Colorado Plant Diseases 
23 
Small brown or black spots are formed on the leaves. They are 
first evident on the upper surfaces. The spots are often very 
numerous, causing the leaves to drop off. 
Leaf-spot is caused by a fungus. The black spots are the cup- 
shaped spore cases bearing a large number of spores. These spores 
live over the winter on dead leaves and are scattered about to the 
new leaves the following spring. 
No practical control measures are known. The disease is con- 
sidered as one of the necessary evils of alfalfa culture. Early 
cutting of heavily infected fields may reduce the loss of leaves. 
Yellow-Leaf blotch (Pyrenopezizza medicaginis) — This disease is 
similar in many respects to leaf spot. It is not as common, 
however, and has been known to occur in this country only since 
1914. In certain localities where it is abundant it is said to produce 
a large loss of hay. Its distribution in Colorado is not known. 
In appearance it differs from the leaf spot chiefly in that the 
spots are smaller and grouped together in large yellow blotches. 
The affected leaves become water soaked during rains and later 
I 
shrivel and fall. 
Yellow-leaf blotch is produced by a fungus very similar to the 
one causing leaf spot and is almost as difficult to control. Careful 
mowing so as to leave no stray plants along fence rows and irrigation 
ditches will probably reduce the amount of the disease. 
Downy Mildew (Peronospora trifoliorum) — Downy mildew is 
very injurious in certain localities under favorable conditions. It 
was first noticed in Colorado in 1906 when it was confined to a few 
localities. It has since spread over practically the entire state. 
Only the leaves are affected. Light green or yellow green spots 
and blotches are produced. A gray downy growth appears on the 
under side of the affected leaves which changes to purple with age. 
The first crop seems to be more heavily infected than the succeeding 
ones. 
The gray downy growth on the leaves is a mass of fungus growth 
bearing thousands of small egg shaped spores. These spores break 
open in drops of rain or dew, setting free a number of swarm spores, 
which swim about in the water for a while, then germinate and 
penetrate the leaves, producing a new disease spot. 
Effective control measures are not known. Since the spores 
require lots of moisture for germination excessive irrigation should 
be avoided. 
