Field Observations on Development of Potato Blight 193 
found to he affected in the same way as A, i.e., discoloured zones 
appeared on the stems or petioles a little distance above soil level. 
Some of the portions under suspicion were removed but only the 
discoloured petioles developed Phytophthora. The fungus was not 
observed to fructify on these black zones in the field prior to the 
leaf infection noticed on J uly 22nd, although in the absence of contin¬ 
uous observation one cannot be certain that it did not. Even during 
an epidemic it is exceptional for the fungus to fructify on the stems 
in the field, and the fact that the lower petioles are often splashed 
with soil may have prevented the observation of fructifications on 
them even if present. Rain fell on July 10th, 11th, 12th, 14th, 
15th, and 17th. 
July 9. Aplant (No. 8) shewing several small blackened areas 
on the branches, was dug up. The tubers were sound but there 
were several small discoloured zones on the stem below soil 
level. 
July 10. A plant (No. 9) shewed blackening of the stem just 
above soil level. This plant was dug up on July 12th and all 
underground parts except the stem immediately below soil level, 
which was discoloured, were healthy. 
July 14-17. Plants 10-27 shewed discolouration on the basal 
parts of the stems and lower petioles, similar to those previously 
described. 
July 22. On this date plants (1), (2), (4), (5) and (13) were 
each affected by a few typical blight spots on the leaves, chiefly 
the lower ones. Microscopic examination confirmed the presence 
of Phytophthora. This was the first occasion on which the fungus 
was found producing spores in the field. The source of this leaf 
infection was not discovered. The blight spots rapidly increased 
in numbers on the leaves of these plants, and the number of discol¬ 
oured areas on the stems and petioles of these plants developed 
considerably after the appearances of the fungus in epidemic form, 
although the fructifications were not seen on these particular parts 
in the open. 
July 23. Plants (6), (11), (14), (15), (19) shewed blight spots 
on the leaves. 
July 24. Plants (23), (24), (26) 
ditto. 
July 25. Plants (7), (10), (20), (21), (22) 
ditto. 
July 28. Plant (12) 
ditto. 
Aug. 1. Plant (25) 
ditto. 
At this date plants (3), (8), (9), (16), (17), 
(18) and (27) which 
had been under suspicion of being affected by Phytophthora on 
account of having discoloured areas towards the base of the plant 
