88 Flag Smut of Wheat. 



CHAPTER XIV. 



Flag Smut of Wheat. 

 (Urocystis tritici. Koern.) 



This smut, as the common name denotes, is most commonly found on the 

 leaf blades and leaf sheaths, but it may also occur on the stem and even on 

 the chaff ; but very rarely in the ovary, as it is seldom formed (Plates IV., 

 VI.). Since it prevents the formation of the ear, this disease is very destruc- 

 tive in its effects, and in some seasons favorable to its development, such as 

 that of 1906, the harvest was considerably reduced by it in several districts 

 of Victoria. The serious nature of the disease and its widespread occurrence 

 demanded a thorough investigation, and, as far as time and opportunity 

 permitted, this has been done. Not only have Laboratory experiments been 

 conducted as to the germination of the spores and the mode of infection, but 

 field experiments are being carried out to determine how far special treat- 

 ment of the soil or a suitable rotation of crops can mitigate the disease, for 

 it has already been proved that the treatment of the seed so successful in the 

 case of stinking smut does not prevent the appearance of the disease. The 

 following account will give the present state of our knowledge with reference 

 to this disease and the measures that have been tried to keep it in check. 



Although it has been known in Australia at least since 1868, it was only 

 in 1873 that Wolffs definitely determined the fungus causing it to be the 

 same as that on rye — Urocystis occulta — but as it is afterwards shown to be 

 biologically distinct from that spacies, the more striking differences between 

 the two may here be given. 



Stem Smut of Rye and Flag Smut of Wheat Compared. 



It is the characteristic of Urocystis on the wheat that it principally attacks 

 the leaf sheaths and the flag or blade, causing the latter to curl up and become 

 variously twisted and distorted, while the ear is very seldom formed (Plate 

 IV.). In the rye, on the other hand, the long grey streaks are formed on the 

 flag without causing much distortion, and even in observed cases, where the 

 whole plant was more or less affected, the lower and older leaves still retained 

 their normal shape, only splitting up towards their tips in lines parallel with 

 the streaks of the fungus and becoming frayed. But it is on the stems that 

 the streaks are principally noticeable, where they run together, more par- 

 ticularly at the base of the inflorescence, forming one dense mass of black 

 spores as the epidermis ruptures to expose them. Not only do the spores 

 escape on the outside of the stem, but the tissue is ruptured on the inside, 

 so that the cavity of the stem is more or less filled with the black spore- 

 powder. The ear is generally formed, but arrested in its development, only 

 becoming a sort of skeleton ear, and it usually droops, as the tissues of the 

 stem immediately beneath it are more or less destroyed by the fungus (Plates 

 V. and VI.). Hence the common names by which these diseases are generally 

 known — the flag smut of wheat, and the stem smut of rye. 



Another feature of the flag smut of wheat is that all the stalks in a stool 

 are often affected, while in the rye, as far as my observation goes, this is 

 not the case. Thus, in one stool of rye there were eighteen healthy stalks 

 and five diseased, and the healthy ears all produced the normal grain. In 

 addition to these visible differences there are others which are microscopic, 

 and are duly noticed in the technical description of the fungus. 



