Aug. 2 7. 191 7 Puccinia graminis on Cereals and Grasses 487 



when they were only a week or two old, they became very heavily in- 

 fected when inoculated with the same rust. Some of these same grasses, 

 however, became infected under natural conditions in the field when 

 quite old. Other grasses, such as Phleutn pratense and Agrosiis alba, 

 seem to be more susceptible when they are older. It was often difficult 

 to obtain normal infection on very young plants, while on older plants 

 very heavy infection resulted from inoculations made under similar 

 conditions. Other grasses again, such as Dactylis glotnerata, seem equally 

 susceptible at all ages. Whether these observations are of any signifi- 

 cance can only be determined by carefully controlled experiments made 

 on an extensive scale under varying conditions and with a large number 

 of species of plants and different forms of rust. It seems quite possible 

 that they may explain conflicting results obtained by the same investi- 

 gators and possibly the discrepancies reported by different investigators. 

 In fact, if the results of investigations by various workers are to be 

 comparable, methods should be standardized with respect to kinds and 

 ages of plants used as well as mechanical methods. 



Temperature is important in determining the rale of development of 

 the various biologic forms. No distinct differences were noted between 

 the different forms; such differences, however, may have escaped obser- 

 vation. Contrary to a fairly general belief, stemrust seems to develop 

 best at fairly high temperatures, provided conditions have been favora- 

 ble for infection. The best infection usually results when the tempera- 

 ture is low enough to cause the condensation of moisture in a very fine 

 film on the leaves. This usually occurs at night. Then, infection 

 having taken place, the uredinia seem to develop more rapidly at higher 

 temperatures up to a certain point, probably about 75° F. During 

 extremely hot weather it is difficult to secure heavy infection, probably 

 on account of the difficulty of inducing a film of water to form on the 

 inoculated plants and on account of the effect of the heat on the vigor 

 of the plants. Tow temperatures, below about 65° F., also inhibit the 

 development of the rust. 



A considerable amount of sunlight is necessary for the best develop- 

 ment of rust. Whether the effect is direct or indirect, the writers are 

 unable to say. During periods of cloudy weather, however, the incuba- 

 tion period may be lengthened a week or more, and the rust does not 

 develop so abundantly as during bright weather, as experiments to 

 determine this showed. Plants were inoculated under the same condi- 

 tions, placed under bell jars for the same length of time and kept side 

 by side, except that some were partially shaded and the others kept in 

 direct sunlight. The shaded plants invariably were more weakly in- 

 fected than the others. Partially etiolated plants were infected with 

 difficulty and the rust developed very weakly on them. No rust de- 

 veloped on etiolated plants. Careful and extensive experiments are 

 desirable. 



