298 



Diseases of Economic Plants 



Black-stem-rust {Puccinia graminis avence E. & H.). — 

 The black-st em-rust on oats shows but sUght differences from 

 the black-stem-rust of wheat, but it is racially 

 distinct (see p. 277) and a field of one of these 

 crops may be badly affected by its fungus, 

 while adjacent fields of the other crop show 

 no rust or indication of infection such as they 

 might be supposed to do were the two rusts 

 identical. 



As with the wheat black-stem-rust, its at- 

 tacks fluctuate greatly in abundance and 

 destructiveness from year to year. 



Crown-rust (Puccinia coronata Cda.). — 

 This rust is found only upon species of oats 

 and closely related grasses and in the telial 

 stage clearly differs, even to the naked eye, 

 from the oat rust mentioned above. 



It occurs upon oats in practically all fields 

 but is more injurious in its southern range. 

 The damage is not serious, or very rarely so. 



The cluster-cup stage is known to grow 

 upon the common buckthorn {Rhamnus cath- 

 artica) and the lance-leaved buckthorn {R. 

 lanceolatus) . The yellow uredinial stage is 

 most conspicuous. Later the black telia ap- 

 pear under the epidermis as long streaks 

 which, unlike most rusts, do not rupture the 

 epidermis. 



Marked varietal resistance is shown. In 

 California black oats are reputed to be less 

 subject to rust than white oats. In the 

 North, the White Russian is said to be resist- 

 ant, while in the South, the red oats resist 

 attack. 

 Minor diseases 



Scab. See wheat. Leaf-spot (PhijUosticta sp.). — Browned 

 or reddened leaves, with the black pycnidia profuse upon the 



Fig. 157.— Cov- 

 ered-smut on 

 oats. After 

 Jackson. 



