Recognizing, Understanding and Controlling Virus Diseases of Hostas Ben E. Lockhart, Professor Department of Plant Pathology University of Minnesota, St. Paul, MN Part I: Understandinv Viruses A.brief overview of viruses, their basic characteristics and special nature, how we recognize and identify them, and how we control the disease they cause. ~ Nucleic acid Because of their extremely simple structure viruses, unlike other disease causing organisms (bacteria, fungi, nematodes etc.) multiply in cells they invade using the same mechanisms and metabolic processes that the plant or animal cell normally uses for its own maintenance. It is therefore very difficult tot attack the virus chemically without doing equal damage to the host cell. For this reason prevention rather that cure is the principle on which virus disease control is based. Bacterial disease Antibiotic II~l isease iminated Fungal disease Fungicide Disease eliminated Vira! disease No chemical treatment available Disease not eliminated How virus diseases spread and why plant virus diseases are different. All viruses must enter into living cells to survive and multiply. Most animal
viruses need no help. Plant viruses need help because plant cells have cellulose cell walls. Know the enemy- Why controlling a plant virus causing the disease requires detailed and exact information about the virus. Testing for hosta viruses: Testing is done by serology (ELISA), electron microscopy (EM) and polymerase chain reaction (PCR) amplification. Part II: Understanding Hosta Viruses Viruses found in hostas to date, the symptoms they cause, their characteristics, and how best to deal with them. 1. Hosta Virus X (HVX) Spread by contact involving non-lethal injury to tissue. Not spread by insects, nematodes, seed or pollen.
Infects only hostas and can be spread only from hosta to hosta. spreading 2. Tobacco Rattle Virus (TRV) Spread by contact unlikely. Spread by nematodes (Trichodorus spp) especially in sandy soils. Infects a very wide range of plant species. Can spread to hostas from a variety of sources. Not native to North America. Introduced frequently in ornamental species such as Epimedium, Dicentra, Anemone etc. Occurs sometimes as NM form which must be detected by PCR. Eliminate infected plants of all species to avoid spread. Nematicides may be used to control nematodes.
Protein Coat Ne gp~viruses (N_.gmatode and P ~ollen transmitted viruses) This group of viruses are spread by soil-inhabiting nematodes belonging to the genus Xiphinema. They can also be spread by pollen. They are not spread by insects and are unlikely to be spread by mechanical contact. They all infect a wide range of species and can spread to hostas from a variety of sources. Best controlled by eliminating infected hostas and looking for other possible sources for infection. Tomato Ringspot Virus (TomRSV) Causes bleaching of leave varying from spots to complete leaf. Native to N, America and occurs naturally in a wide range of both herbaceous and
woody plants (e.g. grapevine, buckthorn, elm). Spreads slowly and is likely to be a home-garden rather than a commercial nursery problem. Eliminate infected plants to prevent spread. Tobacco ringspot virus (TobRSV Same as TomRSVg Arabis Mosaic Virus (ArMV) Not native to N. America. Introduced from Europe in hostas and other perennials. Causes leaf-blanching symptoms similar to those caused by TomRSV and TobRSV. Eliminate infected plants to prevent spread. 3. Tospoviruses: Tomato Spotted Wilt virus (TSWV) and Impatiens Necrotic Spot Virus (INSV) Infect many plant species. Can spread to hostas from many other perennials. Spread very efficiently by thrips (Thrips tabaci and Franklinielli Schultz), Not an outdoor garden problem in northern
climates. Virus particles are large, enveloped and very labile. Little possibility of spread by mechanical contact. Very damaging to many plant species, but hostas seem to escape permanent damage. 4. Cucumber mosaic virus (CMV) This virus infects a wide range of cultivated crops (vegetables, annuals and perennial ornamentals) weeds~ and been found only three times in hostas. ( Sun Power from New Zealand, Zounds from Washington, and recently H. undulata in Germany). CMV is normally potentially dangerous because it is normally easily spread by aphids, the strain of the virus found in hostas to have lost some of the characteristic CMV properties, and attempts to transmit it have been unsuccessful. It would be prudent to keep an eye on this virus. Non-Viral Abnormalities which may be Confused with Viral Diseases Genetic (Chimeric) Effects o Usually manifested as bleached-out (achlorophyllus) spots on leaves. Seen in Sea Sprite, Moonshadow etc. Sunlight Exposure Effects ~ Plants growing under sub-optimal exposure conditions e.g. Sum and Substance, Gold Standard and Striptease. ~ Sun-scald injury with or without subsequent fungal invasion. Cold o Causes leaf crinkling and distortion when plants emerge in spring and encounter a cold snap. Leaves that emerge later are normal. Herbicide injury
Caused by herbicide (e.g. 2.4-D) drift. Symptoms are thickened leaf veins, chlorosis (yellowing) between veins and reduced leaf size. The time taken to grow out of herbicide damage depends on dosage received. Part III: Globalization and the internet phenomenon, and the present and future of viruses of hostas and other perennial ornamentals. Two things are certain. Globalization of trade in ornamental plants has resulted in increased international movement of hostas and other perennials, thereby increasing the risk of movement of diseases and pests. Much production has been shifted from home base (e.g.u.s., Holland) to overseas (Latin America, Africa, Asia) resulting in less stringent monitoring of diseases and pests. Universal internet access has resulted in rapid spread of information about virus diseases in hostas and other ornamentals, and producers and distributors are being increasingly obliged to respond to consumer concerns about virus diseases. Part IV: Hosta Virus X and other hosta viruses in the context of breeding and selection of new hosta cultivars. Questions: o Are there viruses that can be transmitted during crossing? Not been shown experinaentally, but Nepovirus are potential candidates for pollen (and seed) transmission. o What do we know about the genetic basis for resistance to HVX? We know from the three year study that hosta genotypes differ in response to infection with HVX and fall into one of the following categories: Very susceptible (sensitive) Tiaras, Honeybells, Royal Standard.
Tolerant Moderately susceptible Resistant Immune These data indicate that Hosta spp. possess a gene(s) that confer resistance to HVX, How can hosta hybridizers take advantage of the genetic trait(s) for resistance? o Take an empirical (or trial and error) approach, look at response of progeny of parents with different resistance levels. o Take a fundamental approach to identify gene(s). o What governs plant response? Is there one gene or more than one? Is it dominant or recessive? Is HVX variable?