262 The Spread of Fungus Diseases. [aug,, 



necessary ; that factor is suitable weather. The practical 

 potato grower can predict with almost unerring certainty the 

 advent of potato disease, the indication being what he terms 

 "stuffy" or "blighty" weather, which being interpreted, means 

 damp, dull warm weather. The same conditions are illustrated 

 on an exaggerated scale in the experiment with potatoes 

 described above, in the warm, imperfectly-lighted, damp house. 

 On the other hand the potatoes in the cool, dry, well-lighted 

 house, although proved to be infected, remained perfectly free 

 from disease in an obvious form. So with crops on a large 

 scale ; no one ever saw an epidemic of wheat rust, or of any 

 other crop during bright clear weather, yet this does not prove 

 that the plants were free from the germs of disease, but that 

 such germs were held in abeyance by the weather conditions 

 favourable for the growth of the host-plant, which conditions 

 are those most unfavourable for the fungus. 



It will probably be proved in course of time, that a sudden 

 outbreak of potato disease and other epidemics is not always 

 primarily due to infection by spores, — where are these spores to 

 come from if there has been no previous evidence of disease ? — 

 but rather to a sudden stimulation of mycelium already in the 

 plants, and only awaiting suitable conditions before assuming 

 an aggressive attitude. It is generally admitted that para- 

 sitism in fungi is an acquired habit. This being so, as would 

 be expected, there are many degrees of parasitism ; some fungi 

 have become so highly differentiated as parasites that they can 

 only attack one particular kind of plant, or even only one 

 variety of a plant. In cases of parasitism, the advantage is 

 all in favour of the parasite, which lives entirely at the expense 

 of the plant attacked, the latter deriving no benefit from the 

 presence of the parasite. 



In the potato disease the parasite is very destructive in its 

 work, during a severe attack completely destroying every 

 portion of the host-plant within a few days of its appearance. 

 In the case of cereals, the fungus, although yet a parasite, 

 behaves in a very different manner, whether infection takes 

 place through the flower or in the seedling stage in the ground ; 

 the fungus grows up with the plant in its tissues for six months 

 or more, without causing any apparent injury or inconvenience; 



