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Why do you say it's a pathogen?

First, how do we know RRD is a pathogen rather than the plant reaction to the mite (as stated in the ARS "Consulting Rosarian's Manual") and second, why is it now called "virus like" when in the 1960's and earlier some scientists called it a virus?

Why RRD is a Pathogen: Suggestions that RRD symptoms could result from a toxic response to the mites feeding originated with a comment (Oldfield, 1970) about Allington, Staples, and Veihmeyer’s 1968 article identifying Phyllocoptes fructiphilus as the vector of RRD.  His comment is included here:

    “Recently, Ailington et al. (1) reported that they had transmitted rose rosette virus in Nebraska with Phyllocoptes fructiphilus Koch. (The correct name is Phllocoptes fructiphilus Keifer 1940.) In one series of tests, ten P . fructiphilus from infected wild rose were transferred to each of ten healthy Rosa eglanteria. According to the authors, five plants became infected with rose rosette virus. The authors state that several species of Rosa were proved to be infected with rose rosette virus "either by grafting or by mite transmission"; however, they did not specifically state that the virus was graft-transmitted from those test plants to which P. fructiphilus had been transferred. This may only be an error of omission. Unless they graft-transmitted a virus from the plants that received mites, the appearance of symptoms on plants to which P. fructiphilus was transferred could be attributed to a mite-induced toxemia that resembled rose rosette. A definite statement that both types of transmission were accomplished in sequence would greatly substantiate their claim of transmission of a virus by P. fructiphilus. Also, a comparison of the effects on healthy rose plants of populations from infected and healthy roses would further substantiate their case.” (Oldfield, 1970)

Allington et al (1968) and Thomas & Scott (1953) had already demonstrated graft transmission of RRD.  Oldfield's comment was only a comment.  Work was not done to support the suggestion that RRD was a toxic response.  The above speculation was referenced by Jeppson et al (1975) who listed some RRD symptoms and offered his opinion that "Most of  these symptoms are typical of those produced by bud mite feeding on plants (Allington, et al 1968; Oldfield, 1970)".  It was not in reference to any new work, however.  That was, in turn, referenced by Slykhuis (1980).  Oldfield's “could be attributed  to a mite induced toxemia that resembled rose rosette” by 1980 was being published as fact without benefit of supporting tests.   

By 1996 the weight of evidence against the suggestion that RRD “could be attributed to a mite-induced toxemia” rather than from a pathogen was overwhelming due to scanning electron microscopy (SEM) work (Kim, et al, 2000) as well as mite transmission tests (Allington et al 1968) (Amrine et al. 1988 and earlier work).  In 1996 Oldfield wrote the following:

    Rose Rosette

    Although the identity of the causal agent of rosette is unproven, Gergerich et al. (1983) and Gergerich and Kim (1983) found in the cytoplasm of cells of diseased roses spherical virus-like particles measuring 120-150 nm with a 16 nm thick trilaminar wall, which resemble particles associated with two other eriophyid-bome diseases, fig mosaic and wheat spot. Ailington et al. (1968) reported graft transmission and transmission by the eriophyid mite, Phyllocoptes  fructiphilus Keifer, to Rosa eglanteris L., Rosa suffulta Greene, Rosa woodsii Lindley, Rosa multiflora Thunberg ex. J. Murray and Rosa rubrifolia Villars. Recently, Amrine et al. (1988) clarified the role of P. fructiphilus as a vector of the rose rosette pathogen, rather than as an incitant of rose rosette, by demonstrating that symptoms resulting from feeding of the mite on healthy rose after transfer from symptomatic roses persisted and continued to develop after killing the mites with Temik. Mites held 14 days at 4°C (at which temperature they were immobilized) subsequently transmitted the rosette pathogen; single mites were able to transmit.” (Oldfield and Proeseler,1996:p.268-269)

Thus, Dr. Oldfield, who had experience with mite-feeding induced toxemias as well as with mite transmitted virus-like diseases, the person who made the origional suggestion back in 1970, reported in 1996 that it was a pathogen rather than a feeding effect.

  Why RRD is not called a virus:

Once upon a time, certain sets of symptoms were recongnized and they were called viruses, because they were smaller than bacteria and fungi (both of which could be seen with conventional microscopes) and they could be graft transmitted.   As the diseases were studied further, there were found to be differences, some of which were based on SEM (scanning electron micrographs).
To be identified as a virus, a pathogen must meet Koch's Postulates (see glossary).  There is a similar disease to RRD reported in eastern Europe that is being called a phytoplasma. The website

is an introduction to phytoplasmas.

If you are interested in plant pathology, there is a site on the internet that presents class notes with extensive bibliographies for a graduate course in Plant Viruses from Dr. Xiong at Arizona.

References Cited:

Allington, W.B., Staples, R., and Viehmeyer, G., 1968, Transmission of Rose Rosette Virus by the Eriophyid Mite Phyllocoptes fructiphilus, Jour. Econ. Entomol. 61(5): 1137-1140.

Amrine, J.W.,1996, Phyllocoptes fructiphilus and Biological Control of Multiflora Rose.  in E.E. Lingquist, M.W. Sabelis, and J. Bruin, (Eds.). Eriophyoid mites - Their Biology, natural enemies, and control.  Elsevier Science Publ, . Amsterdam, The Netherlands. pp. 741-749.

Amrine, J.W., Hindal, D.F.. 1988, Rose Rosette: A Fatal Disease of Multiflora Rose, West Virginia University Agricultural and Forrestry Experiment Station, Circular 147: 4p.

Amrine, J.W., Hindal, D.F., Stasny, T.A., Williams, R.L. and Coffman, C.C.1988. Transmission of the Rose     Rosette Disease Agent to Rosa multiflora by Phyllocoptes fructiphilus (Acari: Eriophyidae). Entomological News, 99(5):239-252.

Epstein, A.H., Hill, J.H., and Nutter, F.W., 1997. Augmentation of rose rosette disease for biocontrol of multiflora rose (Rosa multiflora). Weed Science 45:172-178.

Gergerich, R.C. and Kim, K.S., 1983, A Description of the Causal Agent of Rose Rosette Disease. Arkansas Farm Research. 1983 (May-June):7.  

Kim,K.S., Ahn, K.K., Gergerich, R.C., and Kim, S.B., 2000. Possible Etiology of Eriophyid Mite-Borne Patholgens Associated with Double Membran-Bound Particles.  In Harris, K.F., Smaith, O.P. and Duffus, J.E, eds.Virus-Insect-Plant Interactions. Acad. Press. NY:29-50.

Oldfield, G.N., 1970. Mite Transmission of Plant Viruses. Ann. Rev. Entomol. 15:343-380.

Oldfield, G.N. 1975, Mites  and Plant Diseases, in  Mites Injurious to Economic Plants, in  Jeppson, L.R., Keifer, H.H. and Baker, E.W, University of Calif. press, Berkeley: 99.

Oldfield, G.N., and  Proeseler, G., 1996, Eriophyoid mites as vectors of plant pathogens in E.E. Lingquist, M.W. Sabelis, and J. Bruin, (Eds.). Eriophyoid mites - Their Biology, natural enemies, and control.  Elsevier Science Publ, . Amsterdam, THe Netherlands. pp. 259-275.

Slykhuis, J. T. (1980) Mites in  Vectors of Plant Pathogens, K.F. Harris and K. Maramorosch eds.Academic Press New York: 325-356.