The Bulletin. 15 



streaks. It is by far the most destructive type of rust, and at the same 

 time the one least commonly seen. The three leaf rusts, as their name 

 implies, attack the blades most conspicuously. Their later stage of 

 development consists of very minute black dots that are buried just 

 beneath the surface of the leaves. 



Life Histories. — The life histories of the four types are similar. In 

 the bright colored spots the socalled summer spores are formed. These 

 are rather short-lived, and can infect other grain plants of the proper 

 species, as well as certan wild grasses. On these last, or on volunteer 

 grain the fungus may persist until the winter grains are up. Inside 

 the tissues of these or of overwintering wild grasses the rusts can sur- 

 vive until the next season, growing slowly, forming spores, and causing 

 new infection as the weather permits. Rapid spread comes again with 

 moist spring weather. 



The dark spots on the maturing plants produce the so-called winter 

 spores. These are long-lived and regularly survive the winter. On 

 germination the next spring they give rise to delicate infection spores 

 that have the peculiarity of being able to infect, not the proper grain or 

 grass species, but some other entirely different plant, as the barberry, on 

 its leaves, in the case of the stem rust. On such a plant still another 

 type of spore is produced rather early in the season, and it will carry 

 the disease back to the grain crops. However, this more complicated 

 cycle is not necessary for the persistence of the disease, and in most 

 cases the overwintering is simply in infected plants as indicated in 

 the preceding paragraph. 



Wind is the important agent in disseminating the spores of these rusts. 



Infection. — Weather conditions influence infection greatly. Cool, 

 dewy nights and humid, misty days, when plant parts remain wet for 

 twelve hours or more, furnish the best conditions for spore germination. 

 Luxuriantly growing plants, because of their succulence, longer growth 

 period and slow drying after showers, suffer much from rusts. Low, 

 heavy, moist places in the field are favorable for rust infection. Some 

 varieties, as the durum wheats of the semi-arid West and the rust-proof 

 oats of the South are satisfactorily resistant; and careful breeding 

 work is now in progress to develop other resistant types. 



Control. — Try to secure early and uniform maturing of the crop, 

 before the period of greatest rust development. Fall planted grain may 

 escape when spring planted crops would be severely affected. Select 

 early varieties, sow plump, uniform seed of good vitality in land that 

 has been well prepared. Avoid excessive nitrogenous fertilizing that 

 will produce rank, succulent growth. Do not sow thick enough to pro- 

 duce excessive shading. Sow on land of proper soil texture and drain- 

 age. Keep down as much as possible volunteer grain and such wild 

 grasses as favor the development of any rust. 



GRAIN SMUTS. 



Four grain smuts are considered here: Oat smut, stinking smut of 

 wheat, loose smut of wheat, and com smut. All are caused by distinct 

 fungi. N"o one of these fungi can attack other plants than the host 

 indicated. 



