144 Dale. — A Bacterial Disease of Potato Leaves. 
In November of the same year (1910) some potato and tomato seed- 
lings were infected with Bacteria obtained by carrying on the original cul- 
tures from the potato leaf. In these later experiments the Bacteria were all 
laid on the surface (either upper or lower or both) of the leaf, which had pre- 
viously been wetted with boiled distilled water. No 'mechanical injury 
whatever was done to the tissues of the host by stabbing. 
The plants were covered with bell-glasses and placed in a cool green- 
house. The infections were made on November 16. In two days brown 
spots began to appear on the infected leaves of those plants which had been 
kept most damp. Some which were drier were at first unaffected until 
sterile distilled water was put upon the leaves. Both the tomato and potato 
plants excreted large drops of water round the edges of their leaves, and it 
was observed that some of these became cloudy with Bacteria. This fact 
suggests the reason why the leaves begin to wither first at the apex and 
round the edges. Most probably infection takes place where there is the 
greatest amount of water, though, as stated above, it is quite independent of 
the stomata. Several leaves were fixed three days after infection. The 
infected leaves later became brown in patches and along the veins, and some 
appeared glassy-looking in patches. By no means all the leaves showed 
signs of ‘ curl ’ even when badly infected. 
2. In 1911, infection experiments were begun earlier in the open at the 
University Farm, in a field of potatoes which was kindly put at my disposal 
by Professor Biffen. 
This year the original organism was used, and also those obtained from 
the diseased plant cut down and examined in February, 1911. 
The first experiments were made on J une 8, when the weather was already 
very hot and dry, but there was some rain at the end of the month. With this 
exception there was hardly any rain at all until the end of the season, and 
the temperature not infrequently rose to over 90° F. in the shade 1 (35 0 C.), 
and was of course much higher in the sun. 
The method used was slightly altered, and was as follows : The lower 
leaves were cut off to prevent their being confused with those which had 
been infected. The young upper leaves were wetted with distilled water. 
In the case of very hairy leaves it was found necessary to rub the water 
gently on the leaf, especially on the under surface, to ensure its being 
thoroughly moistened. The Bacteria were laid on the wet leaves and 
no punctures were made until quite late in the summer, when, owing to the 
drought, the Bacteria could not enter the host. 2 Each of the infected shoots 
was covered with a bag made of thin typing paper, waterproofed with 
paraffin and petrol, and then sewn up. By this means the plant was pro- 
1 It is important in this connexion to remember that the organism will not grow above 35 0 C. 
2 The drought would tend to make the epidermal walls of the host thicker and firmer, and 
therefore more difficult for the Bacteria to pierce. 
