Oct. II. 191S Phytophthora injestans in Irish Potato 83 



check rows, and as there was no other evidence of infection in this entire 

 row it seemed quite certain that the spores had come from the hill pre- 

 viously mentioned. On August 5, six days after this stray infection 

 was first noted, 14 others were found immediately below it on the leaflets 

 in the same hill. It seemed quite apparent that the spores had fallen 

 from the infection above and infected the leaves below. The disease 

 continued to spread rapidly until August 10, when a period of hot, dry 

 weather for 10 days checked its development temporarily. At the end 

 of this dry spell, however, it resumed activity, and an epidemic of blight 

 was well under way in this portion of the field. All the plants in the plot, 

 except those on a few short rows of a foreign resistant variety, were 

 killed by late-blight before frost. Four other cases, similar to the one 

 just described, developed between July 25 and August 4. The symptoms 

 in all cases were the same and need not be repeated. In each case the 

 spores produced by the initially diseased shoots infected adjoining 

 foliage and became centers for the spread of the disease. 



The plants in the three alternating rows planted with healthy seed 

 were watched for evidence of stem and foliage infection as carefully as 

 those planted with infected seed, as was also the rest of the 2-acre field, 

 but in no case did any infections develop that could not be traced to the 

 centers in the rows planted with infected seed. Of course, after the epi- 

 demic was well xmder way, the source of any single infection was un- 

 known. The significant point and the one on which information was 

 desired was the origin of the very early stages in the development of an 

 epidemic and not the late. 



The results of the field tests of 191 3 may be briefly summarized as 

 follows: (i) Only 63 per cent of the whole infected tubers and 49 per 

 cent of the cut infected seed grew; (2) the mycelium in infected seed 

 tubers responded the same way in the field as it did in the laboratory 

 experiments; (3) shoots were found that became infected before they 

 reached the surface of the soil; (4) others infected were able to break 

 through the soil and become centers of foliage infection. On these 

 dwarfed infected shoots the fungus fruited and infected the foliage, first 

 in the same hill and later in those adjoining. In this way these hills 

 became the centers for the development of an epidemic. 



FIELD STUDIES IN 1914 



It is well known that too much reliance can not be placed on the 

 results of i-year trials under field conditions. This is especially true 

 when dealing with a fungus like P. injestans, w^hich is very much influ- 

 enced by environmental conditions. In view of this fact, it seemed 

 desirable to repeat the field trials of 191 3. In 1 914, a plot of ground 

 was chosen at Caribou, about 60 miles north of Houlton, Me., where 

 conditions are fully as favorable for the development of late-blight as at 



