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Figure 2. Romaine lettuce plants in a field<br />
showing variation in severity typical of lettuce<br />
dieback including stunted growth, as well as<br />
yellowing and necrosis of outer leaves.<br />
Since the late 1990s, lettuce dieback has<br />
been believed to be caused by infection of<br />
lettuce plants with either of two viruses<br />
from the genus Tombusvirus; tomato<br />
bushy stunt virus (TBSV) and Moroccan<br />
pepper virus (MPV). These viruses are<br />
absent from healthy lettuce but have<br />
been found regularly in association with<br />
lettuce dieback disease. However, there<br />
have been numerous situations in which<br />
neither virus was found in association<br />
with obvious disease symptoms. Furthermore,<br />
it has not been possible to<br />
consistently and easily reproduce disease<br />
symptoms when lettuce is inoculated<br />
with either virus in a laboratory setting,<br />
raising the possibility that an additional<br />
virus may contribute to causing lettuce<br />
dieback disease.<br />
In an attempt to identify a possible<br />
additional virus contributing to lettuce<br />
dieback disease, high throughput<br />
sequencing (HTS) was used on several<br />
lettuce plants exhibiting dieback symptoms,<br />
which led to the identification of<br />
a new virus consistently associated with<br />
diseased plants but not with healthy<br />
lettuce plants. This novel virus was most<br />
closely related to a recently identified<br />
and poorly characterized virus from watermelon<br />
in China, watermelon crinkle<br />
leaf associated virus, which was found<br />
using the same HTS approach.<br />
The newly identified lettuce virus, tentatively<br />
named lettuce dieback associated<br />
virus (LDaV) shares an extremely low<br />
genetic relationship with the watermelon<br />
virus, which suggests that although the<br />
two viruses are related, they are very<br />
distantly related to one another. Using<br />
a combination of HTS and traditional<br />
DNA sequencing the genome of the<br />
new virus, LDaV, was assembled and<br />
Figure 3. Romaine lettuce leaf from the inner<br />
portion of a head showing star-shaped chlorotic<br />
flecking in veins characteristic of lettuce dieback<br />
disease on romaine.<br />
methods were developed to allow rapid<br />
detection of the virus from lettuce leaf<br />
extracts using RT-PCR, a routine laboratory<br />
diagnostic method. LDaV has now<br />
been found not only in lettuce showing<br />
dieback symptoms collected recently, but<br />
it has also been found in older archived<br />
samples of lettuce nucleic acid collected<br />
from plants showing dieback symptoms<br />
over the past 20 years, including many<br />
that also contained MPV or TBSV. To<br />
date, LDaV has not been found in healthy<br />
lettuce plants. Interestingly, genetic<br />
comparison showed that LDaV isolates<br />
collected from coastal California production<br />
regions are closely related to one<br />
another, and desert isolates from Arizona<br />
and Imperial Valley, California also are<br />
closely related to one another. However,<br />
coastal and desert isolates differ genetically<br />
from one another, suggesting<br />
perhaps some regional adaptation of the<br />
virus to plants grown under the different<br />
climatic conditions.<br />
Further research will clarify the role of<br />
LDaV in lettuce dieback disease and how<br />
it relates to the two tombusviruses, MPV<br />
and TBSV, that have long been linked<br />
to the disease. Studies to date, however,<br />
strongly suggest a role for LDaV in<br />
lettuce dieback disease development, and<br />
research is in progress to clarify the ability<br />
of LDaV to produce lettuce dieback<br />
symptoms when inoculated to lettuce<br />
plants, as well as whether or not the new<br />
virus can infect lettuce plants carrying a<br />
gene for resistance to lettuce dieback.<br />
Comments about this article? We want<br />
to hear from you. Feel free to email us at<br />
article@jcsmarketinginc.com<br />
<strong>July</strong> / <strong>August</strong> <strong>2020</strong> www.progressivecrop.com 31