Oct. II. 1915 Phytophthora injestans in Irish Potato 99 



It is to be hoped that the recent researches on this problem will afford 

 an angle of approach that will yield positive evidence in the near future. 

 In closing it should be pointed out that, although P. injestans rarely 

 produces oospores in the potato plant, this should not be looked upon as 

 abnormal. As shown in this paper, the production of resting organs is 

 not necessary for the hibernation of the fungus. The mycelium is quite 

 sufficient. There are many species closely related to P. injestans that 

 produce few resting spores on certain of their hosts. These may per- 

 petuate themselves from one season to another by means of the living 

 mycelium in the perennial parts of the host plant in much the same way 

 as already described for P. i?ijestans. The sparing production of oospores 

 and the hibernation of the mycelium are therefore not uncommon in 

 several species of this family. 



SUMMARY 



It is clear from the author's experiments that the mycelium of Phy- 

 tophthora injestans spreads in the tissues of the potato tuber and finally 

 reaches the sprouts. The growth of the fungus is retarded when diseased 

 tubers are held in dry soil or at temperatures below 5° C. Infected 

 tubers rot rapidly when placed in warm wet soil. This explains the 

 wide variation in stand obtained by earlier writers. A temperature of 

 23° to 27° C. and a well-watered soil were found to be the most favorable 

 for the mycelium to spread in the tuber and grow out into the sprouts, 

 both when partially and when wholly covered with soil. Under these 

 conditions the sprouts may become infected from 4 to 20 days after 

 planting, regardless of their size and age. The time required is doubt- 

 less influenced by the proximity of the mycelium to the buds and the 

 external conditions. 



The mycelium of P. injestans may remain alive in seed tubers planted 

 in the soil for at least 45 days, and it is very possible that under con- 

 ditions less favorable for the soft rots which follow P. injestans in the 

 tuber the fungus may live much longer. None of the author's results 

 or observations tend to show that the potato fungus is late^it in the stems 

 and leaves of plants growing from diseased tubers, as stated by Massee 

 (20). 



Laboratory tests showed that the fungus infects not only the sprouts 

 but also the shoots that break through the soil. The mycelium grows 

 from the tuber into the stem, where it travels up to the surface of the 

 soil and sporulates, as held by De Bary (4) and Jensen (14). This 

 usually takes place in the small dwarfed shoots in a hill. 



Potato tubers infected with P. injestans used for seed purposes and 

 planted under field conditions may cause the development of an epi- 

 demic. The mycelium grows from the parent tuber up into the stem 

 exactly as shown in the laboratory experiments. It later sporulates 

 and foliage infection results. Ten such cases were found and followed 



