308 OHIO EXPERIMENT STATION: BULLETIN 214 
cal agents such as strong acids and alkalis. Quick growing plants 
appear to fall in drought, as with cucumbers when started during 
a period of excessive rains. Plants, and especially trees, may be 
locally injured by winter freezing, by hail, by overbearing with ex- 
haustion of water supply, and by a variety of causes. 
While we must keep our minds open to these varying causes of 
impaired vigor, by far the larger number of the diseases described 
in this bulletin are directly attributable to parasitic fungi which 
attack the plant or host in some vital part and rob it of its substance. 
The conditions of injury arising from the attacks of insects alone 
are not included. These fungus parasites of particular plants are of 
differing sorts, which produce, each, its more or less particular 
effects. It must follow, therefore, that the diseases produced 
differ in nature and that the names applied will vary accordingly. 
The names are not simply blight, rust, etc., indiscriminately 
applied—they are given with reference both to the parasite and its 
effect on the host plant.* 
Parasitic fungi and bacteria which cause disease, being plants, 
though of lower class, have differences among themselves which may 
be clearly designated and defined. ‘The names applied to them are 
accompanied by specific and generic descriptions which mark off 
the sort as definitely as do the descriptions on higher plants such 
as ferns, flowering plants and trees. The extreme minuteness of 
the parts of parasitic fungi and bacteria make necessary the use of 
the microscope in their description and detection. ‘The parts called 
spores which reproduce these minute plants have special form, size, 
etc., by which these are recognized when found. 
The agencies for the spread of parasitic diseases are those 
operations in which we engage or those which surround and envelop 
the plantsas well as ourselves. Light spores will be carried by 
currents of air like particles of dust. Allspores or germs of these 
lower plants may be carried by numerous agencies such as insects, 
higher animals, and man. They willalso find entrance into plants 
by whatever openings exist at the time. The epidermis of a green leaf 
or stem has breathing pores or stomates init; the leaves of mustard 
plants have water pores in them and wounded plants have these 
freshopenings to invite the entrance of the disease conveying 
spores or germs. 
The remedies for plant diseases are based upon the character 
and life history of the particular parasitic growth with which we 
have to deal and upon the nature of the host plant itself—some 
hosts being very different from others in respect to permitting of 
sprays of fungicides or insecticides. Common sense inferences are 
* See naming of diseases. 
