| Abstract |
- Management factors used for studies on potato scab
control were evaluated for their effect on tuber russeting
and related biochemical and anatomical parameters
during 1971 and 1972.
Russet Burbank potatoes were irrigated in 1971 to
maintain high available soil moisture (between 90% and
field capacity) for 0, 3, 6 and 9 weeks, beginning one
week after plant emergence. Soil moisture was maintained
between 60% and field capacity during the time it was not
held above 90%. In 1972, plots for four soil-moisture
treatments were irrigated to maintain minimum levels of
90%, 75%, 60% and 45%, respectively, throughout the
season beginning one week after emergence.
Subplot treatments in 1971 were soil compaction,
pentachloronitrobenzene (PCNB) at 28 kg/ha, Super-X
(mixture of PCNB and 5-ethoxy-3 trichlornmethyl 1, 2, 4
thiadiazole) at 28 kg/ha, sulfur at 896 kg/ha, and Nserve
[2 chloro, 6 (dichloromethyl) pyridinel at 1.7
kg/ha, and an untreated check. In 1972, subplots were
PCNB at 28, 22 and 17 kg/ha; sulfur at 896, 672 and 448
kg/ha, and an untreated check.
Reduction of tuber russeting (net) resulted from increased
length of high-moisture period in 1971, and increased
available soil-moisture percentage in 1972. Soil
compaction at the high-moisture level reduced net in 1971.
PCNB, Super-X and N-serve did not change net significantly,
but sulfur increased net. In 1972, PCNB reduced net, and
sulfur with low soil moisture reduced net.
Net was found to consist of collapsed, lignified,
unsuberized cork cells above columnar layers of slightly
flattened, suberized cork cells. Together these form the
periderm of Russet Burbank tubers. Periderm areas with
net had more layers of suberized cork cells below the net
than did areas without net. The number of cork-cell
layers increased with visual estimates of the amount of
net.
Histological stains showed that lignin content was
highest in net tissue, lower in suberized cork, and absent
beneath the cork. Suberin was present in cork cells below
the net and absent in other tissue. Pectins were
evident beneath the cork, but were found only in small
amounts in cork tissue. Histochemical stain tests for
cellulose were negative in walls of suberized cork, presumably
due to a suberin covering. Histochemical evidence thus indicates that lignin is responsible for intercellular
adhesion in net tissue.
Thin-layer chromatography of tuber periderm extracts
showed a predominance of chlorogenic acid, caffeic acid
and coumarin. Other unidentified phenols were found in
smaller amounts. The concentration of phenolic acids was
reduced by high soil-moisture treatments and PCNB.
It is suggested that reduced tuber net was due to
reduced lignin in net cells. Reduced net from high soil-moisture
treatments and from compaction was probably due
to inhibition of lignin synthesis in the net as a result
of low soil oxygen. Water extraction of phenolic precursors
from periderm may be a contributing factor. Reduction
of net by PCNB is apparently due to inhibition of
lignin synthesis.
On the basis of these results, soil moisture, sulfur
and PCNB may generally be used in pathogen control programs
without seriously reducing net if excesses are
avoided.
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