Epidemic Plant Diseases. F. T. Brooks. 229 



EPIDEMIC PLANT DISEASES. 



By F. T. Brooks. 



All of us are familiar with disease in an epidemic form as re- 

 gards illness to which hmiian beings are liable. The widespread 

 occurrence of influenza a few years ago need only be recalled to 

 indicate the chief characters of an epidemic. At that time, many 

 people simultaneously became ill with influenza, which spread 

 with great rapidit}' from person to person. Thus the principal 

 features of an epidemic are the large numbers of indi\iduals in 

 a population affected by the disease at one and the same time, 

 and the amazing speed with which the malady spreads from 

 the centres where it lirst occurs. 



Plants of economic importance, in which large numbers of 

 indi\iduals of the same kind are usuall}' densely aggregated, are 

 also Uable to diseases of an epidemic nature. As in hinnan 

 diseases, plant maladies are frequently caused by parasitic or- 

 ganisms, and epidemic diseases are almost invariably of a 

 parasitic nature ; but whereas in animal diseases, these parasites 

 are usually either bacteria or protozoa, the organisms which 

 are the chief source of trouble to plants are of a fungoid nature. 

 Fungoid diseases of animals axe known, but most are compara- 

 tively infrequent and relatively unimportant. The reasons for 

 this difference in character of plant and animal parasites are 

 by no means clear, but it mav be pointed out that bacteria 

 usually thrive best in a slighth' alkahne medium that is well 

 pro\ided with complex nitrogenous compounds, and kept more 

 or less constantly at a lukewarm temperature. Blood is such 

 a medium. Fungi, however, are not so exacting in their re- 

 quirements ; they can exist with smaller and simpler supplies of 

 nitrogen especially if carbohvdrates are abundant ; they flourish 

 over a wider range of temperature, and usually prefer a medium 

 which is slightly acid. Needs of this kind are pro\ided better 

 by plant than by animal tissues. The intercellular spaces with 

 which plant tissues abound are more readilv permeated by 

 fungal hyphae than by bacteria. 



Not all diseases of plants are of an epidemic nature. Some 

 are sporadic, i.e. occur only here and there and not universally 

 over a wide area. Through neglect, however, a disease which is 

 usually sporadic, may assume an epidemic character. Thus silver- 

 leaf disease of fruit trees is usually sporadic in a plum plantation 

 upon first appearance, but if neglected so that the causative 

 fungus, Stereum purpureiim, fructifies upon the dead branches, 

 the disease may become so prevalent as to be epidemic. \\ ith 

 wound parasites there must clearly be widespread opportunity 



