THE POTATO DISEASE. 145 



such as the Fusisporium Solani ( Spicarla solani), so often de- 

 scribed, these are moulds which are nourished by diseased tissue, 

 and do not affect the healthy tubers ; it can be easily verified 

 that their vegetation is of no force, or very tardy, upon the normal 

 tissue, and that it never determines a symptom of the disease 

 which now occupies us. 



The vegetation of the Peronospora, then, alone determines the 

 redoubtable epidemic to which the potato is exposed. Is the inva- 

 sion of the parasite favoured by any predisposition whatever of the 

 affected plant ? It is said that the different varieties of the potato 

 are not equally exposed to the malady. I will not deny that 

 assertion, without however being able to confirm it. There are 

 certainly some doubts on the subject, because frequently assertions 

 advanced on the same variety contradict each other. Nevertheless, 

 in admitting different predispositions in different varieties, one 

 ought to arrange them among the specific predispositions of which 

 we have already spoken, the existence of which cannot be contested. 

 As to the individual and unhealthy predisposition that we have in- 

 dicated so frequently, it must be first remarked that nearly all the 

 authors that admit it have positively ignored or denied the determ- 

 ining influence of the parasite, and it is upon this last point that 

 their opinions are supported. After what is known, and has been 

 proved, these opinions are of little value. And I deny that the 

 introduction of the parasite is favoured by any predisposition of 

 the affected plant, either of the potato or any other kind whatever. 

 Experiments at least show nothing in support. 



The parasite being sown with the necessary precautions upon a 

 morsel of healthy tuber, this becomes diseased, while the rest of 

 the tuber preserves its normal condition. In making similar ex- 

 perihients upon the leaves analogous results are obtained. Com- 

 parative experiments upon a quantity of plants of the same variety 

 have always given me the same results ; nothing determines the 

 invasion of the parasite except the careful sowing of the conidia. 

 The plants experimented upon always became diseased when 

 treated with conditions indispensable to the vegetation and propa- 

 gation of the parasite ; while those protected from the influence of 

 the conidia remained healthy. In very numerous experiments I 

 have never found that one individual was more favourable to receive 

 the parasite than another, provided that the cultivation was carried 

 on under equal external conditions. 



JouRN. Q. M. C. No. 22. l 



