430 PLANTS AND MAN 



wheat rust, a very common and destructive parasite of wheat 

 crops throughout the world (fig. 260). The myceHum of this 

 fungus ramifies throughout the tissues of the wheat plant during 

 the growing season, erupting at intervals on the surfaces of leaves 

 and stems to produce the red rust stage which consists of single 

 celled, thin walled spores which spread the infection to other 

 wheat plants. This general infection of the wheat plant results 

 in interference with its food manufacturing activities, and a 

 subsequent much reduced yield, from the standpoint of both 

 quality and quantity of wheat grain produced. In late summer or 

 autumn, another spore form is produced by the same mycelium, 

 perhaps even on the same plant, or in threshed straw or stubble in 

 the field. This is a heavy- walled, two-celled spore which is the 

 overwintering form of wheat rust. In the spring, each cell 

 germinates to form a four celled basidium, each cell of which 

 produces one basidiospore. These basidiospores are incapable 

 of infecting wheat plants, and will produce an infection only when 

 they alight upon the leaves of the common barberry. Here they 

 germinate and produce a mycelium within the barberry leaves, 

 and here they produce two different spore forms, one of which is 

 responsible for renewing the infection in wheat plants. By remov- 

 ing the common barberry plants in wheat growing regions, one 

 source of wheat rust infection is removed. All important wheat 

 producing states have laws forbidding the growing of common 

 barberry. Formerly used as a hedge plant, it has been replaced 

 by the lower growing, rust immune, Japanese barberry which is 

 wholly as satisfactory in its growth and ornamental value: 

 Unfortunately the disease is often not entirely eliminated by 

 barberry eradication, due to the ability of the first mentioned 

 ''summer spore" form to withstand winter temperatures in milder 

 parts of the wheat growing region. Likewise, these spores may be 

 wind borne from regions of mild winters to more northern cli- 

 mates, since they have been collected at altitudes of several 

 thousand feet, by means of greased plates exposed from aeroplanes. 

 Another important rust is the blister rust of five needle 

 pines, which has helped to make growing of the long used white 

 pine a hazardous undertaking. This rust enters the tree by way 

 of the foliage, working back into the stem where it girdles the 



