ECONOMIC ASPECT OF PLANT DISEASES 51 



appears to be that of exercising the power they possess, of 

 converting nitrates into nitrites, or ammonia, or of going to 

 the extreme and liberating free nitrogen. The strong 

 ammoniacal smell arising from a manure heap testifies to 

 the presence of these organisms. An excess of farmyard 

 manure in the field is acted on in a similar manner. 



It may be regarded as practically certain that further 

 investigation of this subject from a purely scientific stand- 

 point will result in the discovery of methods that will be 

 of the greatest value to all interested in the cultivation of 

 plants. 



Beijerinck, Verslag K. Akad. Wetensch. Amsterdam 

 (1900-01). 



Berthelot, Chimie Vegetale et Agricole (1899). 



Hall,ybwr. Agric. Set., i. p. 241 (1905). 



Oberlin, ' Bodenmiidigkeit u. SchwefelkohlenstofP ; Mainz, 

 1894. Journ. Agr. Prat., 59 (1895). 



Voorhees and Lipman, 'A Review of Investigations in Soil 

 Bacteriology,' U.S. Dept. Agr., Bull. 194 (1907). 



Winogradski, Comp. Rend., 118, p. 353 (1894). 



ECONOMIC ASPECT OF PLANT DISEASES 



As a rule it requires nothing short of an epidemic to 

 convince farmers and gardeners that a disease exists. 

 During those seasons when the loss does not exceed three 

 to five per cent., disease is considered to be absent, or if its 

 presence is actually realised, it is only looked upon as a 

 reminder that matters might have been much worse. This 

 condition of things is most unfortunate; the three or four 

 per cent, loss which undoubtedly occurs every year, and in 

 every country, far exceeds, say during a quarter of a century, 

 that due to sporadic epidemics. An epidemic appeals be- 

 cause there is a sudden, more or less heavy loss, due to 

 an obvious cause, whereas when the same cause is at work 

 in a quiet unobtrusive manner, the loss is not generally 

 realised. 



Comparatively few countries furnish statistics dealing with 

 the loss caused by preventable diseases ; whether this omis- 

 sion is due to indifference, ignorance, or diplomacy, remains 

 to be ascertained. 



