PCC July-August 2020

Lettuce Dieback: New Virus
Found to be Associated with
Soilborne Disease in Lettuce
By WILLIAM M. WINTERMANTEL | USDA-ARS, Salinas
Lettuce dieback is a soilborne virus
disease known to cause significant
losses for lettuce production throughout
all western growing regions. The
disease was originally described in the
Salinas Valley in the late 1990s following
severe flooding along the Salinas River
but has now been found throughout
coastal and inland lettuce production
regions of California, the winter production
region in southwestern Arizona and
Imperial Valley, California.
The disease is most prevalent on romaine
lettuce but is known to occur on all
non-crisphead (iceberg) lettuce types.
Most modern crisphead lettuces are
resistant, and an increasing number of
romaine cultivars now carry resistance
as well. Symptoms of lettuce dieback
include yellowing and necrosis of outer
leaves, stunted growth and death of
affected plants (Figure 1). Plants infected
young may fail to develop beyond the 8
to 10 leaf stage, but symptoms can develop
at any point in the growing season,
and fields often exhibit a range of plant
sizes with some plants appearing healthy
and maturing normally, while others
became stunted and never fully develop
(Figure 2, see page 31).
Initial symptoms begin with yellowing
and necrosis (death) of small veins in
outer leaves, with the necrosis expanding
into larger areas within and between
veins. Inner leaves of the head usually
retain their color, but some romaine
varieties may also exhibit bright chlorotic
flecks within veins of leaves at the center
of the head that resembles tiny stars.
These are most visible when affected
leaves are held up to a light source (Figure
3, see page 31).
This vein-flecking symptom is not always
present on infected romaine, but when
observed it is an excellent diagnostic
indicator. The vein flecking symptom is
less common on other types of lettuce
and is more difficult to observe on red
lettuce. Losses resulting from lettuce
dieback can range from a few plants to
complete loss of crop. In most severely
affected fields lettuce heads are not
harvested because the plants will not
meet quality standards. Symptoms of the
disease are frequently found in low lying
areas with poor drainage, in areas near
rivers, on recently flooded land, and in
areas where soil has been dredged from
a river or ditch and spread onto adjacent
fields.
Figure 1. Romaine lettuce plant near maturity showing
classic symptoms of outer leaf yellowing and necrosis.
Symptoms may develop at any growth stage (all photos
courtesy W.M. Wintermantel.)
Symptoms of lettuce dieback can be
mistaken for those of other diseases, particularly
lettuce drop, a disease caused by
a fungus, and symptoms of two viruses
transmitted by thrips. It is fairly easy to
differentiate lettuce drop from lettuce
dieback because lettuce drop, caused by
fungi in the genus Sclerotinia, results in a
soft rot, outer leaves often flatten against
the ground, and heads easily separate
from the root, whereas with lettuce
dieback the root remains firmly attached
to the head. The two thrips-transmitted
viruses, impatiens necrotic spot virus
(INSV) and tomato spotted wilt virus
(TSWV), also cause necrotic (dead)
patches on leaves of infected lettuce
plants that resemble symptoms of lettuce
dieback, and therefore it can be difficult
to differentiate the two diseases. Diagnostic
tests can be used to differentiate
lettuce plants infected with these viruses
from those with lettuce dieback disease.
Serological detection methods including
commercially available immunostrips
that can be used in the field to determine
infection with INSV or TSWV, but
immunostrips are not available for the
viruses associated with lettuce dieback
disease. Therefore, confirmation of lettuce
dieback requires laboratory testing,
which can include both molecular biology
and serological methods. In some
cases, lettuce plants may be infected by
multiple pathogens simultaneously and
this may complicate diagnosis.
Lettuce dieback is probably a very old
disease of crisphead (iceberg) lettuce
that disappeared for many years before
reemerging with a new name as a disease
of other lettuce types. In the 1930s a
disease known as brown blight devastated
lettuce production in California with
symptoms that closely resembled those
of lettuce dieback based on descriptions
and illustrations at the time.
Iceberg lettuce was the main type of lettuce
grown in the 1930s, and it suffered
severe losses from brown blight for many
years until a source of resistance was
identified by a USDA scientist, Ivan Jagger.
This source of resistance was eventually
bred into all subsequent iceberg
lettuce types, beginning with the variety
Imperial, and this eliminated the threat
from brown blight. In the early 2000s,
after the appearance of lettuce dieback,
USDA scientists identified a source of
resistance to lettuce dieback from the
crisphead lettuce variety Salinas, and
through genetic studies found that the
source of resistance to lettuce dieback is
also present in the brown blight-resistant
lettuces developed by Jagger over 70
years earlier, but was not in earlier susceptible
lettuce varieties. In other words,
only crisphead lettuce varieties that
predate the variety Imperial could develop
symptoms of lettuce dieback. This
suggests the two diseases may actually
be the same. The resistance to lettuce dieback
has been incorporated into several
romaine lettuce varieties, as well as some
leaf and butter lettuce varieties, but there
remain many lettuces that are susceptible
to lettuce dieback disease.
30 Progressive Crop Consultant July / August 2020