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Horticultural
Development
Company
Grower summary
PC 289
Sweet pepper: Securing
knowledge on TSWV and a
potyvirus in an infected crop to
increase understanding of a
potential threat to UK growers
Final Report 2008
© 2008 Agriculture and Horticulture Development Board
Disclaimer
Whilst reports issued under the auspices of the HDC are prepared from the best available
information, neither the authors nor the HDC can accept any responsibility for inaccuracy or
liability for loss, damage or injury from the application of any concept or procedure
discussed.
The results and conclusions in this report may be based on an investigation conducted
over one year. Therefore, care must be taken with the interpretation of the results.
Use of pesticides
Only officially approved pesticides may be used in the UK. Approvals are normally granted
only in relation to individual products and for specified uses. It is an offence to use nonapproved products or to use approved products in a manner that does not comply with the
statutory conditions of use, except where the crop or situation is the subject of an offlabel extension of use.
Before using all pesticides check the approval status and conditions of use.
Read the label before use: use pesticides safely.
Further information
If you would like a copy of the full report, please email the HDC office
([email protected]), quoting your HDC number, alternatively contact the HDC at the address
below.
Horticultural Development Company
Tithe Barn
Bradbourne House
East Malling
Kent
ME19 6DZ
Tel: 01732 848 383
Fax: 01732 848 498
© 2008 Agriculture and Horticulture Development Board
The contents of this publication are strictly private to HDC members. No part of this
publication may be copied or reproduced in any form or by any means without prior written
permission of the Horticultural Development Company.
© 2008 Agriculture and Horticulture Development Board
Headline
Tomato spotted wilt virus (TSWV) was confirmed as the cause of leaf yellowing and fruit
spot symptoms in a crop of sweet pepper cv. Fiesta.
Removal of affected plants and
biological control of western flower thrips (WFT) brought the disease under control.
No
potyvirus was detected.
Background and expected deliverables
In January 2008, individual plants in two pepper crops in the UK developed stunted growth
that progressed to necrotic leaf spots, leaf mosaic, wilted heads and unmarketable fruit due
to irregular ripening.
A yellow variety (cv. Fiesta) was affected on one nursery, a
red/green variety on the other.
On one site, plants developed irreversible wilting in the
heads and flower buds turned brown and died.
Plants were removed from the crop as
soon as symptoms appeared; it was feared that the disease might continue to spread
necessitating early crop removal.
Tests at CSL on samples from one crop confirmed the presence of TSWV (this virus is
usually vectored by WFT in the UK) and a potyvirus (usually vectored by aphids), the
latter as yet unidentified.
This crop was heavily infested with WFT from soon after
planting, and aphids (Myzus persicae) occurred in low numbers; full details of the
outbreak on the second affected crop were not disclosed.
TSWV severely affected some
pepper crops in the Netherlands in recent years, following loss of control of WFT; this
resulted in crop pull-out by mid-summer.
The symptoms in the Netherlands are reported
to include severe leaf necrosis which has not been seen in the current UK problem. Part
of the problem in the affected UK and Dutch crops appears to be that insecticides used to
control leafhopper have inadvertently disrupted biological control of WFT in the previous
crop.
As a consequence, heavy infestation of WFT built up in the old crop and then
carried over to the new crop soon after planting.
© 2008 Agriculture and Horticulture Development Board
TSWV has rarely been seen affecting pepper crops in the UK until this year. There were
a few outbreaks in the early 1990s, shortly after WFT first occurred in the UK, and none
in recent years. The source of the TSWV in the current outbreaks is unclear; both
immigrant WFT and the young plants have been suggested; the plants for both of the
affected crops in the UK originated from propagators in Holland.
carrying TSWV from other infected crops.
Immigrant WFT might be
Or TSWV-infected young pepper plants could
have provided a source of virus for resident WFT larvae to pick up and transmit to healthy
pepper plants during feeding as adult thrips.
Severe wilting and death of flower buds is not a usual symptom of TSWV in pepper
although flower bud necrosis has been recorded in some ornamental species affected by
TSWV.
Possibly there may be an interaction between TSWV and the potyvirus which has
resulted in more severe symptoms than caused by TSWV alone.
Work to identify the
potyvirus is in progress at CSL, funded directly by one of the growers.
Until an effective
method of leafhopper control compatible with biological control of WFT is developed, there
is a significant risk of an increasing number of TSWV problems in UK pepper crops. The
commercial objective of this project is to gain knowledge of TSWV and a potyvirus in
pepper to aid the subsequent development of practical control measures.
The specific objectives are:
1. To determine the association of TSWV and a potyvirus infection with symptom
occurrence in pepper cv. Fiesta;
2. To determine the proportion of WFT carrying TSWV;
3. To determine the proportion of WFT carrying TSWV in an adjacent crop of
cucumber.
Summary of the project and main conclusions
© 2008 Agriculture and Horticulture Development Board
Occurrence of TSWV and a potyvirus in plants
A glasshouse of sweet pepper in Essex showing symptoms of virus infection from early
January 2008 was tested for occurrence of TSWV and a potyvirus in symptomatic and
asymptomatic plants.
Virus symptoms were observed in cv. Fiesta and not in cv. Kelly in
the same house.
On 16 April 2008, one fully expanded upper leaf was collected from each of 20 pepper
plants of cv. Fiesta showing one or more of the following symptoms of virus infection:
stunted growth, fruit spots and blotches, leaf yellowing, leaf bubbling, necrotic leaf flecks
and plant wilting (Fig 1).
A leaf from an asymptomatic plant was also collected from 20
plants each of cvs. Fiesta, and from 20 plants of cv. Kelly in the same glasshouse.
© 2008 Agriculture and Horticulture Development Board
Figure 1: Fruit spotting and irregular ripening, and leaf yellowing in the plant head, in
sweet pepper cv. Fiesta infected with TSWV.
TSWV was confirmed by an ELISA (Enzyme Linked Immuno-Sorbent Assay) test in seven
out of the 20 leaves collected from symptomatic plants.
All of the plants that were
confirmed as infected by TSWV showed yellowing symptoms in the head and most showed
one or more other symptom.
There was no single symptom, or set of symptoms, that
distinguished plants that tested positive for TSWV from suspect-infected plants that tested
negative for TSWV.
No TSWV was detected in any of the leaves collected from asymptomatic plants of cvs.
Fiesta or Kelly.
© 2008 Agriculture and Horticulture Development Board
All 60 leaves (20 symptomatic and 40 asymptomatic) were tested for viruses in the
potato virus Y group (potyviruses) using an ELISA test.
No potyvirus was detected.
The original samples from both nurseries had tested positive for a potyvirus by an ELISA
test. Particles of a probable potyvirus were also seen on electron microscope examination
in both samples. The samples deteriorated quickly and it was not possible to confirm the
original ELISA test result or to identify the virus by PCR and DNA sequencing.
Re-
samples were requested and received, but these were only positive for TSWV.
Distribution of TSWV in plants
The failure to detect TSWV in 13 of the 20 plants showing virus symptoms may have
been due to variation in concentration of virus in different parts of infected plants.
Previous work has shown that concentration of TSWV in different parts of the same plant
can vary greatly and a negative ELISA test result in one part of a plant does not
necessarily mean that the plant is uninfected by TSWV.
Eight leaves from virus-symptomatic plants that tested negative for TSWV by ELISA were
re-tested for the virus by nucleic acid test (TaqMan PCR).
Seven of these samples
were also found to be negative for TSWV by the TaqMan PCR test while one was
positive.
It is considered that the sensitivity of the ELISA test for TSWV is satisfactory
and that the different result on the one leaf was probably due to distribution of virus within
the leaf.
An examination was done of the distribution of TSWV in two pepper plants from the
affected crop of cv. Fiesta taken on 2 May 2008.
One plant had no symptoms and the
other had yellowing leaves in the plant head and virus ring-patterns on fruit.
leaves from the asymptomatic plant tested negative for TSWV.
testing.
All of the
No fruit were available for
In the symptomatic plant, the yellowing leaves in the plant head and the fruit with
ring-patterns tested positive for TSWV by an ELISA test.
Four out of eight green leaves
sampled from lower down the plant, and one fruit, all without virus symptoms, also tested
© 2008 Agriculture and Horticulture Development Board
positive for TSWV.
It was concluded that TSWV is not equally distributed in plants.
The
best chance of detecting TSWV in an infected plant is to test upper symptomatic leaves or
symptomatic fruit.
Occurrence of TSWV in WFT
By 16 April (week 16), the infestation of WFT that had been present in the crop since
November 2007 was largely under control.
Only three WFT adults were collected when
numerous flowers were examined on 16 April.
WFT were therefore collected from sticky
traps in order to test them for occurrence of TSWV.
WFT adults were removed from 10
sticky traps from throughout the house, and also from two sticky traps in an adjacent crop
of cucumber, and bulked for testing.
Four bulked samples of five WFT were removed
from each trap, except trap 2 which did not contain sufficient WFT. TSWV was not
detected in any of 185 WFT tested from the pepper crop by TaqMan PCR.
An internal
control confirmed that RNA was successfully extracted from the WFT, indicating the test
was valid.
The affected pepper crop, cv. Fiesta, was examined again on 17 July.
were found in flowers or on sticky traps at this time.
No WFT adults
Four plants with fruit symptoms of
TSWV were found; leaves from the plant head were taken and tested for TSWV and
potyvirus by ELISA.
All four samples (one symptomatic leaf; three asymptomatic leaves)
tested positive for TSWV and negative for potyvirus.
The crop was examined once more, on 15 October.
traps.
Seven thrips were found on sticky
All were found to be negative for TSWV by the TaqMan PCR test.
Development of TSWV symptoms in occasional plants after April, when all WFT tested were
found to be negative for the virus, could have been due to: 1) slow development of the
virus and symptoms within individual plants; 2) occurrence of TSWV in a very small
proportion of WFT in the crop, which was not detected in the limited number of WFT
found.
© 2008 Agriculture and Horticulture Development Board
Crop losses to TSWV
The crop was planted on 29 November 2007. The disease was first observed around 4
weeks later, week 1 2008. The number of symptomatic plants removed each week was
around 30 in weeks 1 and 2 and increased to around 50 during weeks 3-11, resulting in
a total of 500 plants removed by week 12.
Occurrence of virus-affected plants then
decreased to around 30 per week from weeks 12-15 and to less than 10 per week from
week 16, at which time biological control had brought the WFT infestation under control.
No further virus-affected plants were observed after week 28. At the end of the season,
3.8% of cv. Fiesta plants in the house had been removed due to infection by TSWV.
The grower achieved biological control of WFT in the crop through use of Orius laevigatus
and Amblysieus cucumeris commencing in week 4.
At the height of the problem in week
12, each flower examined by the grower was found to contain four or five WFT insects.
By week 16 only occasional WFT adults were found in flowers.
© 2008 Agriculture and Horticulture Development Board
Conclusions
It should be noted that the conclusions below are based on a small study of one crop.
Further work on other outbreaks would be required to determine their general applicability.
1.
TSWV was confirmed in pepper plants cv. Fiesta showing yellowing of young leaves
in the plant head and ring-patterns on fruit.
2.
Distribution of TSWV in pepper plants appeared to be uneven.
On plants that
tested positive for TSWV, some asymptomatic leaves lower down in the plant tested
positive and other tested negative.
Symptomatic fruit and yellowing leaves in the
plant head appear to be the best samples to take to test for TSWV in pepper.
3.
No evidence was found to support the hypothesis that a potyvirus was involved as
a cause of virus symptoms observed in a crop of cv. Fiesta in Essex in 2008.
4.
No TSWV was detected in adult WFT collected from the TSWV-infected pepper
crop in April and October.
As WFT are the principal vector of TSWV in protected
crops in the UK, this probably explains why the incidence of plants developing
symptoms of TSWV declined as the pest outbreak was brought under control.
5.
At the end of October, the cumulative incidence of plants removed due to TSWV
symptoms was around 3.8% of the crop.
Action points for growers

Be aware that insecticidal control of leaf hopper can disrupt biological control of WFT,
thus increasing risk of TSWV transmission and infection.
© 2008 Agriculture and Horticulture Development Board

If WFT occurs in sweet pepper, examine plants carefully for symptoms of TSWV (Fig.
1).

Prompt removal of TSWV-affected plants from a glasshouse, combined with effective
control of WFT through biological control, are the two principal actions required in order
to bring an outbreak of TSWV under control.
© 2008 Agriculture and Horticulture Development Board