THE FACTORS OF AN EPIDEMIC. 



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potatoes are grown on a large scale, in dull moist 

 weather, especially in fields exposed to mists, 

 heavy dews, etc., about July and August, when the 

 foliage is full and turgid. Similarly on heavj' 

 wet soils, unless the season is remarkably open 

 and dry; but also on dry light soils in rainy 

 seasons. So evident was this that many believed 

 that the mists and dew brought the disease 

 harking back to the superstitions of earlier days. 

 We must remember that prior to i860 the life- 

 history of PhytophtJiora was not known. Since 

 De Bary's proof of the germination of the zoo- 

 spores and of the infection of the leaves, the 

 course of the hyphae in them and in the haulms, 

 the origin of the conidia, etc., and the confirma- 

 tion by numerous competent observers of the 

 true fungus nature of this disease, we are now 

 in a position to understand the principal factors 

 of the various epidemics of potato disease. 



It is not merely that the potato-fields afford 

 plenty of food for the fungus, and that the dull 

 weather causes the tissues to be surcharged with 

 moisture, owing to diminished transpiration, but 

 the mists and dew to say nothing of actual rain 

 and the flapping of wet leaves favour the ger- 

 mination and spread of the zoospores throughout 

 the field. Whether the dull light also favours the 

 accumulation of sugars in the tissues, and the 

 partial etiolation of the latter implies less resist- 

 ance to the entering hyphae, may be passed 

 over here, but in any case it is clear that we 

 have several factors of the non-living environment 



