First Detector News. A Quarterly Pest Update for WPDN First Detectors. In this Issue. Spring 2015 edition, volume 8, number 2.

Transcription

1 Western Plant Diagnostic Network 1 First Detector News A Quarterly Pest Update for WPDN First Detectors Spring 2015 edition, volume 8, number 2 In this Issue Page 1: Editor s Note Pages 2 3: Intro to Plant Viruses Page 4: Virus nomenclature 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: rwhoenisch@ucdavis.edu Web: Editor: Richard W. Regents of the University of California All Rights Reserved Dear First Detectors, 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. 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: Newsletters

2 Image courtesy of APS Photo by Giovanni Martelli, U of Bari Grapevine Fanleaf Virus Plant Viruses Peanut leaf with tomato spotted wilt virus Photo courtesy of APS 2 Squash Mosaic Virus Manitoba Ag,Food, and Rural Initiatives 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. 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.

3 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. 3 Tobacco Mosaic Virus: A Virus with a History.. 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

4 Virus Naming and Classification Binomial nomenclature, with genus and species, is standard in the world of biology except with common 4 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

5 Photos by EP Rybicki The Top World Plant Viruses 5 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 Left, barley yellow dwarf virus affected plant; right a resistant variety Potato leafroll virus Photo courtesy of FarmingUK Potato Y virus Photo courtesy APS Photos courtesy APS Photo courtesy APS African cassava mosaic virus: Severe leaf distortion and mosaic and leaf loss in cassava in western Kenya Tomato yellow leaf curl virus affected tomato 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.

6 Plant Virus Vectors 6 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.

7 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. 7 Grapevine Virus Problems in California grapes By Maher Al Rwahnih University of California Davis/Foundation Plant Services malrwahnih@ucdavis.edu 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. Our author, Dr. Maher Al Rwahnih, is a project scientist at Foundation Plant Services, UC Davis

8 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. 8 Grapevine leafroll virus infected Chardonnay vine Image from Ontario Grape IPM Photo by Maher Al 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. 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. Grape mealybug in adult and nymphal stages Vine mealybug female and winged male Zinfandel infected with grapevine leafroll virus Photo by Maher Al Rwahnih

9 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. 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. 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 9 Leafroll grapevine virus is found the world. With infected vines, sugar production is slowed and fruit maturity can be delayed for weeks. Photo courtesy of the Constellation Academy of Wine

10 Pest Updates Arboreal Camel Cricket in Napa County CA Gammarotettix bilabatus (Orthoptera: Rhaphidophoridae) 10 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) 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. Life cycle of R. majalis "Eurchaferlifecycle" by Art Cushman, USDA The larva of the European chafer Photo by David Cappaert, Michigan State University Adult European Chafer For more photos and means of control, read the WSU Extension Pest Watch: European Chafer Photo by Mike Reding & Betsy Anderson, USDA ARS