Hosta Virus X – A Clear and Present Danger

Bill Meyer

     My experience with this new problem dates back to 2001 when I was visiting a wholesale nursery. I noticed some plants with unusual markings in a batch of 'Sum and Substance'. They had green mottling and looked interesting to me, so I started to pick one up. But as I did I saw that several looked like it. My first impulse was to see which had the best markings, but I realized pretty quickly that something was wrong.

      Sports are genuinely rare occurrences. They don't just appear in large numbers like I was seeing here. In the batch of 100 or so plants, at least 20 had these unusual markings. It dawned on me that I was looking at some kind of new infection I hadn't seen before. When I noticed that a large block of 'Fragrant Bouquet' at the same nursery had similar markings, I began to get concerned.

      I've seen apparent viral infections before in some hostas, but they were always isolated to single, individual plants. Some of these have attained cultivar status with names many of us are familiar with. Plants like 'Lunacy', 'Eternal Father' and 'Leopard Frog' have tested positive for viruses, but this has not really diminished their appeal. Some nurseries have propagated these and have even tried to deliberately infect other plants with the virus. They were unable to do this, so many thought that these plants were safe to grow in gardens. Some viruses are in fact very difficult to transmit, requiring very specific means to move from one plant to another. Unfortunately, though, there are also viruses that can spread quite easily.

     The general thinking about viruses in hostas has been that they were never really a very serious threat. Plants once thought to have viruses like 'Sea Sprite' turned out not to have them after all – and we've grown somewhat complacent. Occasionally nurseries would see an unusual case of a particular virus and destroy the plants, but most discussions in recent years about hosta health problems centered on foliar nematodes and Southern stem blight. Although it is a little frightening to think that there are no cures for most plant viruses and that the plants that are infected must be destroyed, we weren't all that worried about it. None of them seemed to spread much, if at all.

      The disease I saw in those plants in 2001 has been identified by hosta virus expert Dr. Ben Lockhart, University of Minnesota, as a relatively new one. It is of the genus Potexvirus, and named Hosta Virus X, or HVX for short. Bonnie Blanchette, who is well acquainted with Dr. Lockhart’s work, has written articles about this virus in The Hosta Journal and very recently in Fine Gardening magazine (May/June 2004). There are pictures of the symptoms in the AHS booklet The Hosta Adventure (page 28) and now in a link on the front page of Hosta Library (www.hostalibrary.org). They are something we should all get used to looking for in our own hostas and in nurseries.

     In the years since my first sighting of this new problem I have seen more and more plants with the same appearance turning up in nurseries and garden centers everywhere I go. Some people have even named forms of 'Gold Standard' that show symptoms of Hosta Virus X. Chris Wilson, owner of retail hosta nursery Hallson Gardens in Brooklyn, Michigan, became concerned about plants of 'Striptease' and 'Sum and Substance' he had purchased and began having them tested by Dr. Lockhart. The ones showing the classic symptoms tested positive for HVX, but the more frightening news was yet to come. He sent samples of apparently clean and symptom-free plants from the same batches and they also tested positive. What this means is that you cannot visually identify plants carrying the virus, and that all plants in batches that have symptomatic individuals might well be infected too.

     As these obviously virus infected plants are showing up everywhere, the implication is that there are 5-20 times as many out there that do not show the visible symptoms of the virus. At this time there may be tens of thousands of hostas now infected with HVX being offered for sale in nurseries and garden centers around the world.

     The most commonly seen symptoms are found in gold and gold-centered plants. It is generally random green mottling, almost always accompanied by mottling that follows the veins. It often has the appearance of ink on blotting paper soaking out from the line along the vein. Other symptoms may also show, and we are still trying to get a clear idea of what the virus looks like in plants with green leaves and green centers. Dr. Lockhart says there may be a variety of symptoms with very different appearances. Because of this, any plants showing signs of any viral infection should be immediately disposed of. Nurseries that have brought in any batches of hostas that include infected plants should destroy or return the entire batch as infected, not just the symptomatic plants. Testing each one would outweigh the cost of throwing them away.

     It should be obvious to anyone that the sheer number of infected plants we are seeing marks Hosta Virus X as a clear and present danger to the hosta gardener and commercial grower. There simply would not be so many virused plants out there if the virus did not spread easily.

     HVX is spread by contact of the sap of infected plants with healthy plants. This will occur when cutting the infected plants – not just rhizome division but also cutting leaves and scapes. Lawnmowers or string trimmers will also move sap around as will stepping on the plants or injuring them in any way. Pieces of roots left behind from digging the plants may also harbor the virus. All parts of a hosta infected with HVX should be regarded as carrying a highly contagious virus and handled with this in mind. Hands and any tools used on these plants should be sterilized before contacting any healthy plants. The virus only exists in living plant material. It will not be present in the soil, so new plants may be planted where a virused plant was dug as long as there are no still-living roots remaining there.

     It's never good news to find out about a new threat to the health of our plants, but it is never something we should ignore either. With something as easily spread as HVX appears to be, we must be extremely careful handling these plants or we will find ourselves spreading it to the healthy plants in our gardens. It may be tempting to pick up that fancy new "sport" at the local garden center, but if it looks like it might be sick, we should keep our distance from it just like we tell children to stay away from sick animals.

     Never buy a hosta that has a mottled pattern that looks like little separate islands of another color when the leaf should be solid color. Named introductions from large nurseries like 'Revolution' are safe, but do not assume a 'Gold Standard', 'Striptease' or 'Sum and Substance' with irregular mottling is safe to take home unless you are sure you know the difference between mutation-caused variegation and markings caused by plant diseases. Even if you think the plant might just be mottled like 'Cynthia' or 'Filagree', leave it alone as it is too hard to be sure without having it tested for HVX.

     This is a disease we can all watch for and do something about. If you see that friends have purchased a virused hosta, impress upon them the importance of getting rid of it before they start unknowingly spreading it. If you see that a nursery or garden center has plants that show Hosta Virus X, tell the manager or owner that it is a virus that can be spread. All plants infected with Hosta Virus X should be destroyed as soon as possible since there is no cure and it is contagious. Explain that this is a new virus and word is just getting around, and the supplier it purchased the plants from will hear about it soon if it hasn't already. Unlike foliar nematodes, Hosta Virus X can be stopped from becoming the next hosta plague if we all act quickly. Now is the time to start.

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