596 
Journal of Agricultural Research 
Vol. XIV, No. 13 
frames, contained more whitespot lesions than other parts of the beds. 
In a single bed left entirely without shade, most of the seedlings died 
from whitespot. These observations indicated insolation as at least a 
contributory cause of whitespot injury. The nursery practice involved 
as little watering as possible during the damping-off period. In plots 
given somewhat more frequent watering than the general beds, subse¬ 
quent counts showed only three-fifths as much whitespot injury as else¬ 
where. During the three succeeding years, the nurserymen gave the 
seed beds much more frequent watering and more careful shading than 
in 1909. In careful examinations during these three years only occa¬ 
sional cases of whitespot were found. 
The foregoing data all pointed to a physical rather than a parasitic 
cause for whitespot injury. Heat and light were the physical factors 
toward which suspicion was directed. The temperature in the surface 
layer of soil in the seed beds, even under the half-shade of the lath frame, 
was found to go as high as 52 0 C. The^pparent preventive effect of 
frequent watering was believed to be due to the lowering of the soil 
temperature. 
Fig. i.—L esions on seedlings of Pinus ponderosa: Seedlings A and D were injured by the sun’s rays con¬ 
densed by a lens. B was injured by a hot wire, C by an incandescent lamp, and E by the direct sun. The 
leaves remained turgid for 5 to 11 days after the lesions were produced. The horizontal line indicates the 
location of the soil surface at the time the seedlings were subjected to heat. Natural size. 
In tests conducted at Washington, D. C., with seedlings of Pinus pon¬ 
derosa typical whitespot lesions have been artificially produced. Five 
representative experiments are described in the following paragraphs and 
are illustrated in figure r. 
Seedling A.—The stem was subjected for less than two minutes to sunlight passed 
through a condensing lens at the point indicated by the constriction. The stem imme¬ 
diately collapsed and bent over at the point of the lesion. The soil around the root 
at the time was dry, and the seedling was distinctly wilted before it was heated. Later 
the pot was watered, and the seedling restored to a nearly vertical condition by prop¬ 
ping. Turgor returned and was maintained for a week, but was followed by the 
decay of the root and wilting of the plant without extension of the original lesion 
(fig. x, A). 
