BROOKLYN BOTANIC GARDEN 
LEAFLETS 
THE BROOKLYN INSTITUTE OF ARTS AND SCIENCES 
Series III Brooklyn, N. Y., May 19, 1915. Number 4 
THE NATURE OF PLANT DISEASES 
Disease in plants, just as in animals, comes from some dis- 
turbance of normal, healthy living’. Such disturbances may be 
conveniently grouped into two classes. They may arise, first, 
from certain more or less obscure atmospheric, soil or cultural 
conditions which somehowcause sickly, poorly nourished growths, 
or sometimes abnormal or deranged organs. Such diseases, 
classed as the non-parasitic, or physiological, diseases, not only 
occur most abundantly in nature, but they also, by weakening the 
plant and thus lowering its power of resistance, may even en- 
courage, just as in the case of weakened animals, the starting of 
parasitic diseases. 
The second type of disturbance of the healthy life of plants 
arises from the attacks of parasitic organisms. Such infectious 
diseases are very common and their effects are often very con- 
spicuous to the naked eye. Those arising from physiological 
causes, on the other hand, while sometimes conspicuous enough 
in their effects, are quite often only obscurely related to a definite 
cause. The diagnosis of a disease in plants, especially one 
caused by a bad environment, thus becomes sometimes a matter 
of extreme difficulty. 
We may place into perhaps five categories the parasitic or- 
ganisms which cause disease in plants : animal parasites, such 
as nematode worms, and plant lice and other insects; slime molds; 
bacteria; fungi; and certain parasitic flowering plants. The par- 
asitic diseases to which human beings and animals are subject 
likewise fall into similar groups, with the exception of the last. 
Further, just as in the case of the doctors for human beings, it is 
obvious that the plant doctor must have years of training and ex- 
