316 Diseases of Economic Plants 



occurs exclusively at blossoming time; that at all other 

 times the plants are immune. Smut spores from near-by 

 smutted heads are blown into the wheat or barley flowers. 

 Infection follows. The fungus then lies dormant in the 

 grain until the seed sprouts. If the grain ripening from 

 such infected flowers be used for seed, the fungus develops 

 in the seedling, and an infected plant, producing smut but 

 no grain, results, though no external symptom of the disease 

 appears until blossoming time. Conversely, if seed result- 

 ing from blossoms which were not infected be used, plants 

 free from smut will be raised. 



The remedy in this case, therefore, lies in the use of unin- 

 fected seed. To obtain such seed, grain must be secured 

 from a field in which there is no loose smut, or if this is not 

 practicable, seed grain must be raised under such conditions 

 as will afford clean seed. 



It has been found that while the ordinary formalin seed- 

 treatment is ineffective against wheat loose-smut, the hot- 

 water treatment, in modified form, is effective, though some- 

 what difficult of manipulation. See p. 272. 



Rogueing or weeding out all diseased plants from the crop 

 that is to be used for seed, diminishes the smut largely, but 

 does not entirely eliminate it. 



A practicable, effective means, which results in complete 

 and inexpensive eradication of this smut, consists of a com- 

 bination of the seed plat with the hot-water treatment. 



Flag-smut ^^^' ^^^ (Urocystis tritici Koern). — Linear, black 

 streaks in the leaves, much like those shown in fig. 182, 

 occur on wheat. Considerable damage is reported from 

 Australia, India, and Japan. Flag-smut was found in 

 Illinois in 1919. The usual formalin treatment is beneficial, 

 though not so complete in control as with oat smut, since 

 infection occurs, to some extent, from diseased refuse in the 

 soil. If badly infested, the stubble should be burned to 

 destroy as many spores as possible. 



Foot- rots. ''^^' '"-'^^^'-''^ — These diseases are characterized by a 

 brown or black-rot on the lower part of the stem, the diseases 



