318 



Diseases of Economic Plants 



tically four distinct kinds of mildew, although indistinguish- 

 able to the eye. The form upon wheat grows upon all species 

 of Triticum though some varieties show much more resistance 

 than others. 



In damp, shaded spots the mildew is sometimes quite 

 injurious, though it is rarely so over any considerable area. 



The conditions favor- 

 ing the disease should 

 be avoided. 



Scab '-' (Gihherella 

 sauhinetii (Mont.)Sacc., 

 Fusariwn) . — Scab, first 

 described in 1884 in 

 England, seems to be 

 quite generally distrib- 

 uted in America, and 

 is often the cause of 

 large loss which, under 

 conditions favorable to 

 the disease, may reach 

 from 10 to 30 per cent 

 or higher. The loss in 

 1917 was estimated as 

 about 10,000,000 bush- 

 els. Scab appears upon 

 the heads when about 

 half ripe, as yellow or 

 pink incrustations on 

 the spikelets, on the bases of the glumes, or covering the 

 rachis. The affected spikelets ripen prematurely and turn 

 yellow or brown before the normal parts mature. After 

 ripening of the head, the diseased parts appear shrunken. 

 The grain itself is shrunken, covered with a thick felted 

 mycelium, and is incapable of germination. 



Only a few spikelets upon the head may be affected, these 

 occupying any position — basal, terminal, or intermediate 

 — or the whole head may be diseased. The loss consists in 



Fig. 166. — Anthnicnose on wheat stems. 



