560 SPECIAL PLANT PATHOLOGY 



in number, olivaceous, 10 to 17/1 by 40 to 60/i, exclusive of the isthmus, 

 which is 3 to s;t by 3 to 25^1. "^ 



To prevent the disease, only healthy vigorous stock of known par- 

 entage should be grown. These plants should be propagated at the 

 season most favorable to the growth of the violet. The frames, glass 

 houses and conservatories should be kept scrupulously clean. 



Wheat {Triticum sativum Lam.) 



Black-rust {Puccinia graminis, Pers). — Before the rise of modern 

 scientific investigation in botany, the farmers of Germany believed that 

 there was some connection between the rusted condition of their wheat 

 plants and the barberry bushes in proximity to their fields. It re- 

 mained for de Bary in 1865 to give scientific demonstration of the life 

 cycle of the rust fungus by experimental methods. He found on the 

 branches and leaves of the wheat plant rust-red Unes, which represent 

 cracks in the epidermis through which the summer spores known as 

 uredospores, or urediniospores, project. These together form the ure- 

 dinial sorus, or uredinium. The spores, as they rise from the inter- 

 cellular mycelium of the leaf, or stem, are ovate, yellowish-brown, spinu- 

 lose and measure 10 to 15/iby 20 to 35^- They may be repeated, as long 

 as fresh blades and branches are provided for infection and spread 

 to new parts, but these spores are specialized, as they cannot infect any 

 other host plant Uke oat, rye, barley and so forth, but only wheat. 

 Later the rust-red sori are replaced by brownish-black sori, which repre- 

 sent the teUum composed of teliospores, or teleotosjMjres, which project. 

 The teliospores are spindle-shaped, two celled, thick-walled and deep 

 brown in color. Theymeasure3S to 60^1 by 12 to 22/i. Germination 

 consists in the formation of a four-celled promyceUum, or basidium, each 

 cell of a stalk gives rise to a single sporidium, or basidiospore. These if 

 blown to the barbery enter the barberry leaf by the formation of a germ 

 tube and the intercellular myceUum develops a flask-shaped pytnium 

 (spermogonium) with small, spore-Uke bodies abstricted off from vertical 

 hyphas known as spermatia and aecia, or cluster cups on the under leaf 

 surface, which give rise to aeciospores. These carried to the wheat 

 infect the wheat and the cycle is completed. The aeciospores germi- 

 nate irregularly and capriciously, the process being accelerated to some 



» DORSETT, P. H. : Spot Disease of the Violet, Bull. 23, U. S. Division of Vegetable 

 Physiology and Pathology, 1900. 



