8o Journal of Agricultural Research voi. v, no. 2 



In order to test still further the effect of moisture on the growth of the 

 fungus up into shoots, 12 vigorously germinating tubers of the Green 

 Mountain variety were planted in only slightly moist, steam-sterilized 

 sand. These tubers grew rapidly, and in six days some of the sprouts 

 began to break through the surface of the sand. Twelve days later 2 

 of the 12 tubers were dead. The remaining 10 were potted in steam- 

 sterilized soil and placed in a glasshouse where the soil was well watered 

 and the humidity high. Nine days later one shoot of one of the tubers 

 was badly discolored near the surface of the soil. The discoloration 

 spread up the stem, and after two days the infected area bore conidio- 

 phores of P. injestans in considerable abundance. When the tuber was 

 dug up, the shoot was found to be diseased throughout its whole length 

 below the surface of the soil. Six days later another tuber show^ed an 

 infected shoot like the one just described. The remaining 8 mother tubers 

 were dug up two weeks later and found to be entirely decayed. These 

 results tend to show that continuous high moisture content of the soil 

 is not necessary for the growth of the mycelium in the tuber up into the 

 stems. According to the results obtained in these experiments, the soil 

 may be kept comparatively dry until the plants are up. Furthermore, 

 under these conditions the tubers do not rot as rapidly, and a larger 

 number of shoots are produced by each. 



INFECTED SEED POTATOES THE CAUSE OF AN EPIDEMIC OF PHYTOPHTHORA 

 INFESTANS 



The relation between seed potatoes infected with P. injestans and the 

 development of epidemics of the disease under field conditions has received 

 consideration both in Europe and in America, but no one has yet been 

 able to trace and establish beyond doubt the existing relationship. Both 

 De Bary (1,4) and Jensen (14) claim to have done so, but they made 

 only limited tests in the open in gardens, where conditions are not always 

 comparable to those existing in the field. A large number of field trials 

 having been made with only negative results, coupled with the fact that 

 the mycelium grew up into the stems under laboratory conditions, led 

 the writer to make field trials. For this purpose a section of the country 

 was chosen where this disease occurs annually — namely, northern Maine. 

 Such a section should afford the environmental conditions suitable for 

 the development of all phases of the disease. 



FIELD STUDIES IN- X913 



The land selected for the experiment had not grown a crop of potatoes 

 for at least five years and had been in hay for the preceding four years. 

 The infected seed planted was selected in the spring from five bins 

 (1,200 bushels each) of potatoes, Irish Cobbler and Green Mountain 

 varieties, grown in the vicinity of Houlton, Me., and held in storage 



