234 



Perpetuation of Potato Disease. [JULy, 



showed no trace of disease at the end of two months, when one 

 of the plants was removed to the warm house and placed under 

 a bell-jar. Within nine days this plant was blackened and 

 killed by the fungus. A fortnight later a second plant was 

 removed from the cool to the warm house and placed under a 

 bell-jar. Within a week of the removal of this plant it was also 

 covered with PJiytophthora, The third plant continued growing 

 in the cool house for thirteen weeks, and remained perfectly free 

 from obvious disease. 



Similarly marked results were obtained by using potato 

 tubers produced by a plant that was badly infested with potato 

 " leaf-curl " {Macrosporium solani^ Cooke), proving that this 

 disease can also be perpetuated by hybernating mycelium 

 present in the tubers. 



The above experiments, in addition to proving that the 

 diseases indicated can be transmitted from one generation to 

 another by means of mycelium present in the tubers, also 

 denionstrate another point of much practical importance, 

 namely, that the absence of obvious disease in the crop does 

 not necessarily prove the absence of such disease in a latent 

 form. 



In the experiments described above, it was known at the 

 commencement that the six half-tubers were all diseased. The 

 three plants grown in the hot, damp, badly-lighted house were 

 promptly destroyed, simply because the conditions indicated 

 were detrimental to the growth of the potato but highly favour- 

 able to the rapid development of the fungus, which soon became 

 dominant and destroyed its host-plant. On the other hand, the 

 three potato plants in the cool house grew normally under the 

 lower temperature, less atmospheric moisture, and better light, a 

 set of conditions very detrimen^-al to fungus growth ; hence, 

 although the parasite was present, it remained entirely in abey- 

 ance, and the practical man would, without hesitation, have 

 pronounced the plants free from disease. 



Every potato grower of experience can predict almost with 

 certainty the moment when potato disease will appear ; the 

 necessary conditions are warm, damp, dull weather, but instead 

 of the sudden outbreak being due to the rapid diffusion of 

 spores, as has hitherto been believed, it is far more probable 



