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Rose Mosaic, part 2 (see part one, here, for the whole story)
The University of California at Davis has a much older and very much larger program than ours, and they have specialized predominantly in modern roses and rootstock varieties. Our program has always emphasized heritage rose varieties, so there is relatively little overlap between the programs. Between the two programs, we've enabled nurseries to clean up their stock of many rose varieties. Of course not all nurseries are interested in participating; some don't even really believe in rose mosaic, or they don't believe it to be a problem. And so there remains a percentage of roses on the market that continue to be infected.
It is true that in many varieties, you only see leaf symptoms in the spring, often on just a few leaves. For the rest of the year, the plant appears to be perfectly healthy. There are those who say, therefore, that it's not a disease about which we should be concerned. Unlike rose rosette virus, it does not just kill your rose bush outright.
But early work by George Nyland, at UC Davis, indicated that infected plants were less vigorous, produced fewer flowers, were less cold-hardy, and in general, did not live as long, as healthy plants of the same variety. There are also rumors constantly circulating that rose mosaic is contagious, and that trying to keep a plant healthy is therefore futile.
So in the late 1980s, we decided to set up a research plot on the Florida Southern College campus, to test plant performance, and the potential for contagion. We planted a row of 34 plants of 'Double Delight' rose (in FSC's colors of red and white) in the college garden. Going down the row, they alternated -- one healthy, one infected with prunus necrotic ringspot virus, and one infected with apple mosaic virus (we purposely infected them by grafting in pieces of bark from known infected plants, and later tested them to be sure they were actually infected). Then the series repeated. The plants at each end and bordering the central aisle were not counted when data were collected, so there ended up being 10 healthy plants, 10 with PNRSV, and 10 with ApMV, each plant growing next to plants of the other two treatments, from which data were collected. We then observed them in 1989, 1990, 1991, and 1992. We counted number of flowers produced and stem lengths of cuttable flowers. We also retested periodically, to look for contagion. No attempt was made to control leaf-feeding insects or mites or pollinators, and when we pruned, we went directly down the row, not sterilizing clippers between plants. The plants were in sunken 20-gallon pots, so root-grafting between plants was not likely.
After 4 years, most ApMV plants were dead or near death. Most PNRSV-infected plants were still there and often looked healthy, and all healthy plants were still there. We saw no contagion at all, supporting the concept that rose mosaic disease is not contagious in the field, by insect, pollen, or mechanical means. Both viruses reduced number of flowers and average stem length, and ApMV ended up producing a very unhealthy plant that died early.
In the thirty years since then, the story has remained pretty much unchanged. To date, no case of contagion has ever been documented in a garden. Grafting remains the only known way by which a rose can "catch" the disease. And the only documented case of spread by natural grafts was in a research plot at UC Davis, where they planted rootstock plants very close together (inches), and the disease did spread down the row. Tests with glyphosate (RoundUp) indicated that natural root grafts had formed. However, root grafting apparently does not happen at garden spacing. Some have suggested that simply growing ungrafted, own-root roses would solve the problem. Unfortunately, that's not the case -- once a plant is infected, all parts of the plant are infected, so if cuttings are rooted from it, they will also be infected. On the other hand, an own-root plant made from a healthy parent plant should remain free of mosaic, just as would a grafted plant started from uninfected starting materials.
It has been suggested that the disease spreads in northern, cooler climates. But again, there is no documented case of that having ever happened. And when someone sees a "new" infection in the garden, it is very likely due to the fact that the disease may remain symptomless for years, until just the right combination of weather and growth stage occur. It was likely infected when propagated at the nursery.
So the take-home message here: If you have a plant infected with rose mosaic, there is no emergency to remove it; it will not spread the disease to its neighbors. On the other hand, if you can get a disease-free plant of the same variety, it will likely grow better, live longer, and produce more flowers on longer stems. And for those reasons I would always opt for a clean plant if I could get it.
Rose Mosaic, part 1 (then continued in part 2, here, since there is a length limitation on a post)
Rose Mosaic Virus Disease and the Heat Therapy Program
at Florida Southern College
I recently posted some old photos on my Facebook page, of some research we did on rose mosaic, long ago. Several people said that information on that research and our program should be made available to current rosarians, since there is a whole new generation of growers since any of that was published. Hence, this article.
Rose mosaic is really a set of symptoms, rather than a specific virus. It's rather like the common cold or influenza -- numerous different viruses cause similar symptoms, so we name the symptom set. In the case of rose mosaic, Prunus Necrotic Ringspot Virus (PNRSV) is by far the most common cause. Apple Mosaic Virus (ApMV) is also a possibility, and in Europe, Arabis Mosaic Virus (AMV) can cause rose mosaic. In the case of PNRSV, plants may remain symptomless for years, or may show symptoms only when the weather is just right (usually in the spring, while it's still cool and the plants are growing rapidly). Notice that rose mosaic is not at all related to rose rosette disease, caused by a very different virus, and which is deadly to roses.
I started growing roses in the early 1980s. I was not particularly interested in roses at the time, but my boss was an American Rose Society judge and district director, and was very much into roses! He invited me to a local rose show. I didn't really want to go, but to please the boss, I did go. I did not find the show very interesting until I discovered the Old Garden Roses section, and smelled a rose that gave me a fragrance memory from early childhood, at my grandmother's house. I immediately decided I needed to grow heritage roses! In those pre-Google days, I eventually discovered the Roses of Yesterday and Today nursery, in Watsonville California, and I ordered my first 13 roses. I planted my new garden in the autumn and waited. They all grew well. In the spring, I started noticing interesting variegation on the leaves of many of them -- little yellowish or whitish lines, often zigzagging across veins. It was interesting, but I realized, not normal. So I took a sample to show my boss, who immediately recognized it as rose mosaic virus disease. I asked what could be done about it, and he said there wasn't much I could do in the garden, but that it was possible to use heat therapy to clean up (cure) a variety, if you had the right equipment. I noted that we had a large growth chamber in the room 2 doors down from my office, not currently in use, that could provide the conditions needed to cure a rose. So I suggested that we try it. He said he did not want to take the time or effort, but that I was welcome to try it myself.
By then, I was also joining groups interested in old roses -- various round-robins and the Heritage Roses Group. Through those exchanges, I got to know Carl Luhn, who had worked in the University of California at Davis's rose virus program. Carl coached me on how to go about the process, and Florida Southern College's heat-therapy program was born.
Basically, heat therapy involves holding a plant at a high enough temperature that the virus particles degrade, but hopefully not hot enough or long enough to kill the rose bush. We generally treat a 2-gallon potted plant. We stress it ahead of time -- growing it hot and relatively dry -- we want it to wilt. We also starve it of fertilizer until the leaves turn yellowish from lack of nitrogen. That prepares the plant for the extreme stress to which it's about to be exposed, and increases its chance of survival.
The plant is placed in a lighted growth chamber, and held at 100°F/38°C, 24/7, for at least 4 weeks. While there certainly are many places where daytime summer highs may exceed that temperature, there is nowhere on the earth where that temperature continues day and night, without a break, for 4 weeks. So it's a very unnatural situation. Some roses die quickly, and we've not been able to successfully heat treat them. But if a rose does survive the treatment, we cut buds from the upper portion of the plant, bud-graft them to known virus-free rootstock plants, and grow them off in the greenhouse as we would any other budded rose. The plant in the growth chamber is not cured -- every time we water it, the roots drop below 100°, and if the plant is placed back into the greenhouse, it will usually still be infected. On the other hand, those buds that were continuously hot are nearly always disease-free. So if we can get a plant to survive its time in the heat, and then get some buds to survive off of it, we can nearly always end up with a healthy plant.
There is an alternative method of cleaning up a variety, which is being used at U.C. Davis. In a vigorously growing shoot, the very tip of the shoot (the meristem) may outgrow the virus, so that it can be cut off and grown, under sterile tissue culture conditions. It is a challenge, since the clean area is tiny -- a very small fraction of a millimeter. So the trick is to cut a piece small enough to be free of the virus, but large enough that it actually has the ability to survive in tissue culture and grow back into a normal plant. The method has been used successfully in many varieties of fruit trees, but it remains a challenge for roses; they are not easy to culture from such tiny pieces, so the results have often been disappointing. While we have tissue cultured a few roses at FSC over the years, we've not used that method for mosaic treatment.
Once the plants grow out, in the greenhouse, we have them tested to be sure they really are healthy. There are several methods that can be used to test them:
• 'Mme. Butterfly'. The old florist's cut-flower rose 'Mme. Butterfly' is known to show unusually bright leaf patterns if a healthy plant is graft-inoculated in the fall, and then observations are made of the spring growth flush. We used to do a lot of these in our own greenhouses, but we don't use the method anymore.
• 'Shirofugen'. This test is done at U.C. Davis. They bud the variety to be tested onto the branches of a 'Shirofugen' flowering cherry tree. Such grafts always fail and die, since roses are not graft-compatible with cherries. However, if there was virus present in the rose, there is a high probability that it will migrate into the cherry stem before the rose bud dies. If no virus was present, the cherry twig will heal over, forming a healthy bark between itself and the dead or dying rose bud. If one of the viruses causing mosaic is present, that area of the stem will die back, oozing a dark, sticky sap. So we send test budwood to Davis in the spring or early summer, they bud the trees, and they make the observations in the early autumn.
• ELISA. Enzyme-linked immunosorbent assay is a method commonly used in medicine (home pregnancy tests, HIV tests, etc.), and can be done quickly and cheaply in a lab. For this test, we send our sample sticks to Washington State University's ELISA lab, in Prosser. Not only can they detect the presence of virus, but they can tell us exactly which virus(es) they found. Until recently, this has been our go-to method of testing.
• qPCR. This is the newest method, rapidly replacing the other methods, in that it is quick and highly accurate. It uses the same sort of DNA-analysis technology as is used in medicine and forensics. Since viruses have different DNA (or RNA) from a rose bush, the viral material can be detected and identified. Currently the pathology lab at Texas A&M University does that testing for us.
When we are sure we have a healthy plant, it goes into the college rose gardens, beautifying the campus and serving as a source of cuttings or buds, which are distributed to the commercial nursery industry. Over the years, we have supplied propagating material to more than 20 US rose nurseries, as well as to Bermuda, the UK, South Africa, and several other countries.