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Internal quality of harvested potatoes is a key issue in
determining the market value of the crop. Therefore, it is important that we have a
good understanding of some of the current problems being encountered in the
harvested crop.
A problem that is being widely reported during this current harvest season is net
necrosis caused by potato leaf roll virus. This disorder can be easily confused with
stem end discoloration. Leaf roll necrosis can be carried into production via seed
and more importantly spread from plant to plant by aphids, particularly green peach
aphid. Generally, when the green peach aphid population remains high there is an
increased incidence of leaf roll necrosis in potatoes. Although the exact population
counts of aphids, month by month, is not available yet, there is a perception that
several production areas had a higher than normal population of aphids. This may
explain why leaf roll net necrosis is more prevalent this year.
One question frequently asked is what will necrosis or stem end discoloration do to
stored potatoes? Both leaf roll and stem end discoloration are known to increase
during storage. Leaf roll necrosis can increase substantially, affecting the
marketable quality of the potatoes. Neither of these disorders can cause decay or
degradation of tubers in storage and very little is understood about storage
condition that may enhance or stop these problems. It may be in the best interest
to have a tuber sample evaluated for net necrosis, and if detected, early marketing
of the potatoes must be considered.
Leaf roll net necrosis usually penetrates deeper into the tuber and its brown
necrotic strands involve the phloem, either inside or outside of the xylem. Xylem
and phloem are conducting tissues of higher plants. Leaf roll necrosis can be
confused with stem end discoloration. Stem end discoloration is a very shallow
discoloration centered around the place where the stolon attaches to the tuber. It
is occasionally found before harvest, but more often it is detected at harvest or
soon after that. It is known to increase in length from the affected area into the
tuber during storage. But in large measure, the discoloration will reach about an
inch from the stem end of the tuber. Usually the brown discoloration is restricted to
the xylem of the vascular ring. What causes stem end discoloration? The facts
are not clear yet, however it is perceived that rapid vine killing of immature vines
may cause the problem. University of Idaho research has shown that vine killing
alone can not cause stem end discoloration but certain production factors before
vine killing may predispose tubers to this problem. These factors are being
examined currently.
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