i45 
Dale . — A Bacterial Disease of Potato Leaves. 
tected to some extent from dryness and excessive light. Some leaves were 
infected on the upper surface only, some on the lower surface only, and 
others on both sides. Several varieties of potato were infected, including 
f White City ’, ‘ Up-to-Date ’, ‘ British Queen and ‘ Maresland Queen ’. 
The results of these experiments will not be given in detail, partly 
because later in the season so many gave no results, and also because the 
plants were attacked by a fungus, apparently Cladosporium. The symptoms 
of the disease caused by this fungus are externally so like those due to 
Bacteria that only a microscopic examination shows which is present. 
Both diseases may occur in the same leaf. 
The positive results, though few in number, are indisputable, as well- 
developed infection tubes were found in the leaves. These will be described 
in the next section of the paper. 
With regard to Cladosporium it may be mentioned in passing that this 
'fungus attacks the potato leaves chiefly after the plants have been earthed 
up. It is one of the common Fungi of the soil, and, when the soil is brought 
into contact with the leaves in the process of earthing up, the spores attack 
the lower leaves and the fungus gradually passes upwards, until the whole 
plant may be involved. Each spore and the hyphae formed from it kill a 
small area of the leaf, causing a yellow or brown spot. The veins are also 
very frequently attacked and become brown. The diseased areas gradually 
spread over the leaf, which becomes yellow, then dries up, and finally 
falls off. 
Some experiments, both with seedlings and with plants from tubers, 
were also made during the summer in a greenhouse, but gave no results, 
without any doubt because it was not possible to prevent the temperature 
from rising far too high. On one occasion, with open windows and white- 
washed roof, the temperature was 104° F. Since the maximum growing 
point is 35 0 C. (95 0 F.), the optimum temperature for growth is of course 
lower. This fact, as well as the excessive drought, accounts for the 
fewness of the positive results obtained in the summer of 1911 after artificial 
infection. 
5. Examination of the artificially infected Plants. 
1. 1910 Material. Few sections were cut through the material infected 
by puncture, because one of the chief objects in the experiments was to deter- 
mine whether the infection took place through the uninjured epidermis, and 
this of course could not be seen when the leaf had been pierced artificially. 
As in the case of naturally infected plants, no infection was seen 
to take place through either the water stomata or the ordinary stomata, 
though diseased leaf edges were specially examined. The fact that the 
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