III.] EXCESS OF NITROGEN AND DISEASE 87 



on normally manured 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 Rothamstcd 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 about 

 by an excess of nitrogen is perhaps most strikingly seen 

 at Rothamsted on the mangold plots, though the man- 

 gold is a plant which, as a rule, suffers but liule 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 manured plots, although they are in close 

 proximity and equally exposed to infection. The 

 association of high nitrogenous manuring with suscepti- 

 bility 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 



