314 OHIO EXPERIMENT STATION: BULLETIN 214 
the mycelium. These organs called haustoria penetrate the leaf 
epidermis and must do this for the purpose of food extraction—it is 
needless to add that all food extraction from the plant acts as 
robbery. f 
Furthermore, the mildew-covered leaves drop to the ground in 
fall and there afford the fungus the ‘needed conditions for the 
development of the resting or winter stages of its course by which 
‘it is again ready to attack the plants the following season. Because 
so largely external in development these powdery mildews are 
usually comparatively easy of control. 
WILT DISEASES—SEEDLING COLLAPSE 
The stems or branches of plants may suffer from localized 
attack by parasitic fungias wellas from hail, insect attack and 
_mechanical agencies. The symptoms which follow will be found 
characteristic. In certain ones as in the clover anthracnose and in 
the fusarium of clover stems, we have the lesions accompanied by 
discolorations in which the fungus occupies a subordinate place 
outwardly. ‘On the other hand the spots or sorz of the rusts upon 
grains and grasses and the spots caused by the anthracnose of 
wheat, oats, rye, etc. show commonly a crowded occupation of the 
area by the parasitic fungus. 
_ ‘There are many examples of the effects of such lesions. 
Fuller discussions will be found under the description of the par- 
ticular diseases. ‘The anthracnose of the bean as well as that of 
the pea are good illustrations where these attack seedlings. Even 
-clearer symptoms come out in potato rosette where the fungus 
parasite at early stages of growth may kill off the stem attacked, 
while in later attack will cause such impaired development of the 
plant that stem or axial lengthening is arrested and a “rosette” 
appearance results. A still more striking arrest of stem elongation 
takes place in lettuce rosette wherein the roots are destroyed so 
largely by the fungus in the soil. (See soil infesting diseases.) 
In cankers of branches upon orchard trees the final death of 
the immediate branch is preceded by a depressed area invaded by 
by the parasite. 
PLANT DISEASES NOT BEYOND EXPLANATION 
The old mystery attached to disease prevalence can scarcely be 
maintained in our day. We have worked out in recent years or 
had determined for us the causal relations between the ferment or 
parasite and the effects upon the host plant or crop. So far as we 
can now discover the reason for the spread of diseases, or of a 
