Oct. II, 191S Phytophthora infestans in Irish Potato 75 



This experiment was repeated and has been reported in full in an 

 earlier paper (21). Except in one particular, the results were, in general, 

 alike. In this case a sprout grew out near the surface of the soil from one 

 of the infected tubers. This sprout became infected and the mycelium 

 of P. infestans grew out from it into the soil for a distance of about i cm. 

 This is not a usual occurrence and happens only when conditions are very 

 favorable for the growth of the fungus. A slight decrease in the moisture 

 content of the soil and the fungus is no longer in evidence, nor does it 

 return if the original moisture condition is restored. 



This experiment was again repeated on January 29, but only two sets 

 of temperatures, 15° to 20° and 23° to 27° C, were used. The other set 

 of temperatures was omitted because the supply of tubers was rapidly 

 becoming exhausted, and, besides, it had been shown that temperatures 

 between 15° and 27° were the most favorable. The results were, in 

 general, like those already recorded and need no further consideration. 

 From this series of three experiments, in which infected tubers were par- 

 tially buried in moist, sterile soil, it is clearly shown that the mycelium of 

 P. infestans in infected tubers spreads from the parent tuber into the 

 sprouts, where it may sporulate freely. 



Naturally the next step was to learn something as to the behavior of 

 the infected tubers when wholly buried in the soil. To this end 12 sound 

 tubers of the Irish Cobbler variety were artificially infected with a zoo- 

 spore suspension held in contact with a sprout about one-fourth of an inch 

 long by means of a ring of paraffin, as shown in Plate IV, figure 2. These 

 tubers, together with 6 sound ones as controls, were buried 2 inches deep 

 in a box of wet sterilized soil and placed in a saturated atmosphere at 23° 

 to 27° C. The tubers had gone through the rest period, and in some cases 

 the sprouts were i inch long. Eleven days after planting, 4 of the tubers 

 had thrown up shoots. The remaining 8 were dug up to learn their con- 

 dition, and it was found that in every case the fungus had spread into 

 sprouts other than the one originally infected. Plate IV, figure 2, shows a 

 tuber with the paraffin about the infected eye and the cluster of 5 

 sprouts at the bud end of the potato. One of the cluster, it should be 

 noted, is free from infection. After the tuber was photographed it was 

 cut and the discoloration typical of P. infestans was found at the base of 

 the sprouts. That it was P. infestans was further shown by the pro- 

 duction of spores and conidiophores on the discolored tissue. The fungus 

 had spread from the initial point of infection over to the point where the 

 cluster of infected sprouts originated from the parent tuber. The four 

 shoots that came through the ground were allowed to remain until 

 April 30, when they were dug up. These were found to be sound, while 

 the parent tubers were totally decayed. The controls remained free from 

 infection by P. infestans throughout and developed into normal plants. 

 5771°— 15 2 



