Field Observations on Development of Potato Blight 105 
discoloured areas before July 22nd, none exhibited typical blight 
spots, ont he leaves up to the end of the period. 
From an examination of the plan of the plot it is evident that 
there was no regularity in the sequence of the plants shewing first, 
suspicious symtoms, and then definite blight spots, except that the 
plants first affected were grouped rather towards that part of the 
plot which contained plant A. After the end of July the production 
of spores on the plot was so great that there would be no lack of 
material to infect the remaining plants under favourable conditions 
It is thus uncertain how the plants in Plot 1 were infected. 
The phenomena observed are capable of explanation either by 
infection via the soil or via the seed tubers. 
PLOT 2. 
This Plot, which was about a mile from Plot 1, contained about 
60 plants of each of two varieties—Great Scot and Dunbar—and 
120 of Arran Chief, all Scotch seed and planted in April. The land 
was under strawberries during 1917 and before being planted with 
potatoes was dressed with farmyard manure. On a part of the area 
planted with the variety Dunbar many blighted tubers of the 1917 
crop had been thrown from a shed near by, in which the tubers had 
been sorted the previous autumn. A search in the top layer of soil 
revealed many decayed tubers and other tubers partly shrivelled but 
apparently sound, giving rise to weak shoots. Several of the latter 
were incubated but Phytophtliora did not develop. 
A plan of this Plot is appended : 
During the period of detailed observations, none of the Great 
Scot plants and only two plants of Arran Chief came under 
suspicion of being affected by Phytophtliora , but it was subsequently 
concluded that these were not so attacked. 
The following record was taken of the Dunbar potatoes:— 
July 9. A plant B in the area (cross shaded in the plan) 
containing last year’s blighted tubers shewed one stem blackened 
all round from soil level upwards for about two inches. Two other 
stems of the same plant were healthy. All the neighbouring plants 
were sound. 
July 10. No change. 
July 11. No change. Another plant E in the same area 
shewed blackening of one of the lower petioles. 
July 21 (morning). No change observed. 
July 22 (evening). The plant B noted on July 9 as having a dis¬ 
coloured stem now shewed many petioles blackened and numerous 
typical blight spots on the leaves, especially the lower ones. This 
