Oct. II, 1915 Phytophthora infestans in Irish Potato 85 



hills in the four check rows were watched as carefully as those in the 

 two rows planted with infected seed, but no infections with P. infestans 

 were found. 



The development of foliage infection from the three centers described 

 was gradual and wholly comparable to that described in considerable 

 detail in the studies of 1913. It should probably be said in this con- 

 nection that the first foliage infection was found on July 27, five days 

 after the first case was discovered. By August 14 leaves within a radius 

 of 10 to 20 feet from each center or station were infected with P. infes- 

 tans. A bad epidemic of late-blight was in full swing throughout the 

 whole 2-acre field by September 10. It is plain that the three centers 

 above described formed the starting points for this epidemic. Other 

 centers of infection may have developed subsequently, but no attempt 

 was made to follow the later developments because of the constant 

 recurrence of new foliage infections resulting from the infections about 

 the original centers. The results of the field studies of 19 14 confirmed 

 in every way the results obtained in 1913. 



The fact that a tuber infected with late-blight may cause an epidemic 

 of the disease raises the question as to the role of infected tubers left in 

 the field at harvest time. The majority of these are killed by frost, 

 but a few remain in the soil or get covered during the digging of the 

 crop and may pass through the winter in a living condition. Observa- 

 tions showed plainly that many tubers survived the winter of 1913 in 

 Aroostook County, Me. The fields planted to oats in 19 14 that had been 

 in potatoes the previous season were well sprinkled with volunteer 

 potato plants. It is common knowledge among the growers of northern 

 Maine that some seasons volunteer potato plants are very plentiful. 

 Their presence or absence is determined largely by the season, especially 

 by the time and amount of sno\\1"all. 



POSSIBII^ITY OP CONIDIA OP PHYTOPHTHORA INFESTANS BORNE) ON TH^ 

 SEED TUBER REACHING THE SURFACE AND CAUSING FOLIAGE IN- 

 FECTION 



In 1876 De Bary (4) called attention to the possibiHty of conidia on 

 the seed tuber being able to reach the surface and cause foliage infection. 

 Hecke (12) and Clinton (8) are incUned to believe they function more 

 extensively than the mycelium in the seed tuber. lyittle is known about 

 the production of conidia on infected potato tissue in the soil or their 

 relation to renewing infection from one year to another. For this reason 

 it was thought advisable to learn something about the possibility of the 

 fungus fruiting on cut seed in the soil and whether the spores functioned. 



To this end 31 infected seed pieces were planted in the usual manner 

 on June 22, 1913, at Houlton, Me. The soil was quite dry, and the soil 

 temperature ranged from 10° to 14° C. Three days later they were dug 



