Disease Symptoms 



sudden as, for instance, in the case of the too well- 

 known potato-bUght, where a whole field will seem 

 to be smitten and dead within a few days. It is 

 true the fungus spreads rapidly and works quickly 

 when the conditions which induce the develop- 

 ment of thunderstorms prevail, but it has been 

 growing on the lower leaves of the plants earher, 

 and has not been noticed because it has been 

 hidden. 



Perhaps a word of caution in interpreting 

 sjnnptoms should be given. When a fungus is 

 found growing on a dead leaf or a dead spot on the 

 leaf, it is not a certain sign that the fungus is the 

 cause of the death of the leaf or of the part of it. 

 The proof of the fungus being the cause is not a very 

 easy matter to establish. The spores of the fungus 

 in question must be obtained free from all mixture 

 with other spores, or indeed with other organisms ; 

 they must germinate upon and attack an otherwise 

 healthy plant kept from chance infection by any 

 other organism of any kind whatever ; this attack 

 must produce the same symptoms as were present in 

 the original attack ; and finally the fungus must 

 fruit and prove its identity with that which was used 

 to infect. 



Only when the fruits and spores of a fungus are 

 obtained can its identity be established. 



We have already alluded to the fact that not all 

 the varieties of any one kind of plant are equally 



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