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ETIOLOGY 
The pathogene causing corn smut is Ustilago Zeae (Beckmann) Unger, 
one of the Ustilaginaceae. It was first described as one of the puff-balls 
by Beckmann in 1768 and has been, since that time, a frequent subject 
for investigation by mycologists and phytopathologists. 
Life-history. It differs markedly in its life-habits from most of the com- 
mon cereal smut pathogenes. There are normally, in temperate climates 
at least, only the primary cycles. 
The Primary Cycles are initiated at any favorable time during the 
period of active growth of the host. Overwintered chlamydospores, 
usually in the manure on the field, are the sources of inoculum. 
Pathogenesis. Some of the chlamydospores from an old smut-boil 
have been germinated in manure-extract on the slides provided. Examine 
under the microscope and OBSERVE :— 
13. The large globose spiny dark-brown chlamydospores. 
14. From some of these, the protruding hyaline promycelium 
(basidium). 
15. The crack in the epispore through which the promycelium 
is protruded. 
16. The septa in the promycelium, from below each of which 
arise one to several slender pointed sporidia. They are often produced 
in great numbers; sometimes in chains. 
DRAW a germinating chlamydospore showing abundant sporidial 
production. These sporidia are carried by the wind to the growing corn. 
Some of them lodge on the growing tissues of the host near the joints 
within the leaf-sheath, the emerging silks, or the blossoms on the tassels. 
Here they germinate, sending out germtubes that penetrate these embry- 
onic tissues, and give rise to the mycelium. This mycelium does not spread 
through the tissues to any great distance from the point of infection; 
every boil probably arises from a separate infection. The hyphae grow 
in great masses into the hypertrophied tissues of the boil and form the 
greater bulk of the excrescence. Chlamydospores are soon developed in 
chains within short branches of this mycelium. The rest of the mycelium 
gelatinizes and disintegrates more or less. 
To study the mycelium in the host-tissue about the base of the boil, 
make thin longisections of the stalk, just beneath the boil. Stain with 
methyl blue or eosin. Examine and in the pith-tissues, OBSERVE :— 
17. The slender hyphae in the intercellular spaces or penetrating 
the cells; often in strands of several hyphae or gnarled in the intercellular 
spaces. 
18. Short knotted or irregularly swollen short branches sent 
into the cells,—the haustoria. 
DRAW to show the mycelium in the host-tissue. 
To study the distribution of the mycelium in the boil and the formation 
of the chlamydospores, examine prepared sections through a partly ma- 
tured boil. Make prawincs to show the structures studied. 
Saprogenesis. Mount some of the black spore-mass in potassium 
hydroxide. Study with the high-power and oBSERVE:— 
19. The shape, size and color of the mature chlamydospores; 
the short spines which thickly beset the spore-wall. 
