POTATO-ROT. 217 



them ; but if potato plants happen to be near the corrosive in3-eeruiui, 

 this [)enetrates at once into the roots or tubes and stems, and when 

 they have extended into the parts above ground, as the stem and 

 leaves, l)ranches are sent out through the breathing pores into tiie 

 air, and upon these are formed the conidia and zoospores winch re- 

 enter the breathing pores and develop their mycelium within the 

 tissues of the potato leaf, and the whole history of the fungus is 

 repeated again. 



It is a well known fact that the potato disease is most abundant 

 in warm damp seasons. The summer of 1844 was unusually cloudy 

 and moist, and so far as I am able to learu, all the great outbursts 

 of the disease occurred in seasons which were unusually moist and 

 warm, especiall}' if this condition prevailed in July and August. A 

 short time onl_y is quite sufficient for the development of the asexual 

 spores and the complete destruction of the potatoes as we well 

 know, and since the parasite is an internal one there seems to be no 

 good and practical method by means of which the malady ma}- be 

 arrested, when once the potatoes are infested. If, however, the 

 tops alone are affected, as would be the case when asexual spores 

 are brought by the wind from some other infested field, we may 

 save the potatoes in the ground to an extent by pulling up the tops 

 before the mycelium has had time to penetrate down through the 

 stems to the tubers. Even in this case great care should be used 

 to prevent, as far as possible, the conidia and zoospores from falling 

 upon the potato hills, since they have been known to be w'ashed 

 down through the ground far enough to reach the tubers and infest 

 them. It is plain, therefore, that we should carefully burn all the 

 infested tops, for should any of them fall into water or into damp 

 places, even, the very conditions would be fulfilled which would 

 cause the development of prodigious numbers of resting spores 

 capable of infesting the crop of the following year. 



It is an easy matter for us to conjecture how the resting spores 

 may be so widely distributed as we know they must be. Should 

 infested tops and tubers be allowed to remain in the field in damp 

 places, or in damp weather, we can understand how the resting 

 spores would be formed and matured. Later in the season, in 

 September or October, the weather becoming dry, these tops would 

 have decomposed and the resting spores liberated, and taken u^) by 

 the winds and scattered over the fields where they would hibernate 



