2 BULLETIN 629, U. S. DEPARTMENT OF AGRICULTURE. 



the stem rust of oats, Puccinia graminis avenae Erikss. and Henn. 

 (PL I, fig. 1, and PL III). 1 These rusts are both rather common in 

 the United States. ' The stem rust probably causes the greater loss 

 in the Northern States and the crown rust in the Southern States. 



THE CULTURE OF CEREAL RUSTS IN THE GREENHOUSE. 



Obligate parasitism is a well-known characteristic of the rust 

 fungi, and all attempts to grow them in artificial media have failed. 

 They are easily cultured on the living host, however, and, as pointed 

 out by Carleton (2), 1 it is possible to make many interesting studies 

 by this method. Carleton described the method of inoculation used 

 by him and gave suggestions concerning work with rusts. 



Evans (1) has tested the rust resistance of oat varieties in the 

 greenhouse and concludes that the Indian varieties are far more sus- 

 ceptible to the crown rust than to the stem rust. 



Melhus (8) has described and illustrated the apparatus used and 

 has given his methods in culturing parasitic fungi on the living host, 

 including notes on culture work with the crown rust of oats. 



Fromme (5) has published a comprehensive paper dealing with 

 the culture of cereal rusts in the greenhouse, in which he briefly 

 reviews the work of previous writers and describes in some detail 

 his own methods, particularly the results obtained with the crown 

 rust of oats. 



Stakman (12) briefly describes culture methods used in his green- 

 house studies of biologic forms of the cereal rusts and calls attention 

 to the effects of temperature, humidity, and light on the incubation 

 period. In a second paper (13) he has briefly described similar 

 methods used in the additional study of cereal rusts on plants grown 

 in the greenhouse. 



Melchers (7) has suggested the use of galvanized-iron tubs with 

 window-glass covers as moist chambers. He also advocates wetting 

 the leaves to be inoculated by the " finger-rubbing " method instead 

 of with an atomizer and keeping the pots bearing the rusted leaves 

 in shallow pans of water, to avoid the necessity of overhead watering. 



EXPERIMENTAL METHODS. 



In the greenhouse experiments of the writer, methods similar to 

 those above cited were emploj^ed. For the infection studies on seed- 

 ling oat plants 4-inch pots were used, and 5-inch pots for the plants 

 inoculated at heading time. Greenhouse potting soil of approx- 

 imately the same make-up was used throughout the work. For the 

 studies of plants at heading time four seeds of a variety were sown 

 in each pot. When the plants were 6 to 8 inches high they were 

 thinned to two in a pot, the number usually reserved for subsequent 

 inoculation. 



1 The serial numbers in parentheses refer to " Literature cited," p. 16. 



