POTATO DISEASE. 99 



Having endeavored to convey the well and repeatedly observed 

 facts upon which is based the conclusion that the fungus, designat- 

 ed by botanists as Peronospora infestans, is the immediate and only 

 cause of the potato disease, it remains, in the next place, to point 

 out the harmony of this conclusion with the facts familiar to every 

 one who cultivates potatoes, and finally, to indicate the means of 

 checking or suppressing the ravages of the disease, as far as this 

 is, at present, practicable. 



It is universally observed that the leaf blight and the rot attack 

 the potato most destructively in localities where the atmosphere 

 and the soil are most liable to be impregnated or saturated with 

 moisture. Hence we find that low lands, lying along a stream and 

 sheltered by forest or hill, are visited by the disease when more 

 elevated and airy positions escape. Potatoes on a hill are often 

 unaffected, while those of the same kind in a valley a few hundred 

 rods distant, are totally destroyed. In dry seasons, especially 

 in those which are dry in August and September, the disease is 

 less prevalent than in wet years. When sultry, showery weather 

 succeeds warm and dry days, or when by a storm the air is rapid- 

 ly cooled, so that heavy dews or fogs supervene, and evaporation 

 is checked, it often happens that a field, healthy to the ordinary 

 observer at night, is black and ruined in the morning. 



This influence of moisture may be exhibited in the following 

 manner, at any time when potato tops are at hand, on which, 

 though their appearance is fresh and healthy, by close inspection 

 may be found minute patches of the fungus. Two portions of such 

 infected foliage are taken, and the stems of each placed in the neck 

 of a glass or bottle containing water. One portion is exposed free- 

 ly to the air, the other is covered with a bell-glass. Other 

 things being equal, it will be seen in these few hours that while 

 the exposed foliage has not perceptibly altered in appearance, that 

 which is under cover exhibits a large growth of the brown fungus- 

 stains, and in a day or less is black and blasted. 



De Bary found that very high temperature is of less influence in 

 developing the fungus, than an atmosphere saturated with mois- 

 ture. The Peronospora grew with equal rapidity at temperatures 

 of 65° and 80°, when the air is fully charged with vapor. 



The fact that the conditions which develop the potato disease are 

 precisely those which produce the fungus, is in harmony with and 

 a consequence of the theory we believe to be the true one. 



