July 2022 - 8

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Tips for managing tomato spotted wilt virus
LAURA INGWELL,
ELIZABETH LONG &
DAN EGEL
Purdue University
Tomato spotted wilt virus (TSWV)
is a plant disease that infects more
than 1,000 species of plants, including
ornamentals and vegetables. While
this disease affects many crops in
the southeast U.S., in the Midwest
TSWV is most often observed on
tomatoes in high tunnels. This has to
do with survival of the insect vector
of TSWV. This article will discuss the
biology and management of TSWV
on tomatoes in high tunnels or
greenhouses.
Identifying the pathogen
Visual symptoms of TSWV on
tomatoes vary depending on the
plant that is infected, but general
characteristics include yellow or brown
ringspots on fruit and small, dark
ringspots on foliage that may make the
entire leaf appear bronzed in severe
cases (Fig. 1). In some instances,
TSWV may cause stunting of tomatoes.
The virus is moved (vectored) from
plant to plant by the piercing-sucking
Figure 1. TSWV symptoms on tomato plant (left) and fruit (right).
feeding of tiny insects called thrips.
Managing this disease requires strong
and continued efforts to: Manage the
source of the virus (other plants that
are already infected), and manage
thrips.
The insect that moves TSWV
from plant to plant: Thrips
Thrips feed on plants by puncturing
the outer layer of leaf cells and sucking
up the contents, which results in
silvering, stippling and discolored
flecking of the surface (Fig. 2). Thrips
feeding may also cause leaves and
flowers to be deformed in appearance.
Larval thrips are the only life stage
that can pick up TSWV when they feed
on infected plants, but adult thrips are
the only life stage that can transmit
Figure 2. Silvering and flecking caused by
thrips feeding on tomato leaves.
the virus to new plants. This means
adults can only infect healthy plants if
they fed on an infected plant as a larva.
TSWV is not transmitted vertically
from adult thrips to offspring.
Thrips lay their eggs inside leaves or
flower petals where they are difficult
to reach with insecticides. Once eggs
hatch, the larvae often remain in
protected areas, like inside flower buds
or within leaves at the top of the plant.
After two larval stages (Fig. 3), thrips
move down to the soil or leaf litter
and enter a pupal stage where they are
inactive (Fig. 4). During this phase, the
insects are not managed by insecticides
that are directed at the leaves or
require ingestion. Adult thrips live for
30-45 days and can lay 150-300 eggs.
The development from egg to adult
ranges from 7.5-13 days depending on
temperature. A warmer environment
leads to faster development.
Several species of thrips transmit
TSWV, but the most important ones
are the western flower thrips, tobacco
thrips and onion thrips. Because
thrips are so tiny, they can be carried
by the wind from surrounding areas
or on your clothing as you move from
outside to inside.
Keeping TSWV out of your
greenhouse
Start with healthy, virus-free plants
and inspect all incoming plant material
for TSWV symptoms and thrips
infestation. Vegetatively propagated
ornamentals can be an important
source of TSWV! For example, you
could accidently introduce TSWV
into your greenhouse from propagated
plants or transplant seedlings that are
already infected. Once thrips arrive,
they will spread the virus to other
plants. Do not buy vegetable starts that
are produced in the same greenhouse
as ornamentals. Seed transmission
is not considered important for the
spread of this disease.
Manage weeds, both inside and
around your greenhouse, because they
are an important source of the virus
each year. Many perennial ornamentals
and weeds can be infected by TSWV
and these plants can become a source
of the virus at any given time during
the current or following year.
Once TSWV is confirmed in your
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Figure 4. Diagram of the thrips life cycle.
Photo: vegetablemdonline.ppath.cornell.
edu/factsheets/Virus_SpottedWilt.htm
8 | VegetableGrowersNews.com
Figure 3. The two nymphal stages of
thrips: smaller, first instar (on right) and
larger, second instar (on left). Adult thrips
are less than 1/10 of an inch in length.
Photo: John Obermeyer, Purdue University.
greenhouse, remove and destroy all
infected plants because they cannot be
cured and will serve as a source of new
infections.
What to do RIGHT NOW
Sanitation! Remove and destroy all
plant material from this year's crop
promptly, as soon as you are done
harvesting. This is especially important
if you have confirmed TSWV and
thrips in your crop. Physically
remove all old crops from controlled
environments and destroy them away
from the controlled environment by
plowing, disking, etc. Whether in a
field or controlled environment, allow
the ground to lay fallow for 2-3 weeks
if possible, to break the life cycle of any
remaining thrips.
Remove all weeds in and around
the space (Fig. 5). Almost all of the
weed species that are commonly
encountered in the Midwest can host
these pests, and many of these weeds
have been reported to host the virus as
well. The weed hosts don't necessarily
show the same symptoms as infected
tomatoes, so there is no way to
visually inspect the plants to know
if they are carrying the virus. Any
weeds that overwinter in high tunnels
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July 2022

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https://www.nxtbook.com/greatamericanmediaservices/VGN/seedguide-2021
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