326 DISEASES OF FIELD & GARDEN CROPS. [on. 



time, and that a considerably higher temperature does 

 not impair the germinating power of the potato. Mr. 

 Jensen states that the exposure of tubers to a dry heat of 

 100 or 105 F. for four or five hours is the best mode of 

 disinfecting them. This disinfection, it appears, should 

 be carried out in an oven, which has to be carefully 

 watched the whole time, and it is necessary that the heat- 

 ing process be continued for some hours to ensure the 

 deeper parts of the tubers being raised to the requisite 

 temperature. Mr. Jensen recommends for small experi- 

 ments the construction of a double box capable of hold- 

 ing water in its interior ; the space between the inner 

 and the outer box being filled with some non-conductor 

 of heat, as chaff. The inner box is filled with water at 

 a temperature of 100- 120 F. Into this are placed tin 

 or zinc cylinders, about five inches in diameter, containing 

 the tubers to be disinfected, and in these cylinders the 

 tubers are allowed to remain. By the employment of 

 narrow cylinders like the above, the potatoes are more 

 readily heated than is the case if wider ones are used. A 

 thermometer in the water is essential to ascertain its 

 temperature. It may require an addition of warm water 

 once or twice, according to the heat of the surrounding 

 atmosphere, and the efficacy of the non-conducting medium 

 employed. In a later communication Mr. Plowright 

 states that the tubers must, for not less than four hours, 

 have a temperature not below 104 F., and if this rises^ 

 to 115 F., no harm will be done to them, and that it is 

 not safe to go beyond 130 F. 



The apparatus has, of course, been founded on the idea 

 that many potatoes apparently sound at the time of plant- 

 ing, yet contain living hibernating fungus mycelium, and 

 that this mycelium grows with the young potato plant, 

 and such diseased plants act as centres from which the 

 potato fungus spreads. Our observation and experience 

 of growing potatoes does not lead us to look upon this as 

 always the case ; and we must leave it for practical men 



