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Net necrosis of potato is the result of infection by potato leaf
roll virus (PLRV). This symptom is caused by the selective death and damage to
cells in the vascular tissues of the tuber. The fact that only specific cells within the
tuber are affected by this problem while others remain normal causes the
characteristic “net” symptom . Infection by the virus may
directly cause the damage to and death of the vascular tissues or the presence of
the virus may make these sensitive tissues more susceptible to damage from
other stresses. There is a strong resemblance between PLRV net necrosis and
another tuber defect known as stem end discoloration (SED). Unlike PLRV, SED
is believed to be a physiological disorder.
The virus itself is an extremely small, nearly spherical particle (its diameter is only
0.000001 inch) which can be spread by several aphid species that colonize potato,
with the green peach aphid being the most efficient. The insect vector is absolutely
essential to spread because mechanical transmission, like that which occurs when
the leaves of an infected plant rub on a healthy one, simply does not occur with
PLRV. The infection process is actually quite complicated with this virus. First the
aphid must acquire the virus by feeding on a PLRV infected plant. Then the virus
must circulate from the gut of the aphid, through the circulatory system until it finally
gets into the salivary glands, from which it can be excreted when the aphid feeds
on healthy plants. Only after this has happened can the aphid spread the virus.
This sequence of events may require 24 hr. or more to occur. Unfortunately, once
an aphid becomes infected, it remains so for the rest of its life. Spread of the virus
between plants within a field and between fields can be done by the winged forms
of the aphid but most spread within a field, especially from infected plants to
nearby, healthy ones, is accomplished by the wingless forms.
Seed certification programs allow only a very small level of PLRV in certified seed.
In Idaho, for instance, during the second field inspection the allowable amount of
PLRV is only 0.05% for G4 seed, 0.01% for G3 and G2 and none at all allowed in
nuclear and G1. Very small percentages of PLRV in seed potatoes do not
normally pose any risk for the commercial producer. However, even very small
percentages of virus can be a problem if green peach aphids appear very early
and in abundance. In years that are very favorable for insects, like the 1996
season was, the aphid population can become so large that even a very low
percentage of PLRV infected seed could result in sufficient spread to cause a
problem, because as the aphid population increases, so does the probablity that
they will encounter an infected plant. Control of aphids with insecticide application
is the only means of managing this problem in production years that are highly
favorable for aphids.
Seed borne infection generally results in small, stunted, badly impaired plants which
have reduced yield both in tuber numbers and in tuber size. Large tubers that
show the typical net necrosis symptom may well be the result of current season
infection.
This information was prepared for the PGI Magazine, “Seed Hotline,” Friday
October 11, 1996 deadline.
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