LEAF-SPOT OF ALFALFA AND RED CLOVER. 3 
The amount of damage which the fungus may cause appears to de- 
pend on several circumstances relating to the development of the 
crop and the weather. Under ordinary conditions the incubation 
period of the disease is more than a week. If for any reason the 
plant is growing slowly, the stand is thick, and the weather is fre- 
quently wet, only a few of the upper leaves reach full development 
before they are covered with the disease. Thus young stands which 
grow slowly before becoming firmly rooted and old stands which are 
retarded for any reason are likely to show bad attacks, while stands 
which are growing rapidly keep most of the upper leaves well above 
the rising invasion of the fungus and show little harm. Thus, in 
most cases where the fungus is found in great abundance, apparently 
defoliating plants, it will be found that some condition has reduced 
the normal rate of growth of the plants and is in part responsible 
for the resulting damage. When plants are vigorous, infection must 
be heavy indeed to cause extensive yellowing and falling of leaves, 
though this may occasionally occur. 
Nevertheless, the fungus is present in almost every alfalfa field, 
if not in all fields, taking a small toll of the foliage under even ay 
conditions and a large toll under more humid conditions. Since it 
‘rarely produces great loss at one time it has come to be regarded as 
one of the unavoidable evils to which the alfalfa plant is subject. 
The leaf-spot of red clover caused by the fungus Pseudopeziza 
trifoliz has not frequently been reported as occasioning great loss. In 
Russia Jaczewski (1912, p. 98) speaks of it as causing appreciable 
damage. Blasdale (1902, p. 75) states that it injures nearly all the 
clovers of the stock ranges of northwestern California. Freeman 
(1905, p. 809-310) notes that it causes local epidemics in clover fields 
in Minnesota. In fields in northern Wisconsin and in Maine in the 
summer of 1915 it was observed by the writer to be so abundant as to 
cause appreciable loss of foliage. From the evidence at hand it 
appears that the disease is not of great significance to clover and 
that this significance is only in northern regions. However, the 
destroyed foliage is so much less conspicuotis than that on alfalfa 
that the amount of damage is more likely to be underestimated than 
overestimated. 
DESCRIPTION OF THE DISEASE ON ALFALFA. 
There are two characteristics of the leaf-spot caused by Pseudo- 
pexiza medicaginis which usually serve to distinguish it from spots 
caused by other parasitic fungi. The first of these is the circular 
shape and limited size of the spot. (Pl. I, A.) The second is the 
presence of a smal] raised disk (PI. II, B) that appears in the center 
of the spot when it has reached full development. The edge of the 
spot may be smooth and definite, especially if the leaf has been much 
