HOW FUNGUS DISEASES ARE DISSEMINATED 17 



or trampled into manure. In either case myriads of spores 

 escape destruction, and are eventually returned to the land in 

 a condition favourable for infecting any suitable crop. The 

 same remarks apply to rusted straw. 



Passing to the spread of disease from one country or even 

 from one continent to another, we have mostly to deal with 

 economic plants that have been introduced to distant coun- 

 tries from Europe. It is too frequently the case that either 

 from the first, or after an interval of time, such introduced 

 plants are attacked by the same kind of fungus disease from 

 which they suffer at home. 



Three different reasons have been advanced in explanation 

 of these outbreaks of disease in a distant country, caused by 

 fungi that are natives of Europe. The first, which is now 

 almost universally discredited, assumed that the spores of 

 fungi were carried immense distances by wind, as from Europe 

 to Australia for example. The second reason is based on the 

 assumption that those particular kinds of fungi that attack 

 plants of economic importance are widely distributed, are in 

 fact practically everywhere, lying in wait as it were for the 

 advent of introduced plants. There are many grave reasons 

 against this view. Fungi conform to the laws of geographical 

 distribution as other plants do, and if we admit this reasoning, 

 we must also admit that it applies mainly to those fungi that 

 happen to cause damage to economic plants. No other kinds 

 of fungi are known to be cosmopolitan. 



The fact that wheat rust occurs now on some indigenous 

 Australian grasses, or that broad bean rust is now met with on 

 indigenous New Zealand weeds, does not prove that these 

 fungi are indigenous to Australia or New Zealand respec- 

 tively, or that they existed there before the advent of wheat 

 and broad beans, as is advocated by some authorities. 



Every one is agreed that the hollyhock rust (Puccinia 

 malvacearum) is an alien in Europe, and that by some means 

 it followed its host-plant to that continent. At the pre- 

 sent day this rust has attacked practically every kind of 

 European wild plant belonging to the hollyhock family. 

 Now suppose this indisputable fact not to have been known, 

 it might, and probably would, have been argued that the 

 hollyhock was attacked on its arrival in Europe by a fungus 

 common on European mallows. The same remarks apply to 

 the fungus causing the destructive potato disease. The 

 fungus by some means followed its host-plant, and has since 



B 



