Chapter XVIII. 



Diseases of Field and Forage Crops. 

 Jff 



The rusts of wheat and other cereals [Puccinia graminis 

 Pers., P. rubigo-vcra (DC.} Wint., and P. coronata Cda.]. What 

 is commonly known as wheat rust may be due to one or more of 

 a considerable number of rust fungi. These fungi, moreover, 

 may be found on a large number of grasses. The three most 

 important forms of cereal rusts are : the Black or Stem Rust (P. 

 graminis), the Orange Leaf Rust (P. rubigo-vera) and the 

 Crown Rust (P. coronata). In all of these forms, the sum- 

 mer and winter spores are formed on the plants of cultivated 

 cereals or of wild grasses and in the former case (i. e. sum- 

 mer- and winter-spore stages), cause annually an enormous 

 amount of damage. The summer spores first appear in early 

 summer and are formed with great rapidity so that as the 

 grain is growing the disease is also rapidly gaining ground. 

 These spores occur in red lines, crowded between the parallel 

 veins of the leaf. This form is commonly known as red 

 rust and is particularly in evidence after very moist weather con- 

 ditions, for these are very favorable to the rapid development of 

 the fungus parasite. Toward autumn the production of the 

 summer spores decreases and the formation of winter spores be- 

 gins. These are produced in long black lines, chiefly on the 

 stalks and form what is commonly known as black rust. 



The orange leaf rust makes, as one might say, a specialty of 

 the red rust, or summer spore stage, so that this is the prominent 

 feature of this particular rust. On the other hand the most 

 abundant spore of the black rust is the winter spore, hence its 

 common name. But it must be understood that the orange leaf 

 rust also produces winter spores, and that the stem rust 

 produces summer spores and that both have cluster cups. 

 The crown rust also produces three kinds of spores on its para- 

 sitic mycelium. The three rust species can be distinguished first 



