76 



Journal of Agricultural Research 



In this experiment 4 of the tubers produced healthy plants, while the 

 8 others were completely overrun before any of the sprouts could reach 

 the surface of the soil. This explains why seed potatoes infected with 

 P. infestans give a poor stand. It also shows that the relation of the 

 fungus to the sprouts is the same whether the tubers are wholly or only 

 partially buried. Another significant fact brought out in this experi- 

 ment was the presence of spores on the surface of the infected sprouts 

 in the soil. This was especially true on sprouts attacked but not wholly 

 killed. 



GROWTH OP the; MYCELIUM UP INTO THE SHOOTS 



When it became evident that the fungus could grow out into the 

 sprouts from an infected tuber partially or wholly buried in the soil, 

 experiments were outlined to ascertain whether it might not also grow 

 up into the shoots. Thirty tubers were artificially inoculated by intro- 

 ducing living mycelium from pure cultures of P. infestans into a wound 

 in each, and all were imrriediately planted in pots in the greenhouse, the 

 samxC number of healthy tubers being planted as checks on the same date. 

 None of the plants growing from these tubers showed any signs of infec- 

 tion with P. infestans, although they were watched carefully for 71 days, 

 after which the experiment was terminated. 



In another experiment 12 naturally infected tubers were planted in 

 pots of steam-sterilized soil. The same number of healthy tubers were 

 planted at the same time as checks. Only 4 of the 12 infected tubers 

 came up, and 3 of these were much less vigorous than the controls. The 

 spindly, sickly looking shoots that grew from the diseased tubers were 

 watched for 47 days, but no sign of P. infestans was noted. The tubers 

 were then dug up and found to be wholly decayed, but the stems were 

 sound. 



In a later experiment 200 naturally infected tubers were divided into 

 four equal lots and planted directly on the greenhouse bench i, 2, 3, and 

 4 inches deep, instead of in pots. An equal number of sound tubers were 

 planted in a like manner as checks. Conditions were made highly favor- 

 able for the growth and development of the plants. Seven days after 

 the tubers were planted, a few shoots were noted coming through the 

 ground. The following germination was obtained (Table I). 



Table I. — Percentage of germination of potato tubers infected with Phytophthora infestans 



