NOTES AND ABSTRACTS. 
505 
tion was very scanty below I5°C. Above 2o°C. it became most abundant, 
increasing with the temperature. The minimum is probably between io° and 
I3°C, the optimum about 24°C, and the maximum very near 3o°C. Indirect 
germination occurs generally in a 10 per cent, dextrose solution, sparingly in a 
16 per cent, solution, and not at all in a 20 per cent, solution. In the last, direct 
germination occurs. 
The time required for spores of P. infestans to germinate depends upon (a) 
the viability of the spores, (b) the external influences. The shortest period for 
indirect germination was 45 minutes, though it usually requires two or three 
hours. The time decreases as the temperature increases up to I3°C. Above this 
the ratio is reversed. Direct germination is a slower process. The number of 
spores germinating was also dependent upon the temperature, 80 per cent, 
germinating at io° to I3°C. 
The motility of the zoospores was also influenced by temperature. Its 
duration varied inversely with the temperature, ranging from 22 hours at 5°-6°C. 
to 19 minutes at 24°-25°C. 
The spores are killed in from 6 to 24 hours when exposed to such dry 
atmospheric conditions as exist in an ordinary room. 
Leaf juices from the softening of infected tissues have an inhibiting effect 
on germination. 
Increasing the amount of nascent oxygen in the medium containing the spores 
inhibits germination. 
When the spores were subjected to optimum temperature conditions for 
indirect germination, '0159 per cent, of copper was necessary to prevent germina- 
tion. Infection of the potato plant with P. infestans takes place at conditions 
favourable to germination. Plants chilled for periods of from 12 to 24 hours 
at io°-i3°C. showed a greater amount of infection than the controls held at 
higher temperatures. 
Infection becomes visible in two or three days at temperatures between 
23 0 to 27°C. It requires a longer period at lower temperatures. Foliage 
infection may only take place by direct germination of the spores, and may take 
place either through the upper or lower surface of the leaf. Usually, however, 
infection occurs through the lower surface of the leaf, and is due to the presence 
of the stomata. 
The most favourable temperature for the growth of the mycelium in the 
tissue (probably about 24°C.) is about the same as the optimum for direct germi- 
nation in water, and considerably higher than the optimum temperature for 
indirect germination. 
A list of references is attached. — A . B. 
Potato Leaf-roll Disease : Nature, Mode of Dissemination, and Control of 
Phloem-necrosis (Leaf-roll) and Related Diseases. By Dr. H. M. Quanjer 
(Wageningen, 1916). — The disease known as leaf-roll, in which the leaf, instead 
of being flat as in healthy plants, is rolled or curled in various ways, frequently 
discoloured, and far less efficient as a food-making organ than it ought to be, 
as is evident from the small number of tubers which the plant produces, is very 
common in Potatos in England. Various suggestions have been made with regard 
to the cause of the disease, and three or four forms have been distinguished. 
(See Journal R.H.S. xli. p. 383, 1916.) Dr. Quanjer here deals with what is 
probably the most important of these. 
The characteristics of the disease are these : Young plants do not show signs 
of the trouble, but when about a month old the lower leaves become rigid and 
pale yellowish in colour, while the margins are rolled upwards. Later the upper 
leaves also show symptoms of the disease. The discoloration is confined at 
first to the tips, but later spreads gradually over the whole leaf ; while in some 
varieties a red or violet tinge is evident along the edges of the discoloured part, and 
the under-sides of the curled leaves often have a bluish gloss, while the plants 
rattle as one passes among them. The affected plants are scattered here and 
there in the field, and the disease does not spread from one centre as would be 
the case with ordinary fungus diseases. Later still the leaf tissue dies here and 
there, starting from the tips and margins and causing brownish -black spots to 
appear, which afterwards spread. Diseased plants grow slowly, and unfavourable 
conditions accelerate the development of the symptoms. When the conditions 
are particularly unfavourable the plant remains quite small, and the seed tubers 
will be found unexhausted at the time the crop is lifted. Some plants are attacked 
only after considerable growth has been made, and these may produce a fair crop. 
Some varieties seem to be more susceptible than others . Most of the experiments 
performed by the author have been done with the variety ' Paul Kruger ' (raised 
in Holland in 1896 by crossing ' Imperator ' with ' Wilhelm Korn '). This variety 
