ISO DISEASE IN PLANTS. 



dangerous one : any pest may become epidemic if 

 the conditions favour it ! 



In 1844 and 1845 the potato disease assumed 

 an epidemic character so appalling in its effects 

 that it is no exaggeration to say that it consti- 

 tuted a national disaster in several countries. It 

 was stated at the time that this disease had been 

 known for some time in Belgium, in Canada and 

 the United States, in Ireland, in the Isle of 

 Thanet, and in other parts of the world. Similar, 

 but less devastating epidemics have occurred in 

 various years since. It was generally noticed 

 during such epidemics that the plants themselves 

 were full of foliage, surcharged with moisture, and 

 of a luxuriant green colour promising abundant 

 crops. The now well-known spots, at first pale 

 and then brown and fringed with a whitish mould- 

 like growth the conidiophores of the Phytophthora 

 were observed during the dull cloudy and wet 

 weather, cooler than usual, when the atmosphere 

 was saturated for days together, in July and 

 August. The actual amount of rain does not 

 appear to have been excessiv^e, but most observers 

 seem to agree that dull weather with moist air 

 had succeeded a warm forcing period of growth. 

 So rapidly did the disease run its course that in 

 a few days nearly all the plants were a rotting 

 blackened mass in the fields, and the potatoes dug 

 up afterwards were either already rotten or soon 

 became so in the stores. Further experience has 

 confirmed this, and we now know that the epi- 

 demic is very apt to appear in any region where 



