SECTIONAL TRANSACTIONS.—K. 493 
in certain potato diseases and mosaic diseases of various plants. Curly-top of 
beet is spread by leaf-hoppers sucking the phloem. Where infection by means 
of juice can be easily performed, as in tobacco and cucumber mosaics, biting 
insects also act as carriers. 
The potato diseases of this group spread more in warmer and sheltered 
regions where aphides flourish than in colder and rougher climates. 
Since the term ‘ virus diseases’ tends to cause confusion, the author calls 
them ‘phloem diseases.’ The idea that they are caused by specific micro- 
organisms spreading in the phloem has served him as a working hypothesis for 
the last ten years, but their etiology will be a subject of discussion so long as 
successful inoculation experiments with pure cultures of these organisms have 
not established their exact relation to these diseases. 
‘Their economic importance in the light of this hypothesis, especially as 
regards the cultivation of potatoes, is explained either by the slowing-down of 
the functions of the phloem, or by its ré/e in generalising infection not only in 
‘plant ’ but in ‘ clone.’ 
A considerable degree of resemblance exists between sieve tubes and latex 
vessels. A number of diseases induced by protozoa, spread through latex vessels, 
and carried by sucking insects, have now been detected; these results are 
stimulating to workers on ‘ phloem diseases.’ 
(c) Dr. W. B. Brierey. 
The Virus group of plant diseases is an important limiting factor in the 
world’s agriculture, and general experience indicates that the several diseases 
are rapidly spreading. The only statistics available are contained in the 
publications of the Plant Disease Survey of the United States. These date 
from 1918 and show that in the succeeding four years an average of twenty-two 
States lost nearly a million tons of potatoes from mosaic disease alone. In 
energy values this is food for about 170,000 people during that period. From leaf- 
roll disease the loss of potatoes in twelve States was about half a million tons. 
In an average of four States the loss of beans during 1918-21 was approxi- 
mately 850,000 tons. There is little doubt that, were statistics available for 
other crops and other countries, similar losses would be found to occur. 
(d) Mr. T. WurrEHeap. 
(1) Relative importance of different potato-virus diseases not yet ascertained, 
though leaf-roll, crinkle, and stipple-streak are probably the most serious. 
(2) Leaf-roll resulted in a loss of 55.8 per cent. and 51.8 per cent. of the 
crop in 1921 and 1922 respectively. In some localities loss in second year may 
amount to over 80 per cent. 
(3) As a result of infection by leaf-roll there was no appreciable reduction in 
size of tubers, but the total number produced was reduced by 54.4 per cent. 
(4) The effect on the yield was not seen during the year in which infection 
took place, but only in the crop from the infected tubers. 
(5) Transmission may be by aerial insects or through the soil by some means 
unknown. 
(6) Rate of spread varies in same ground in different seasons; in 1921 disease 
spread only from plant to plant in the same row, but across the rows in 1922. 
(7) Virus frequently fails to reach all tubers. 
(8) Rogueing, selection of healthy plants, and early lifting may enable 
healthy ‘seed’ to be maintained in many localities. 
(e) Mr. Hotmss Sirs. 
(f) Dr. R. N. Savaman. 
In regard to Mosaic Disease and Leaf-Roll amongst potato seedlings :— 
(1) Seedlings receive no protection against mosaic disease or roll by 
periodic nicotine spraying from the day of sowing. 
(2) Infection is only very exceptionally congenital. 
(3) Solanum nigrum, so far as ascertained, does not appear to be a source of 
infection. 
