74 Journal of Agricultural Research voi. v, no. j 



SPREAD OF THE MYCELIUM INTO THE SPROUTS 



When it had been shown that the myceUum was aUve in the tuber, at 

 least at some point, its spread into the sprouts was studied. Three boxes 

 (i8 by 1 8 by 6 inches) were filled half full of soil which had never grown a 

 crop of potatoes and which had been steamed for 40 minutes in an auto- 

 clave at 15 pounds' pressure. Twelve tubers were partially buried in 

 each box, four of which were sound, the remaining eight being infected 

 with P. infestans when harvested. The soil was well moistened with dis- 

 tilled water, and each box covered with a pane of glass. Each box in the 

 series wa's held at a different temperature — that is, 15° to 20°; 20° to 22°; 

 and 23° to 27° C. 



The 8 infected tubers subjected to a temperature of 15° to 20° produced 

 many sprouts, 5 of which became infected during the period under obser- 

 vation. The tubers subjected to 20° to 22° also produced 5 infected 

 sprouts, these appearing during the first 14 days after planting. The 

 greatest number of infections were obtained from the 8 diseased tubers 

 held at 23° to 27°, 13 sprouts becoming infected during the first 14 days 

 after planting. The checks remained free from infection. P. infestans 

 seldom sporulated on the parent tuber unless the corky layer was broken, 

 but it was very common on the basal portion of the sprouts growing from 

 infected tubers. In many cases the eyes producing infected sprouts were 

 cut out to learn whether the fungus was present in the tissues imme- 

 diately surrounding them, and in every case it was found. This showed 

 that the sprout infection was due to the spread of the mycelium and not 

 to spores present in the air, for had the infection been due to spores the 

 checks would have shown as high a percentage of infection as the diseased 

 tubers. Infection by P. infestans occurred on sprouts of all sizes, from 

 those barely visible to those nearly i inch in length. It was a very 

 common occurrence to find the fungus sporulating first on the lower 

 third of the sprouts, while on the upper two-thirds it was not apparent, 

 but it required only one or two days for the remaining portion to become 

 covered also, which indicates the rate of spread of the mycelium in the 

 sprout tissue. 



Naturally discoloration and decay followed the fructification of the 

 fungus. Plate IV, figure 2, shows a potato with diseased and healthy 

 sprouts. This is a late stage of sprout infection, and the tissues of the 

 two infected shoots have blackened. The healthy sprout stands on a 

 portion of the tuber showing no external evidence of the disease, while 

 that part surrounding the diseased sprouts is infected with P. infestans. 

 The fungus sporulated only on the sprouts of the diseased tubers, while 

 those arising from the healthy tubers in the same box remained sound 

 throughout, which makes it certain that infection was not by spores 

 present in the air or soil, but by the migration of the mycelium in the 

 tissues of the parent tuber. 



