Feb. x6. 1934 
Flag Smut of Wheat 
453 
general surface and are usually first apparent on the lower side of the leaf. 
They vary from a few millimeters to many centimeters in length (PI. 1, B) 
and usually are between the vascular bundles, although occasionally they 
appear between the vascular bundle and the epidermis. More rarely the 
peduncle (neck) and inflorescence are attacked (PI. 1, C). The stripes 
change from white to dull gray in color, usually from three to four days 
after the first appearance of the lesions. 
The stripes represent individual sori of spores. These sori may remain 
unbroken for several weeks or less, depending on environmental com 
ditions. The epidermis eventually ruptures and exposes the black mass 
of spores within. The lesions may appear on plants of practically all 
ages. McAlpine (29) reports the first production of new spores on plants 
40 days old. The earliest production of spores observed by the writer 
occurred on the fifth leaf of a young seedling 29 days after inoculation. 
Occasionally the parasite gains the ascendancy just as the culms of the 
host begin to elongate rapidly. 
Usually the organism causes considerable stunting of the plant. The 
leaves become characteristically incurled and twisted, and the plant may 
be considerably deformed (PI. 1, D). Even if heads are formed on 
affected culms, they usually are empty or produce only badly shriveled 
kernels. Plants which are severely attacked when young may be killed 
before the unaffected plants are mature; hence their loss easily may be 
overlooked at harvest time. Frequently not all of the culms are equally 
severely affected. Many instances have been observed in which normal 
culms have grown to maturity, although others have been completely 
destroyed by the parasite. In studies on such plants, under green¬ 
house conditions, it was observed that these unaffected shoots were 
those first produced by the plant and that the later-tillering shoots 
frequently became infected. 
THE CAUSAE ORGANISM 
Flag smut of wheat is caused by Urocystis tritici Koern. It was first 
designated by Wolff (57) in 1873 as U. occulta (Wallr.) Rab., and the 
early records of the disease refer to it under this name. Koernicke {23, 
Bd . 16 , p . 33-34) , in 1877, first described it as a separate species. The 
morphological and physiological differences between U. occulta , which 
causes stem smut of rye, and U. tritici were again noted by McAlpine (29). 
Koernicke (23 ), McAlpine (29), and others have described the morpho¬ 
logical characters of the spores of Urocystis tritici. The latter may occur 
singly or in spore balls, which contain a variable number of spores, al¬ 
though from two to three spores are found most frequently in the ball. 
The spore balls are dark brown in color and variable in shape, though 
most frequently globose. They usually vary from 18 to 52/x in length 
and from 18 to 45/* in breadth. The individual spores are spherical or 
somewhat oval in shape and vary within the size limits of 12 to i6/x by 9 
to 12 ix. The spores which occur singly, and the spore balls also, are most 
frequently completely invested with a layer of sterile peripheral cells, pale 
brown in color, globose or ellipsoid in shape, and of dimensions 7 to 10y. 
by 5 to gji (PI. 3, H). 
McAlpine ( 29) has described some of the features of the germination of 
the spores. The writer (33) has recently described further phenomena 
