LEAF-SPOT DISEASES 
There are a large number of diseases affecting crops which may be 
designated under the term leaf-spot diseases. In most cases little more 
is known than the names of the fungi associated with the lesions. They 
are, nevertheless, in the aggregate, of considerable economic importance 
and deserve more careful and exhaustive study. 
These diseases take their names from the character of the lesions most 
commonly exhibited, namely, spots on the leaves. These spots vary 
in size from mere specks to extensive dead or discolored areas, often involv- 
ing a large part of the blade. In most cases they are rather definite in 
outline, more or less circular, and necrotic in character. They may, 
however, be angular or linear. They usually exhibit fruiting structures 
of the pathogene either on the upper or lower surface near the center of 
the lesion. Most of these leaf-spots are due to ascomycetous pathogenes 
which produce during pathogenesis the conidial or imperfect fruiting 
structures. Hence one may expect to find on these spots either conidio- 
phores, acervuli or pycnidia. In some cases the fruit-bodies of the sexual 
stage are formed in the lesions. For many of these leaf-spot fungi, the 
sexual or perfect stage is unknown; in many cases because it has never 
been sought for. It will usually be found to develop during saprogenesis 
on the old fallen leaves, either in the areas of the original spots or scattered 
over the dead foliage. 
SYMPTOMS 
Specimens of several leaf-spot diseases have been provided. Study 
the symptoms exhibited by each and make DRAWINGS to show the same. 
Note that the opposite sides of the lesion may differ. The drawings and 
labeling should bring out the different color-zones in the lesion, size, dis- 
tribution in the blade and other characters. 
ETIOLOGY 
The pathogene in the material provided is in the pathogenic phase of 
either the primary or secondary cycle. Remembering that it is probably 
an ascomycetous pathogene, search carefully for the characteristic fruiting 
structures of ascomycetes. Use your hand-lens or place the leaf on the 
microscope-stage under low-power and examine with reflected light. 
If conidiophores are found, wet the scalpel, scrape some of 
the conidiophores from the spot, and mount in potassium hydroxide. 
Study and praw conidiophores and conidia. How are the conidia 
borne on the conidiophores? 
If acervuli or pycnidia are found, make an enlarged sKETCH 
of the lesion as it appears under the hand-lens, showing location and 
distribution of these fruit-bodies. 
To study the internal structures of these fruit-bodies and their relation 
to the host-tissue, make thin cross-sections from dry material and mount 
in potassium hydroxide, or from alcoholic material, in water. Show the 
character of these structures and relations in DRAWINGS. 
Determine the name of the pathogene by using Duggar, Fungous 
Diseases of Plants; Stevens, Fungi which Cause Plant Disease; Cooke, 
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