actual host plants 

 are established. As already stated, a number of instances of repeated 

 experiments on older plants resulted negatively, although they had 

 been successful in the first experiments. 



The ''mycoplasmatic'' theory, advanced by Eriksson (25, pp. 475-477) 

 to exi)lain the manner of perpetuation of Pnccinia ghimarum from one 

 season to another, has alreadj' been mentioned. A description of 

 Eriksson's experiments with this rust seems to reveal conditions differ- 

 ent from those existing in the case of any other cereal rust. Barley 

 plants grown from the seed and completely shut oft" from external influ- 

 ences showed rust aftei a time. The seed was sown in sterilized soil and 

 grown in glass houses, into which air could pass only through cotton- 

 wool filters. From other statements and illustrations in Die Getreide- 

 roste. it seems probable that we have here-at last an instance of the piop- 

 agation of grain rust through the medium of the germinating seed of 

 the host, as in some of the smuts. Eriksson claims that, as he found 

 no mycelium present, the rust exists in the form of a mycoplasma in 

 the cells of the embryo in a latent symbiotic state during the interval 

 between harvest and seeding time. As the rust in question does not 

 exist in this country, the writer has had no opportunity to investigate 

 the matter. 2so other cereal rust is known to live within the seed in 

 any form or manner.^ 



SUMMARY. 



(1) At least six and probably seven distinct rusts affect the cereals 

 of the United States. They are as follows : Orange leaf rust of wheat 

 {Puccinia nihigo-vera tritici), orange leaf rust of rye (P. rnhigo-vera 

 secalis), crown rust of oats (P. coronata Corda), black stem rust of 

 wheat and barley (P. graminis tritici Eriks. and Henn.), black stem 

 rust of rye (P. graminis secalis Eriks. and Henn.), black stem rust of 

 oats (P. graminis avence Eriks. and Henn.), and maize rust (P. sorghi 

 Schw.), 



(2) Though all these rusts except that on maize i>robably cause 

 some injury occasionally, the black stem rusts of wheat and of oats are 

 by far the most destructive. 



(3) All tlie rusts are pretty evenly distributed over the United States, 

 wherever their respective hosts are grown, except the stem rusts, 

 which are perhaps most prevalent in the region between the Alle- 

 ghanies and the ninety-fifth degree of west longitude north of the 



' Cases where rusts other than those of cereals live over in the seed are not, how- 

 ever, unknown. The actual peridiaof the ivciuium of Uromijces euphorhicp C. and P. 

 may often be seen within the seed of Exiphorhia dentata. The writer's investigations 

 concerning this rust are yet to he published. In 1890 Ralph (61. p. 18) also described 

 an ;ccidiiim affecting the see<l of Senecio vnUjaris in Australia, which seems to fur- 

 nish (Hie other instance. 



