July 14,1923 
Degeneration Diseases of Irish Potatoes . 
101 
in the cool place (PI. 15, C). When partly grown, the plants in the warm 
place were put near the others in the cool place. Then the difference 
failed to continue in the upper parts of the plants, since the leaves that 
expanded upon the transferred plants were like the corresponding leaves 
of the plants kept in the cool place from the first/ This experiment 
confirms one of those performed by Freiberg (16, p. 195-202), in which 
potato and other plants showed apparent recovery from mosaic under 
certain conditions of temperature. Murphy has described the seasonal 
masking of mosaic symptoms (. 30 , p . 148), and Johnson has demonstrated 
the suppressing effects of temperatures over 20° C. (19). 
Ten whole tubers from a leaf-roll stock were planted in the Orono green¬ 
house during the winter of 1921-22. Three of the 10 plants were grown 
caged, and of these three, two showed no distinct rolling, though all three 
were chlorotic and burned. The uncaged plants were rolled until 
after the roof was whitewashed in April, after which new leaves that grew 
out were only chlorotic and flat. This diminishing of rolling was noted 
also under field cages in 1921. Such an effect from caging or other means 
of shading is not surprising in view of the evidence that leafroll is asso¬ 
ciated with the abnormal accumulation of food, which is caused by the 
more constant internal symptom of phloem necrosis {38, p. 43-45), prob¬ 
ably only in the presence of sufficient sunlight. 
In 1921 tubers from hills diseased in 1920 were split in two and the halves 
of each tuber were planted, respectively, under and outside of insect cages. 
The disease in 1920 was either streak, curly dwarf, or mosaic, and several 
varieties, including Green Mountains and Rurals, were used. The sum¬ 
mer was warm and dry, so that the cheesecloth cover probably caused 
changes in environmental factors as shown by Bums for wire-screen cages 
(9 ). Comparisons were made several times during the season. The hills 
grown outside of cages usually were smaller than the inclosed sister hills. 
The shading in the cages enlarges healthy plants so that this difference in 
diseased plants is not surprising. Such a difference in size is not due to 
different degrees of dwarfing so much as to the same degree of dwarfing 
acting upon plants of different sizes. In a few cases mottling was present 
in the cages while the chlorosis outside was diffused. Symptoms observed 
in common to these environments were wrinkling (in Green Mountains), 
ruffling (in Rurals and Green Mountains), brittleness (in certain seedling 
varieties), burning (in seedlings, Rurals, and Mountains), and premature 
death (in seedlings). This experiment indicates that a reduction in sun¬ 
light may decrease the apparent dwarfing effect of a disease and may 
increase mottling. It also shows that in certain climatic conditions burn¬ 
ing is not prevented by shading or by protection from .insects provided 
certain diseases are present. 
In 1920 parts of the same stock of a strain of the Green Mountain 
variety were planted in two fields in northeastern Maine and in a field on 
Long Island. The yield rate in one field in Maine was almost double that 
in the other and but little more than that on Long Island. The mosaic 
percentage was low. Another part originally of the same strain but all¬ 
mosaic was planted in the poor field in Maine and in the Long Island field. 
Its yield rate was lower on Long Island, even with field conditions twice 
as good for healthy stock. Thus an all-mosaic condition reduced the 
yield rate of Green Mountains much more on Long Island than in north¬ 
eastern Maine. Stewart reports heavy losses upon Long Island (46, p. 
357). Murphy describes the effects of season, locality, and climate on the 
yield-rate reduction by leaf roll (29, p. 40-44) and mosaic ( 29, p. 67). 
