Cherry leaf roll virus, as a nepovirus with a bipartite genome, can be genetically analysed by comparing the properties of distinct 'parental' strains and the pseudorecombinant isolates generated from them. In the present work, the elderberry (E) and rhubarb (R) strains were each purified and separated into their middle (M) and bottom (B) components by sucrose gradient centrifugation followed by near-equilibrium banding in cesium chloride. RNA was extracted from the separated components by treatment with a dissociation buffer followed by sucrose gradient centrifugation. Extracted M-RNA of E-strain and B-RNA of R-strain were mixed and inoculated to a series of test plants as were M-RNA of R-strain and B-RNA of E-strain. New local lesion types in Nicotiana clevelandii induced by these heterologous RNA combinations were passaged three times through local lesions on N. clevelandii in order to generate pure pseudorecombinant stocks. M-RNA determined serological specificity, the distribution of virus particle components, systemic symptoms in N. clevelandii and N. tabacum cvs. Samsun and Xanthi as well as the local and systemic symptoms in Chenopodium amaranticolor and C. quinoa. B-RNA determined the ability to induce systemic symptoms in Gomphrena globosa and the type of local lesion in N. clevelandii. When the procedure used for originally generating the pseudorecombinants from the parental strains was applied to the pseudorecombinant isolates themselves, isolates were obtained in the predicted manner that were identical to the original parental E- and R-strains. / Land and Food Systems, Faculty of / Graduate

Five virus isolates RS 2, RS 25, RS 26, RS 28 and RS 29, were transmitted by juice-inoculation technique from sour and sweet cherry trees to cucumber. Four isolates were obtained from trees growing in the Kootenay cherry district of British Columbia. Another one was isolated from a tree growing in the coastal area of this province. Viruses known to occur in the source trees are Necrotic Ring Spot Virus, Sour Cherry Yellows Virus, Twisted Leaf Virus and Little Cherry Virus. The relationship and the complexity of the virus isolates was studied with herbaceous hosts, using a mechanical transmission technique. Pincherry (Prunus pennsylvanica L.) was inoculated by the same technique as a means for provisional identification of the virus isolates. The cucumber syndrome of isolate RS 25 was very mild, that of isolate RS 2 mild, that of isolate RS 29 was of medium severity and those of isolates RS 26 and RS 28 were very severe. Inoculates RS 2 and RS 29 varied greatly in symptom expression on cucumber, whereas the symptom expression of the other isolates was less variable. Isolate RS 29 was characterized by symptomless systemic infection of Nemesia sp.. var. Triumph. Isolates RS 26 and RS 28 both infected Petunia hybr., var. Blue Bee, without expressing symptoms, whereas the other isolates did not infect this species. Other host species too carried the isolates without expressing symptoms, whereas symptoms were produced on cucurbit hosts. Isolates RS 2, RS 26, RS 28 and RS 29 appeared to consist of more than one virus. Strains of a virus P occur in all isolates and isolate RS 25 itself is also a strain of this virus. All five strains of virus P express similar very mild symptoms on cucumber, whereas a characteristic severe savoying type of symptom is produced on squash (var. Table Queen). Species susceptible to virus P are cucumber, pincherry, squash, sweet pea, tobacco (under conditions of long day) and other species. Lathyrus odoratus L. and Lens culinaris Medic. are species useful in separating virus P from the other viruses occurring in isolates RS 2, RS 26, RS 28 and RS 29. It is possible that virus P is related to cucumber-mosaic virus as suggested by symptoms on squash and tobacco. In previous work by other investigators a strain of cucumber-mosaic virus was also isolated from Prunus hosts. On pincherry (P. pennsylvanica L.) isolate RS 28 caused acute symptoms of necrosis and shothole. The plants recovered but symptoms of mottling were systemic. Necrotic Ring Spot Virus caused similar symptoms on Prunus hosts and this virus and Sour Cherry Yellows Virus was present in the original source tree. The other isolates in pincherry all caused similar symptoms of mottling on the young leaves. A few necrotic lesions were produced also. On reisolation from pincherry virus P was obtained in case of isolates RS 2, RS 26 and RS 29. No virus was reisolated in the case of isolate RS 25. The complete parent isolate was reisolated in case of isolate RS 28. The results with pincherry suggest that virus P is responsible for the mild symptoms whereas virus P in conjunction with an additional virus as in isolate RS 28 incites the severe shock symptoms. The identification of the viruses present in the isolates can be carried out by scion inoculation of a set of suitable Prunus indicator hosts. / Land and Food Systems, Faculty of / Graduate

Bibliography: leaves 169-185.

Bibliography: leaves 169-185 / viii, 185 leaves, [1] leaf of plates : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Plant Pathology, 1987

Graduation date: 1995