890 
History and Definition 
Diseases in plants have existed as long 
as plants themselves—ages before the ad- 
vent of man. Civilization and agriculture 
have usually developed together in all 
parts of the earth and it is not strange 
that anything that troubled or destroyed 
an important food plant should be ob- 
served and the cause sought. In the earli- 
est historic records as well as in early 
Greek and Roman times some of the more 
destructive diseases of plants, like rust 
and mildew or blight of cereals were 
widely known and discussed. A special 
deity was recognized who ruled these 
phenomena and to whom sacrifices were 
offered. 
Injury due to animals, especially in- 
sects, and to extremes of weather and 
unfavorable soil conditions were early 
often associated with their appropriate 
causes. It was not, however, until the 
latter part of the eighteenth and the be- 
ginning of the nineteenth century that 
the solid foundations of plant pathology 
were laid by the development of anatomy 
and physiology. The early works of 
Unger, “Die Exantheme der Pflanzen,” ete. 
(1833); Weigmann, “Die Krankheiten 
und Krankhaften Misbildungen der Ge- 
wasche” (1839); and of Meyen Pflanzen- 
pathologie” (1841), marked an important 
step forward in the embryo science of 
plant pathology. During this period mi- 
croscopical, chemical and physiological 
work with plants was active. The writ- 
ers of this period rather overworked un- 
favorable nutrition as the cause of dis- 
ease. Maladies that could not be traced 
to visible external causes were usually 
held to be due to unsuitable nourishment 
or the lack of something in the soil. It 
was not believed that the fungi so often 
found associated with diseases had any 
casual relation to them. They were held 
to be abnormal developments of the dis- 
eased cells themselves and not independ- 
ent organisms. It remained for De Bary 
to determine the true nature and habits 
of fungi and bacteria and to demonstrate 
their causal relation to disease in many 
cases. His careful work gave a great 
stimulus to investigation in plant and ani- 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
mal pathology and opened what has 
proved to be the most important field of 
the science. His two most important 
works are “Untersuchungen uber die 
Brandpilze,” ete. (1853), and “Morpho- 
logie und Physiologie der Pilze,” ete. 
(1866). Since De Bary the rapid develop- 
ment of the subject is well represented by 
the works of Hartig (1874-89), Frank, 
(1880-96), Soraurer (1886-8), Marshall 
Ward (1889-1901). The last work “Dis- 
eases in Plants,” is one of the most ex- 
cellent and readable expositions of the 
subject that has appeared. 
Health and Disease Compared 
That there is no defined line between 
health and disease is generally recognized 
by pathologists and physiologists. A 
plant continually varies in response to 
changes in its environment. There is, 
however, for each individual and for a 
given species as a whole a certain accus- 
tomed range or power of adaptation to 
each factor of its environment and to the 
various combinations of these factors. 
The process of natural selection operates 
to perfect this adaptive attuning of the in- 
dividual and the species as a whole to the 
conditions under which they live. If 
these conditions are subject to great ex- 
tremes of moisture and dryness or heat 
and cold, the natural or indigenous vege- 
tation will be found, as a rule, equal to 
the emergency, while an introduced spe- 
cies, if developed under an environment 
not subject to such extremes, might be 
seriously injured or destroyed, and if the 
change is very unusual even the indige- 
nous species may suffer. A moist, warm, 
cloudy spring may be followed by dry, 
hot weather and the tender watery 
growth be so much dried out and checked 
that it may be deformed and abnormal in 
shape, structure and size. This variation 
may be slight or it may be great. If it is 
slight it may have no appreciable effect 
on the vigor and growth of the plant. 
The leaves become a little firmer and 
smaller and more resistant to the hot, 
dry conditions, while the maturer leaves 
that cannot adapt themselves to the 
change turn yellow and fall, cut off by 
the parent plant. The plant is better for 
