Download Spring Issue - National Plant Diagnostic Network

Western Plant Diagnostic Network
First Detector News
1
A Quarterly Pest Update for WPDN First Detectors
Spring 2015 edition, volume 8, number 2
In this Issue
Page 1: Editor’s Note
Dear First Detectors,
Pages 2 – 3: Intro to Plant
Viruses
Page 4: Virus nomenclature
Plant viruses cause many important plant diseases and are
responsible for huge losses in crop production and quality in
all parts of the world. Plant viruses can spread very quickly
because many are vectored by insects such as aphids and
whitefly. They are a major pest of crop production as well as
major pests of home gardens. By mid-summer many fields,
vineyards, orchards, and gardens will see the effects of plant
viruses. The focus of this edition is the origin, discovery,
taxonomy, vectors, and the effects of virus infection in
plants. There is also a feature article on grapevine viruses.
And, as usual, there are some pest updates from the West.
Page 5 – Most Serious World
Plant Viruses & Symptoms
Pages 6 – 7: Plant Virus
Vectors
Pages 7 - 10: Grapevine
Viruses
Page 10: Pest Alerts
Contact us at the WPDN Regional
Center at UC Davis:
Phone: 530 754 2255
Email: [email protected]
Web: https://wpdn.org
Editor: Richard W. Hoenisch
@Copyright Regents of the
University of California
All Rights Reserved
On June 16 – 18, the WPDN is sponsoring the second
Invasive Snail and Slug workshop at UC Davis. The workshop
will be recorded and will be posted on the WPDN and NPDN
home pages. Have a great summer and here’s hoping for
rain!
Please find the NPDN family of newsletters at:
Western Plant Diagnostic Network News
Newsletters
Grapevine Fanleaf Virus
Peanut leaf with
tomato spotted wilt virus
Manitoba Ag,Food, and Rural Initiatives
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Photo courtesy of APS
Photo by Giovanni Martelli, U of Bari
Plant Viruses
Squash Mosaic Virus
Viruses are infectious pathogens that are too small to be seen with a light microscope, but despite their small size
they can cause chaos. The simplest viruses are composed of a small piece of nucleic acid surrounded by a protein
coat. As is the case with other organisms, viruses carry genetic information in their nucleic acid which typically
specifies three or more proteins. All viruses are obligate parasites that depend on the cellular machinery of their
hosts to reproduce. Viruses are not active outside of their hosts, and this has led some people to suggest that
they are not alive. All types of living organisms including animals, plants, fungi, and bacteria are hosts for viruses,
but most viruses infect only one type of host. Viruses cause many important plant diseases and are responsible
for losses in crop yield and quality in all parts of the world.
Image courtesy of APS
Photo courtesy of APS
Electron micrograph of the
actual TMV virus
TMV in action in tobacco
Most viruses are restricted to a particular type of host. Some infect bacteria, and are known as bacteriophages,
whereas others are known that infect algae, protozoa, fungi (mycoviruses), invertebrates, vertebrates or vascular
plants. However, some viruses that are transmitted between vertebrate or plant hosts by feeding insects
(vectors) can replicate within both their host and their vector. This web site is mostly concerned with those
viruses that infect plants but we also provide some taxonomic and genome information about viruses of fungi,
protozoa, vertebrates and invertebrates where these are related to plant viruses. Viruses cause many diseases of
international importance. Amongst the human viruses, smallpox, polio, influenza, hepatitis, human
immunodeficiency virus (HIV-AIDS), measles and the SARS coronavirus are particularly well known. While
antibiotics can be very effective against diseases caused by bacteria, these treatments are ineffective against
viruses and most control measures rely on vaccines (antibodies raised against some component of the virus) or
relief of the symptoms to encourage the body's own defense system.
Western Plant Diagnostic Network News
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Viruses also cause many important plant diseases and are responsible for huge losses in crop production
and quality in all parts of the world. Infected plants may show a range of symptoms depending on the disease
but often there is leaf yellowing (either of the whole leaf or in a pattern of stripes or blotches), leaf distortion
(e.g. curling) and/or other growth distortions (e.g. stunting of the whole plant, abnormalities in flower or fruit
formation).
Some important animal and human viruses can be spread through aerosols. The viruses have the "machinery"
to enter the animal cells directly by fusing with the cell membrane (e.g. in the nasal lining or gut). By contrast,
plant cells have a robust cell wall and viruses cannot penetrate them unaided. Most plant viruses are
therefore transmitted by a vector organism that feeds on the plant or (in some diseases) are introduced
through wounds made, for example, during cultural operations (e.g. pruning). A small number of viruses can
be transmitted through pollen to the seed (e.g. barley stripe mosaic virus, genus Hordeivirus) while many that
cause systemic infections accumulate in vegetatively-propagated crops.
Tobacco Mosaic Virus: A Virus with a History
1.
1.
Adolf Mayer
Martinus Beijerinck
Dmitri Ivanovsky
The discovery of plant viruses causing disease is often accredited to Adolf Mayer (1886) working in the
Netherlands demonstrated that the sap of mosaic obtained from tobacco leaves developed mosaic symptom
when injected in healthy plants. However the infection of the sap was destroyed when it was boiled. He
thought that the causal agent was the bacteria. However, after larger inoculation with a large number of
bacteria, he failed to develop a mosaic symptom. In 1898, Martinus Beijerinck , who was a Professor of
Microbiology at the Technical University the Netherlands, and at the same time Dmitri Ivanovsky in the
Crimea put forth their concepts that viruses were small and determined that the "mosaic disease" remained
infectious when passed through a Chamberland filter . This was in contrast to bacteria microorganisms, which
were retained by the filter. Beijerinck referred to the infectious filtrate as a “Contagium vivum fluidum ", thus
the coinage of the modern term "virus". Plant pathologists are always very proud that the discovery of
tobacco mosaic virus was the first virus in all biology to be so identified. The investigations of tobacco mosaic
disease and subsequent discovery of its viral nature were instrumental in the establishment of the general
concepts of virology. TMV was the first virus to be crystalized in 1935 by Wendell Meredith Stanley at UC
Berkeley, for which he won the Nobel Prize. Stanley found that TMV remained infectious even after
crystallization!
Wendell Meredith Stanley
Western Plant Diagnostic Network News
Virus Naming and Classification
Binomial nomenclature , with genus and species, is standard in the world of biology – except with common
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virus names. Most plant viruses are named by their hosts and symptoms, such as tobacco mosaic virus,
eggplant yellow mosaic, grapevine fan leaf virus, barley yellow dwarf, and peanut stunt virus. However,
there is a method behind this possible confusion. The common names originated from the first plant the
virus symptoms were noted, such as tobacco mosaic virus. However as the science of virology advanced,
similarities and a tremendous diversity among viruses were noted and they were assigned to orders and
family groups, as in binomial nomenclature. Starting in 1971, International Committee on Taxonomy of
Viruses (ICTV) began to standardize virus taxonomy, thus organizing the confusion brought about by only
common names. See also Virus family groups for all the known plant, animal, and human viruses.
Chart courtesy of the Australian Ministry of Agriculture
Note the different symptoms on the same plant cultivar
Western Plant Diagnostic Network News
The Top World Plant Viruses
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1. Tobacco mosaic virus
2. Tomato spotted wilt virus
3. Tomato yellow leaf curl virus
4. Cucumber mosaic virus
5. Potato virus Y
6. Cauliflower mosaic virus
7. African cassava mosaic virus
8. Plum pox virus
9. Brome mosaic virus
10. Potato virus X
11. Citrus tristeza virus
12. Barley yellow dwarf virus
13. Potato leafroll virus
14. Tomato bushy stunt virus
15. Grapevine leafroll virus complex
16. Grapevine fanleaf virus
17. Rose rosette virus
The common names of viruses came from the plant species in
which a virus was first observed. However, this doesn’t
necessarily mean the virus is limited just to that one particular
plant. Cauliflower mosaic virus (CaMV) (notice the shorthand)
was first found on cauliflower, CaMV infects mostly plants of
the Brassicaceae family, but some CaMV strains (D4 and W260)
are also able to infect Solanaceae species of the genera Datura
and Nicotiana . Barley yellow dwarf is a plant disease caused by
the barley yellow dwarf virus (BYDV), and is the most widely
distributed viral disease of cereals. It affects the economically
important crop species barley, oats, wheat, maize, triticale and
rice. As you click on each virus group, notice the crops each
virus infects and the very modern virology nomenclature used.
Photo courtesy APS
Photo courtesy of FarmingUK
Photo courtesy APS
Potato leafroll virus
Potato Y virus
Left, barley yellow dwarf
virus affected plant; right a
resistant variety
Photos by EP Rybicki
Tomato yellow leaf curl
virus affected tomato
Western Plant Diagnostic Network News
Photos courtesy APS
Photo courtesy APS
African cassava mosaic
virus: Severe leaf distortion
and mosaic and leaf loss in
cassava in western Kenya
Plum pox virus: Leaves and fruit showing
chlorotic and necrotic ring patterns, and
chlorotic blotches. A) Chlorotic ring patterns
in peach fruit; B) Chlorotic blotches in peach
leaves; C) Chlorotic ring patterns in plum
leaves D) Necrotic ring patterns on apricot
fruit.
Plant Virus Vectors
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The three forms of viruses. Viruses are shaped like rods or spheres or have twenty sides (icosahedral).
Diagram courtesy of The Gale Group
A virus remains totally inactive until it attaches itself to and infects a host cell. Once that happens, the virus may
follow one of two paths. First, the virus may insert its genetic material (it is always DNA in this case) into the
DNA of the host cell. The combined host-viral DNA is then carried along in the host cell as it lives and
reproduces, generation after generation. Viruses are at the borderline between living and nonliving matter.
When they infect a host cell, they are able to carry on many life functions, such as metabolism and reproduction.
But outside a host cell, they are as inactive as a grain of sand. Viruses cause disease by infecting a host cell and
taking over its biochemical functions. In order to produce new copies of itself, a virus must use the host cell's
reproductive "machinery." The newly made viruses then leave the host cell, sometimes killing it in the process,
and proceed to infect other cells within the organism. Read more about viruses at Science Clarified and
Descriptions of Plant Viruses.
A plant virus usually needs a vector to get from plant to plant and over evolution has made partners with several
species.
Sap Transmission: viruses can be spread by direct transfer of sap by contact of a wounded plant with a healthy
one. Such contact may occur during agricultural practices, as by damage caused by tools or hands, or naturally,
as by an animal feeding on the plant. A snail, Oxychilus draparnaudi , can spread TMV. Generally TMV, potato
viruses and cucumber mosaic viruses are transmitted via sap.
Insects: plant viruses vectored by insects are in three categories; 1) non-persistent, only short-lived on the stylet
of the insect; 2) semi-persistent involves the virus entering the foregut of the insect and remaining until the
foregut is emptied; and 3) Those viruses that manage to pass through the gut into the haemolymph and then to
the salivary glands are known as persistent. There are two sub-classes of persistent viruses: propagative and
circulative. Propagative viruses are able to replicate in both the plant and the insect (and may have originally
been insect viruses), whereas circulative cannot. One class of viruses, the Rhabdoviridae, has been proposed to
actually be insect viruses that have evolved to replicate in plants. Vectors include aphids, whitefly, plant-and
treehoppers, thrips, and beetles. Plant-feeding mites , an acari arachnid, transmit viruses in the general family
Tritimovirus , causing several virus diseases of cereals, such as wheat streak mosaic virus .
Nematodes: Soil-borne nematodes have been shown to transmit viruses. They acquire and transmit them by
feeding on infected roots. Viruses can be transmitted both non-persistently and persistently, but there is no
evidence of viruses being able to replicate in nematodes. The virions attach to the stylet (feeding organ) or to
the gut when they feed on an infected plant and can then detach during later feeding to infect other plants.
Examples of viruses that can be transmitted by nematodes include tobacco ringspot virus and tobacco rattle
virus. Grapevine fanleaf virus is transmitted by Xiphinema index.
Western Plant Diagnostic Network News
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Seed and pollen borne viruses: Many plants species can be infected through seeds including but not
limited to the families Fabaceae, Solanaceae, Asteraceae, Rosaceae, Cucurbitaceae, and Poaceae. When
viruses are transmitted by seeds, the seed is infected in the generative cells and the virus is maintained in
the germ cells and sometimes, but less often, in the seed coat. Lettuce mosaic virus is one major example
of a seed-borne virus, controlled by a certified seed program and the removal of alternate weedy hosts in
the area. Blackline disease in walnut is a pollen-borne cherry leafroll virus (CLRV). Because it is pollenborne, it spread very quickly in California after it was first noticed in the 1980s. It continues to spread.
Vegetative propagation: If the mother plant is infected with virus, then any material taken from that
plant is also infected, because virus persists in the plant. This type of transmission is very common with
commercial cultivars of potato, sweet potato, strawberry, wine and table grapes, roses, fruit and nut
trees, etc. The commercial nursery industry in heavily regulated for this reason. The National Clean Plant
Network (NCPN) and Foundation Plant Services (FPS) at UC Davis provide methods for elimination of virus
from cultivars and maintaining virus-free propagation material.
Grapevine Virus Problems in California grapes
By Maher Al Rwahnih
University of California Davis/Foundation Plant Services
Email: [email protected]
Times are changing in the Napa Valley. The picturesque vineyards still grow their renowned varietal grapes, and the
world-class wineries there producing their famous vintages. But growers are now challenged with a leafroll virus
epidemic that has been slowly spreading across the valley for the last hundred years. The virus is prevalent in older
established vineyards, but the infections there can be asymptomatic. However, new vines on more modern
rootstocks, which when planted were free of the virus, are now becoming diseased.
In the past tending the vines was simpler. The growers could grow their own replacement stock by grafting cuttings
of their choice cultivars, and use that material to renew their vineyards and maximize productivity. But latent
leafroll Grapevine leafroll-associated virus 3 (GLRaV-3) infection can become symptomatic and damaging if grafted
onto some of the modern rootstocks. Healthy-looking, home-grafted propagation stock may turn up infected after
the plantings have become established.
Western Plant Diagnostic Network News
Our author, Dr. Maher
Al Rwahnih, is a
project scientist at
Foundation Plant
Services, UC Davis
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Foliage of red grape varieties turn red between the veins in the fall (white grape varieties turn chlorotic) and the edges
of the leaves roll under. Those vines prematurely loose vigor and suffer a reduction in yield, irregular ripening, and
lower berry sugar content. Furthermore, an infected plant among the rows is a source of disease proliferation.
Mealybug larval instars crawl or blow from one vine to the next carrying the virus. The focal point of the disease
expands, spreading to ruin the productivity of the vineyard.
Photo by Maher Al
Rwahnih
Image from Ontario Grape IPM
Grapevine
leafroll virus
infected
Chardonnay
vine
Grapevine
leafroll virus
infected vine in
red variety
Leafroll disease is caused by old-world viruses that came to the U.S. in imported propagative plant material of the
classic European varietal selections – material that showed no symptoms when it was planted. When the infected
propagation material was transplanted to California vineyards, the insect vectors of the disease picked up the virus and
carried it into the hills overlooking the valleys. There it established further asymptomatic presence in the native wild
Vitis californica grapevines. Now the disease is endemic across the area
There are five main recognized GLRaV species. GLRaV-1, GLRaV-3, and GLRaV-4, are transmitted by mealybugs
and scale insects in California. There is no known vector for GLRaV-2, so despite its adverse effect on grapevine health
and wine quality, is not considered to be as much of a threat because it has not been seen to spread on its own.
GLRaV-7 is a mild form of the virus, the potential of which is still under study. Among these, GLRaV-3 is the major
concern, due to its wider distribution in our grape growing regions.
Grape mealybug in
adult and nymphal
stages
Vine mealybug female and
winged male
Photo by Maher Al Rwahnih
Western Plant Diagnostic Network News
Photo courtesy UCCE
Photo courtesy UCCE
Photo courtesy of UCCE
An adult obscure
mealybug and newly
molted nymph before it
has developed a waxy
coating.
Zinfandel infected with grapevine leafroll
virus
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The Grapevine fanleaf virus epidemic of decades past was addressed by the use of vines carrying genetic resistance to
the disease. But there is no genetic resistance in grape to leafroll virus or to its insect vector. Insects that carry
infections from plant to plant are sometimes controlled by spraying insecticides. But insecticidal control of mealybugs
can be difficult due to the biology of the bug.
Mealybugs are waxy and hard to wet. They retire into cracks in the bark where the spray does not penetrate. And they
have developed resistance to the insecticides. Spraying kills the highly-susceptible parasitic insects that would
otherwise control native mealybug populations. And in a further complication, exotic species such as the vine mealybug
and the obscure mealybug have been introduced to California. They appear to have left their parasites behind, and
have no natural controls in their new range. These are aggressive mealybugs with multiple, overlapping generations, for
which insecticidal treatments may be the only control measures, but then those treatments release the native
mealybugs from their own natural enemies.
The economics of managing the GLRaV-3 epidemic have been studied recently. Kate Binzen Fuller, Julian Ashton, and
Deborah Golino wrote: The Economic Benefits from Virus Screening: A Case Study , analyzed the costs vs. benefits of a
clean planting stock program for GLRaV-3 covering all grape varieties. They modeled the consequences of gradually
replacing all vineyard nursery stock in the entire north coastal region with certified virus-tested material. Costs that
would be added for such a program were found to be in the millions of dollars. But the benefits in productivity of
disease-free vineyards, and the savings of the costs of replacing diseased vines as they appear, out-weighed the
certified nursery stock costs. The calculated net productivity improvement per year for the area was found to be on the
order of fifty million dollars. A similar conclusion was reached by Ricketts et al wrote: Reducing the Economic Impact of
Grapevine Leafroll Disease in California: Identifying Optimal Disease Management Strategies, in a study of the Cabernet
Sauvignon grapevine.
Some of the calculated benefit would arise from the removal of infection sources located at a distance from each
vineyard. The leafroll virus does not respect property lines. The insect vectors that carry infectious virus can blow into
previously uninfected vineyards from sources that may be miles away.
For more information on grapevine leafroll virus management:
Grapevine Leafroll Disease: Management Strategies
Which mealybug is it, why should you care?
Mealybugs in California Vineyards
Vine Mealybug: What You Should Know
Western Plant Diagnostic Network News
Photo courtesy of the Constellation Academy of Wine
Replacement of the infected vines with virus-free plants, in addition to the spraying of insecticides for exotic mealybug
control, reduces the potential spread of GLRaV-3 from inoculum sources within the vineyard, and returns profitability to
the enterprise. Commercial nurseries carry registered planting stock, certified in accordance with the California
Department of Food and the California Grapevine Registration and Certification Program. But the successful
management of this disease will also advance through collaborative involvements among the stakeholders. GLRaV-3
management groups have formed in many parts of the Napa Valley, as well as in other grape-growing regions. Those
groups meet to discuss the successes of their management strategies and share information with other growers who
are dealing with the leafroll disease. Growers who want to adopt a virus management program learn of the successes of
their peers. In this way, they are collectively developing a solution to their collective challenge with this virus epidemic
in California grapes.
Leafroll grapevine virus is found the
world. With infected vines, sugar
production is slowed and fruit maturity
can be delayed for weeks.
Pest Updates
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Arboreal Camel Cricket in Napa County CA
Gammarotettix bilabatus (Orthoptera: Rhaphidophoridae)
Monica Cooper and Lucia Varela UC Cooperative Extension in Napa and the North CA Coast respectively, have
found arboreal camel cricket infesting select hillside vineyards immediately adjacent to oak woodlands in Napa
County CA. Both Drs. Cooper and Varela are quite experience and well-known in the fields of viticulture and
entomology. Grape is not recognized as a major host for this cricket, although vineyards surrounded by suitable
cricket habitat may experience some damage annually. However drought conditions in their arboreal range could
have resulted in more widespread damage. For more on the arboreal camel cricket and the vine, click on:
Napa UCCE Newsletter - Arboreal Camel Cricket
European Chafer found in Washington State
Rhizotrogus majalis (Coleoptera: Scarabaeidae)
"Eurchaferlifecycle" by Art Cushman, USDA
The larva of the European
chafer
Photo by David Cappaert, Michigan State University
Life cycle of
R. majalis
Adult European Chafer
For more photos and means of control, read the
WSU Extension Pest Watch: European Chafer
Western Plant Diagnostic Network News
Photo by Mike Reding & Betsy Anderson, USDA ARS
The European chafer, commonly called June bugs, is a beetle that causes damage to turf and cereal crops.
The damage caused by chafer infestation to residential lawns is exacerbated by the fact that its grubs are an
attractive food source for local fauna such as crows and raccoons, who relentlessly dig up the turf in search
of the morsels. Homeowners often find themselves bewildered by the speed and extent of the destruction
which may ensue. Because it is now confirmed as a problem in southwest British Columbia, Canada, it is
important that Washington State First Detectors, gardeners, and horticultural professionals be aware of this
pest, recognize it various like stages, and know how to report new infestations. The European chafer was
introduced to the US in the 1940s on the East Coast. States currently infested with the pest include NY, MI,
OH, MD, WV, and IN. The life cycle and damage are similar to the Japanese beetle. The name “chafer” is
similar to the German word Käfer (beetle) coming into Middle English as cheaffer, finally chafer.