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Summary The effects of infection by prune dwarf (PDV) and/or Prunus necrotic ringspot (PNRV) viruses upon seed weight and seed germination were studied in Ackermann and Grose Grune Reneklode plum seed trees. The mean weight of seeds gathered from PNRV-infected seed trees was found to be 565.2 mg for Ackermann and 809.7 mg for Grose Grune Reneklode, respectively. The mean weight of seeds collected from Grose Grune Reneklode seed trees infested by PDV and PNRV in mixed infection was evaluated to be 1,010.4 mg. The mean weight of seeds obtained from healthy seed trees of the same two plum varieties was proved to be 552.3 mg for Ackermann and 771.5 mg for Grose Grune Reneklode. The differences in mean weight between seeds obtained from PNRV- or PDV + PNRV-infected and healthy seeds trees were found to be statistically significant with Grose Grune Reneklode variety only. Apparently, in virus-infected Grose Grune Reneklode trees the lower fruit production was correlated with a compensatory increase in the weight of individual seeds. The mean germination capacity of seeds gathered from PNRV-infected seed trees was proved to be 20.6% for Ackermann and 58.0% for Grose Grune Reneklode, respectively. The mean germination capacity of seeds collected from Grose Grune Reneklode seed trees infected by PDV and PNRV in common was evaluated to be 24.5%. The mean germination capacity of seeds obtained from healthy seed trees was found to be 21.6% for Ackermann and 59.3 % for Grose Grune Reneklode, respectively. The differences in mean germination capacity between seeds produced by healthy and virus-infected Ackermann and Grose Grune Reneklode seed trees were statistically significant only with Grose Grune Reneklode infested by PDV and PNRV in mixed infection.

Summary Comparative screenings using ELISA technique were performed to evaluate the incidence of prune dwarf (PDV) and Prunus necrotic ringspot (PNRV) viruses in seed, young two-leaf seedlings and elder field-grown seedlings of plum (Prunus domestica L.) varieties Damas noir, Grose Grune Reneklode and Nancymirabelle. The rate of PDV/PNRV infestation in seeds was significantly greater than in seedlings grown up from the same seed lot. In seeds, two-leaf seedlings and elder field-grown seedlings, rates of infestation with PDV averaged 19.2, 2.0 and 0.4%, respectively. Infestation rates with PNRV were 6.4% in seeds, 0.8% in two-leaf seedlings and 0.5% in elder field-grown seedlings. The rates of infestation with PDV and PNRV in common were proved to be 0.6% for seeds and 0% for each of the two developmental stages of the seedlings.

Summary Enzyme-linked immunosorbent assay (ELISA) was used to evaluate the incidence of prune dwarf virus (PDV) antigen in seed and seedlings derived from nine open-pollinated, spontaneously PDV-infected blackthorn ( Prunus spinosa L.) shrubs. The incidence of PDV antigen in seed lots gathered from the different seed source shrubs ranged between 93.6 and 100%. In seedling progenies arised from seeds harvested from four of those PDV-infected blackthorn shrubs, the rate of plants carrying PDV antigen, however, was ascertained to be 0% in three progenies and 4.8% in one progeny. PDV-infected blackthorn seedlings up to four leaf stage showed neither growth depression nor any other disease symptoms.

Summary The effects of infection by prune dwarf virus (PDV) upon seed germination were studied in mazzard cherry [Cerasus avium (L.) Moench ] seed trees. The mean germination capacity of seeds gathered from PDV-infected seed trees belonging to two different mazzard clones was found to be 69.6% for clone Nr. 53 and 31.1% for clone Nr. 170, respectively. The mean germination capacity of seeds obtained from healthy seed trees of the same both mazzard clones was proved to be 74.5% for clone Nr. 53 and 41.6% for clone Nr. 170, respectively. The differences in germination capacity between seeds obtained from PDV-infected and healthy seed trees were found to be statistically significant in both mazzard clones. The difference in mean germination speed between seeds gathered from PDV-infected and healthy seed trees was statistically significant only with that mazzard clone the seed germination capacity of which has been more strongly diminished by virus infection. In that mazzard clone Nr. 170, the mean germination speed was pointed out to be 139.4 d for PDV-infected seed trees and 123.5 d for virus-free trees.

Summary By using ultrathin sectioning the localization of prunus necrotic ringspot virus and cherry chlorotic ringspot virus (prune dwarf virus) particles in naturally infected cherry varieties was investigated. As alterations caused by the pathogens aggregations of virus particles and tubules were observed in vesicles and cisterns of the endoplasmatic reticulum bundle cells. The virus infections were restricted to the xylem tissue. For detecting both viruses all through the year a method was elaborated using immunoelectron microscopy.

Summary Wild-growing woody plants of species taxonomically related to cultivated pome and stone fruits were surveyed for the presence of fruit tree viruses in order to determine their importance as natural virus sources. Among wild-growing roses ( Rosa canina L., R. corymbifera Borckh. , R. elliptica Tausch , R. rubiginosa L.) no plants investigated were infected by ilarviruses (Prunus necrotic ringspot virus, prune dwarf virus) or nepoviruses (especially Arabis mosaic virus). Wild-growing apple trees ( Malus sylvestris Mill .) proved to be not infected by closteroviruses (apple chlorotic leaf spot virus) or by the apple Spy decline agent. In wild-growing mountain ash ( Sorbus aucuparia L. em. Hedl .) neither closteroviruses (apple chlorotic leaf spot virus, apple stem grooving virus) nor ilarviruses (apple mosaic virus) were detectable, although 7 % of the trees investigated were infested by the mountain ash ringspot disease. Mazzard cherries ( Prunus avium (L.) L. var. avium ) were free from nepoviruses (Arabis mosaic virus, cherry leaf roll virus, raspberry ringspot virus, strawberry latent ringspot virus, tomato black ring virus), but tomato bushy stunt virus was present in this species with 1.2%. Wild-growing European dwarf cherry ( Prunus fruticosa Pall .) proved to be free from ilarviruses (Prunus necrotic ringspot virus, prune dwarf virus), while escaped sour cherry shrubs ( Prunus cerasus L.) were shown to contain Prunus necrotic ringspot virus at a rate of 40%. Wild-growing or escaped rose, apple, mountain ash, mazzard cherry or European dwarf cherry plants therefore seem not to be dangerous virus sources for cultivated apples and cherries, because of lack of virus infestation. The main danger of virus spread seems to originate from cultivated pome and stone fruit trees itself.

Summary Biological and serological methods for detecting PNRV, ApMV and PDV in plum trees were compared with regard to their reliability of diagnosis. Furthermore we carried out measurements of virus concentration in different test materials by means of latex test and ELISA, respectively. In these experiments we could find out relatively high concentrations of PNRV, ApMV and PDV in forced or dormant buds of leaves and/or petals, in petals in May as well as in young leaves. From this resulting both the latextest and the ELISA allowed to identify 100% of all diseased trees that were tested. Using older leaves the reliability of serological detection decreased somewhat, especially in the case of ApMV. The biological detection of PNRV as well as PDV by means of Cucumis sativus L. satisfied only in the period from September to Januar using buds forced in a greenhouse. This method is not to recommend at other times or for detecting ApMV. Other test materials of the tree were also not suitable.