36 



FERTILIZERS AND DISEASE. 



H. S. Hammond, Macdonald College. 



One of the most important effects upon plants of an excess of nitrogen 

 is their increased susceptibility to fungoid attacks of all kinds; for 

 example, rust is always much more abundant upon wheat which has been 

 heavily manured with nitrogen, just as it appears on normally matured 

 wheat whenever the character of the season has been such as to induce a 

 specially rapid production of nitrates while the plant was making its 

 growth, as when great heat and moisture come together in May. In 

 seasons when rust is prevalent the high nitrogen plots at Rothamsted are 

 always markedly the more rusty, and can easily be picked out by their 

 colour ; the grass plots are also marked by their special rusts ; and again, 

 such a characteristic grass fungus as Epichiloe typhina is generally 

 common enough on the high nitrogen plots but absent from the others. 

 But susceptibility to disease brought ab.out by an excess of nitrogen ia 

 perhaps most strikingly seen at Rothamsted on the mangold plots, though 

 the mangold is a plant which, as a rule, suffers but little from fungoid 

 attacks. In September, however, the leaves of the mangolds at Rothamsted 

 that receive an excess of nitrogen begin to be attacked by a leaf spot 

 fungus, Uromyces betae, which develops rapidly until on the worst plots 

 all the larger leaves turn brown and present a burnt-up appearance, 

 because the spots of destroyed leaf tissue have become so numerous as to 

 run together. Where the application of nitrogen has been less heavy but 

 is still high, the severity of the attack is diminished, while the fungus is 

 entirely absent from the leaves of the normally matured plots, although 

 they are in close proximity and equally exposed to infection. The associa- 

 tion of high nitrogenous manuring with susceptibility to disease may be 

 seen in all plants; it is often very manifest in greenhouses where crops 

 are grown in specially rich soil, nitrifying very rapidly owing to the high 

 temperature prevailing. The dark green aspect of the leaves of such 

 plants is generally evidence of the excessive amounts of nitrogen they are 

 receiving, and it is well known that if any fungoid disease makes its 

 appearance it is very difficult to keep in check and often destroys the 

 whole crop with great rapidity ; as, for example, with the leaf spot fungus 

 Cercosporium melonis, which has, of late years, proved so destructive to 

 cucumbers grown under glass. 



Various attempts have been made to get a little nearer to the cause of 

 this association of high nitrogenous manuring with susceptibility to 



