Experiment Station Record, Volume 92, January-June, 1945 Page: 68
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68 EXPERIMENT STATION RECORD [Vol 92
from the field is due chiefly to one condition or set of conditions; this paper considers
primarily the relation of the rot to these conditions. From controlled experiments
with infrared lamps-described in detail-it appeared that there was an
accumulation of heat in the tubers after a prolonged exposure to these rays, and
that the resultant flesh temperature was far above that of the surrounding air; i. e.,
heat injury can be produced in potatoes even when air temperatures are not excessively
high. The symptoms developing resembled very closely those seen many
times in the field on tubers harvested in warm or very hot weather. In potatoes
harvested at weekly intervals over a 5-week period, all exposed so that tissue temperatures
reached 135-140 F. showed the typical symptoms of heat injury. When
tubers with skinned areas were exposed to the infrared rays the air and tissue
temperatures at the skinned places were lower than those where the skin was intact,
probably because some of the heat was absorbed by evaporation from the freshly
exposed surface. When evaporation continued for 2-3 hr. or more, it caused a
marked pitting or sinking of the flesh. Further experiments indicated that the
heat injury and the bacterial soft rot are two different phenomena and that the rot
follows the injury only when the bacteria are present and conditions favor their
development. It is evident that whether or not the bacterial soft rot develops on
tubers in transit is largely a matter of transit temperatures, provided that prior
to shipment the tubers have been subjected to the conditions predisposing them to
the rot. Under field conditions, the soft rot bacteria are almost certain to be
present. It is believed that much of the commercial loss encountered has been due
to unavoidable exposure of the tubers to heat of the sun when help could not be
obtained to get them picked up and hauled from the field promptly after they were
dug. From further tests it seems evident that the reason why skinned potatoes
blown for 12 hr. in moist air do not shrivel when later held in dry air is that they
have begun to suberize; it is not clear, however, why in either still or moving moist
air they brown very little if at all, whereas in dry air they develop browning within
a half hour. The controlled tests' and field observations show that the need of
picking up potatoes promptly after digging applies equally well in preventing both
heat injury and skin browning.
Observations on phloem necrosis of potato tubers, G. B. SANFORD and J. G.
GRIMBLE (Canad. Jour. Res., 22 (1944), No. 4, Sect. C, pp. 162-170, ills. 1) .-In
field tests of random lots of tubers showing phloem necrosis of similar type, the
vines from five developed typical leaf roll symptoms, but those from three others
exhibited no sign of this or other symptoms suggesting a virus etiology. The net
necrosis in one of the latter samples was known to have resulted from vine infestation
by the potato psyllid; in this case the stock returned to full vigor during the
second season. It was further confirmed that once the vines show definite symptoms
of leaf roll, the phloem elements of their tubers henceforth do not exhibit necrosis;
tuber net necrosis as associated with the leaf roll virus is thus considered a transitory
symptom of primary infection. Tissues contiguous to the necrotic phloem
elements in affected tubers were uniformly fluorescent, but the unaffected portion
of such tubers as well as the nonnecrotic tubers from positive leaf roll plants reacted
negatively to ultraviolet light.
Variation and physiologic specialization in the common scab fungus (Actinomyces
scabies), L. A. SCHAAL. (U. S. D. A. et al.). (Jour. Agr. Res. [U. S.1,
69 (1944), No. 5, pp. 169-186, illus. 11).-Single-cell cultures of A. scabies obtained
from potatoes grown on various soil types in different States differed in color
of mycelium and pigment produced on modified potato-dextrose agar. The isolates
were unstable and produced variants often differing culturally from both the parents
and each other; one variant did not sector during a 3-mo. period. No isolate pro
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U.S. Department of Agriculture. Agricultural Research Administration. Office of Experiment Stations. Experiment Station Record, Volume 92, January-June, 1945, book, 1947; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc5064/m1/81/: accessed April 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.