the black stem rust 
385 
per cent of the whole crop. The wheat affected by rust has 
shrivelled grains, which are light in weight, and straws which 
are crinkled and broken. 
The Cause of the Rust. 
— This is a fungus 
which lives as a parasite 
on the stalks and heads 
of grains and grasses 
during a part of its life 
history, and on the 
leaves of the common 
barberry for the other 
part. This is spoken of 
as the alternation of 
hosts, a habit which is 
characteristic of several 
other fungi which cause 
plant diseases (see page 
380). 
How It Spreads. — 
The fungus itself is 
composed of very small 
threads or hyphse which 
grow inside the leaves 
and stems of the host 
plant. It spreads from 
one wheat plant to an¬ 
other by means of spores 
which are carried by the 
wind, insects, or other 
means. In the early 
part of the summer these spores appear in red pustules on 
the stems and leaves of the infested plants, from which they 
fly like dust when the plants are disturbed. On account of 
their small size and large numbers, they fall on other plants, 
Courtesy of Experiment Station, University of Minnesota. 
Figure 353. — Diseased Heads of Wheat. 
