Rootstock: the most likely culprit, not pollen

Simon, I am no fan of RMV and won’t ignore the large body of research about these viruses. But I wonder if your alarm about pollen transmission of PNRSV and ApMV in Rosa may be a bit overblown. We need to take care not to generalize RMV findings from other plant species too much. Both PNRSV and ApMV have been shown to be transmitted in roses by propagation. There is more than enough virused rootstock to explain 99% of the cases of RMV. In Australia, an old study of cut flower producers with virused rootstock in 1986 show R. multiflora 35%, 15%, 65% infected from different growers and Dr Huey 55% infected. I reckon everyone on this forum from North America with ‘Altissimo’ has a virused plant. Usually they are symptom free for a couple of years. A few years later you can see the markings from space. This was probably bad luck with the timing of the release of the rose, when commercial indifference to the spread of RMV was widespread.

The UC Davis study of 651 seedlings of PVRSV and ApMV-infected plants showed not one infected with virus. 173 plants that were pollinated with infected pollen never tested positive to PVSV or ApMV. Whether thrips can transmit remains unknown. The study provides indirect evidence that mechanical transmission by pruning tools does not occur, but that evidence is purely statistical (on some level, isn’t it all!) and confounded with the great unknown: the unexplained high rate of transmission of PVRSV and ApMV in R. multiflora planted very close together, root-grafting being the suspect. I have the published report if anyone wants to read it.

Arabis Mosaic Virus, Tomato Ringspot Virus and Strawberry latent Ringspot Virus do seem to be transmissible by certain nematodes-- X. diversicaudatum, X. coxi and by X. americanum, riversi[/i] and californicum[/i] - - the last 3 in the lab- - not roses to my knowledge.

Did you get the results back from those cultivars you were having tested Cass?

Cass, the UC Davis PNRSV published results so far do not utilize a climate controlled environment. PNRSV is a temperature sensitive virus, to be of general use, pollen and sap transmission studies should also be be done in cooler climates (or a controlled temperature environment).

Link: home.neo.rr.com/kuska/info_about_virus.htm

Older varieties like ‘Cecile Brunner’ are probably universally infected with one or more viruses. You have to remember that during the 1950’s and 1960’s in particular, almost all roses were grafted onto a rootstock like ‘Dr. Huey’ with absolutely no regard to the spread of virus. I am among those who believe that 80% or more of older roses are probably infected with one or more viruses which they almost certainly got through grafting. Personally, I assume that most roses have at least one virus and should be treated accordingly. Its just a fact that we are living with this legacy that the industry has created for us.

Regards,

Paul

I found this interesting reading:

Link: apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO.2001.91.1.84?cookieSet=1

I agree that rootstock is the likely problem. I have used virus infected varieties as both seed and pollen parents (not that I prefer them, just sometimes you cannot find a particular variety that is clean) and have not seen any sign of virus in the seedlings.

With regard to rare transmission via root to root grafting, it makes sense to me when roses are planted closely and deeply cultivated that some adjacent roses may produce root to root unions.

For the most part, I would not worry a bit about pollen transmission. If it happens, it is an extreme rarity. There are too many other things to worry about! BUT, do not assume that your ‘Dr. Huey’ is clean just because it shows no symptoms.

For grafting seedlings, I have resorted to only using one of my own seedlings as rootstock. The mother plant is grown separate from other roses, and only cuttings are taken from the mother plant to root and be grafted with other of my seedling varieties.

Jim Sproul

Henry, I don’t think temperature control is such a serious defect in the Davis study. Yes, it’s is a bit vague about the timing of the ELISA testing, but the pathologists were obviously aware of the temperature issue. In the seedling tests, the study states: "Plants were periodically observed for symptom development and after one year they were tested in the spring when virus is at high titer for ApMV and PNRSV by ELISA." In the case of the pollination tests, the study states “All bushes were ELISA tested in the spring of years four and five.”

Robert, the test results never arrived. I need to harass the lab again.

Cass, your answer only applies to testing, not to transmission.

  1. I feel that the problem with their not using temperature control when studying whether the virus is transfered through infected pollen is that any spring formation of virus infected seeds may be “cleansed” of the virus during the heat of the summer. Thus, they could find that no seedlings have the virus. But, this does not answer the seed transfer question for rose growers in cooler climates.

  2. Regarding sap transmission due to pruning, branches of neighboring bushes hitting each other, insect bites, etc - in hot climates the virus will be in the below ground parts of the plant during the hot summer months. Thus, any transmission from these possible causes would only occur in the cooler months. Lets say that these conditions contribute about 1 % (or less) spread in their climate, but their present experiments do not answer the question of what would the spread (from the above ground routes) be in cooler climates.

Interesting hypothesis, Henry, with many implications. I know that ELISA testers specifically ask for hips to do testing, so I question the “cleansing” of the virus.