58 THE BUSTS OF GRAINS IN THE UNITED STATES. 



these grasses in the Dakotas and Minnesota generally appears later 

 than the uredo in the cereals, so that the first infection, if it comes 

 from them, must come from wintering uredos. 1 



Careful studies of the wintering of Puccinia coronata and P. grami- 

 nis on oats have not been made and a discussion of them is omitted. 

 Undoubtedly the first appearance of these rusts in the spring will also 

 be found to result from wintering uredos and wind-borne spores. 



EPIDEMICS. 



GENERAL DISCUSSION. 



At irregular intervals of several years wheat-rust epidemics, more 

 or less general, occur throughout the country. That these depend 

 to a great extent on climatological conditions is quite generally 

 believed. Periods of excessive rainfall, followed by warm, muggy 

 days, are supposed to be favorable to their development. Why this 

 should be so is not generally understood, and numerous instances 

 where epidemics have not occurred, even after such climatological 

 conditions, might be cited. On the other hand, in some parts of the 

 country, south-central Texas for instance, rust is abundant almost 

 every year in spite of frequent droughts during the maturing period 

 of the grain. 



CONDITIONS FAVORABLE FOR AN EPIDEMIC. 



At least three conditions must be fulfilled before an epidemic can 

 occur: (1) A sufficient number of rust spores must be present on the 

 growing grain to give the fungus a start; (2) the humidity and tem- 

 perature conditions must be favorable for the germination of these 

 spores and consequent infection; (3) the grain must be in a receptive 

 condition. 



The first condition, very probably, is satisfied almost every year in 

 the main grain-growing regions by the presence of overwintering 

 uredos, wind-blown uredospores, or secidiospores. However, if such 

 spores are unusually abundant, as they may be after a favorable 



1 A full discussion and consideration of Eriksson's mycoplasm theory published in Compt. Rend., 1897, 

 pp. 475-477, and further treated in Eriksson's later publications, is omitted for lack of space. In this 

 theory Eriksson holds that the rust fungus "lives for a long time a latent symbiotic life as a mycoplasma 

 in the cells of the embryo and of the resulting plant, and that only a short time before the eruption of the 

 pustules, when outer conditions are favorable, it develops into a visible state, assuming the form of a 

 mycelium." External infection is given only secondary importance. This theory has been severely 

 criticized by H.Marshall Ward in "History of Uredo dispersa Erikss.,and the 'Mycoplasm hypothesis.'" 

 Philosophic Transactions of the Royal Society, series B, vol. 196, pp. 29-46, and in "Recent Researches on 

 the Parasitism of Fungi," Annals of Botany, vol. 19, 1905, pp. 1-45, and Klebahn (63, pp. 72-76). Eriksson 

 defends his position in Arkiv fur Botanik, vol. 3, 1905, pp. 1-54, and in later articles. The subject is still 

 a live one and readers are referred to the various authors cited for full discussions of it. The authors of 

 this bulletin have found no evidence which can be said to substantiate the mycoplasm theory. On the 

 other hand, the wintering over of the rusts, as shown above, can be reasonably explained without the 

 assistance of Eriksson's theory. 



216 



