Response of the pear rootstock to boron and salinity in vitro

The effects of boron and NaCl induced salinity on growth and mineral composition of the pear (Pyrus communis L.) rootstock OH × F 333 shoots cultured in vitro were investigated. Shoots were grown in vitro for seven weeks on a Murashige and Skoog medium containing two B concentrations (0.1 and 2 mM) combined with five NaCl concentrations (0, 10, 20, 40, and 80 mM). The longest shoots were produced at 0.1 mM B and 80 mM NaCl, but highest number of shoots were produced at 0.1 mM B and 0–20 mM NaCl. Inclusion of 20 and 40 mM NaCl in the culture medium significantly increased fresh mass of cultures compared to 0 mM NaCl for all B concentrations tested. The concentrations of P, K, Ca, Mg, Na, Fe, Mn and Zn of plants were affected by B and NaCl concentration of the medium.     

Effects of short light-dark regimes on in vitro shoot rooting of some fruit tree rootstocks

Experiments were carried out to evaluate the effects of 4/2 light-dark cycles (4 h of light followed by 2 h of dark) on the rooting responses of shoots cultivated in vitro of the fruit tree rootstocks GF 677 (peach × almond hybrid), Mr.S. 2/5 (Prunus cerasifera), MM 106 (apple Nothern Spy × Paradise M1) and BA 29 (Cydonia oblonga). Under this light regime rooting percentage of GF 677, Mr.S. 2/5 and MM 106 shoots reached 100 % as in the control treatment (16/8), while in BA 29 shoots, short light-dark cycles increased rooting response by about 65 %. Moreover, the shoots of all rootstocks submitted to the 4/2 cycle showed an appreciable increase in root number and length, and an earlier root emergence of about 4 – 5 d compared to the 16/8 cycle. Finally, rooting percentage of BA 29 shoots submitted to the 4/2 light regime and treated with 0.2 mg dm⁻³ indolebutyric acid (IBA), was equal to that reported with 0.4 mg dm⁻³ IBA under the 16/8 regime, indicating that the former light regime also amplified the rhizogenic effect of auxin.     

Responses of the cherry rootstock to salinity <Emphasis Type="Italic">in vitro</Emphasis>

The in vitro response of sweet cherry rootstock Gisela 5 (Prunus cerasus × Prunus canescens) to increasing concentrations of NaCl (0, 50, 100 and 150 mM) in the Murashige and Skoog culture medium was studied. Induced salinity reduced growth and chlorophyll content in shoots but had no effect on water content. The increase in malondialdehyde content indicated that salinity induced oxidative stress which was accompanied with the visible symptoms of salt injury in the shoots. Antioxidant enzymes, such as superoxide dismutase, ascorbate peroxidase, peroxidase, catalase, and glutathione reductase were also significantly elevated. Although no change was observed in the Cl concentration, Na concentration of shoots significantly rose and NaCl treatments impaired K, Ca and Mg nutrition and induced imbalance in K:Na and Na:Ca ratios.     

In vitro plantlet regeneration from cotyledon, hypocotyl and root explants of hybrid seed geranium

In vitro plantlet regeneration systems for the seed geranium (Pelargonium x hortorum Bailey) using cotyledon, hypocotyl and root explants were optimized by studying the influence of seedling age, growth regulators and excision orientation on organogenesis. Indole-3-acetic acid combined with zeatin yielded the highest rate of shoot production on cotyledon explants (0.2–2 shoots per explant). More shoots were produced on explants cut from the most basal region of cotyledons from 2 to 4-day-old seedlings than from older seedlings or more distal cut sites. Hypocotyl explants produced the highest number of shoots, up to 40 shoots per explant, on indole-3-acetic acid (2.8–5.6 mM) + zeatin (4.6 mM) or thidiazuron (4.5 mM). Maximum shoot formation (0.3–1.4 shoots per explant) on root explants occurred when they were cultured on medium containing zeatin. Regenerated shoots rooted best on a basal medium containing no growth regulators. There were substantial differences among cultivars in shoot formation from each of the explant systems.Abbreviations BA 6-benzylaminopurine - 2,4-d 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - NAA naphthaleneacetic acid - TDZ thidiazuron     

Response to increasing rates of boron and NaCl on shoot proliferation and chemical composition of in vitro kiwifruit shoot cultures

The in vitro response of kiwifruit (Actinidia deliciosa) to increasing concentrations of boron (B) and NaCl in the culture medium was studied. Kiwifruit shoot cultures were grown in vitro for 12 weeks on an MS medium containing two B concentrations (0.1 and 2 mM) combined with five NaCl concentrations (0, 10, 20, 40 and 80 mM). Kiwifruit produced the longest shoots with 2 mM B when NaCl concentration was 0--20 mM. More shoots were produced with 2 mM B for all NaCl treatments. More shoots were produced with 2 mM B and 10 and 20 mM NaCl. High B concentrations in the culture medium significantly increased shoot proliferation. Explants exhibited a moderate chlorotic appearance with 40 mM NaCl and shoots died with 80 mM NaCl. With 2 mM B, the B concentration of explants was 5--9X greater for the various NaCl treatments compared to the control. Increasing the NaCl concentration from 10 to 80 mM, resulted in higher Na and Cl concentrations in explants for all B treatments, while K and Ca concentrations decreased. Phosphorus concentration in the explants was significantly increased by increasing the NaCl concentration reaching a maximum value at 80 mM NaCl for the two B concentrations.     

Use of lysozyme for treatment of bacterial contamination in <Emphasis Type="Italic">in vitro</Emphasis> shoot cultures of fruit plants

Summary Use of lysozyme was tested for treatment of bacterial contaminations in in vitro shoot cultures of quince (Cydonia oblonga) ‘BA 29’ and the hybrid (Prunus persica × P. amygdalus) rootstock ‘GF 677’. Shoots which had been contaminated for about 1 yr by Bacillus circulans and Sphingomonas paucimobilis were treated in liquid culture, at pH 4.5, with 9–36 mg ml⁻¹ egg white lysozyme (EWL), and compared to each other and to untreated cultures for their growth, proliferation, and number of bacterial colony-forming units in the tissues. EWL did not negatively affect shoot growth up to 18 mg ml⁻¹; furthermore, the proliferation rates of EWL-treated shoots were sometimes higher than those of controls. In contrast, the concentration of 36 mg ml⁻¹ had some deleterious effect on the regrowth capacity and shoot production of ‘GF 677’ at the first subculture to solid medium after EWL, treatments. EWL had a simple bacteriostatic effect against Sphingomonas paucimobilis; in contrast, it was effective at 18 mg ml⁻¹ in eliminating Bacillus circulans in both ‘BA 29’ and ‘GF 677’ cultures, after optimal treatment duration.     

Metabolic response in roots of Prunus rootstocks submitted to iron chlorosis

Iron deficiency induces several responses to iron shortage in plants. Metabolic changes occur to sustain the increased iron uptake capacity of Fe-deficient plants. We evaluated the metabolic changes of three Prunus rootstocks submitted to iron chlorosis and their different responses for tolerance using measurements of metabolites and enzymatic activities. The more tolerant rootstocks Adesoto (Prunus insititia) and GF 677 (Prunus amygdalus × Prunus persica), and the more sensitive Barrier (P. persica × Prunus davidiana) were grown hydroponically in iron-sufficient and -deficient conditions over two weeks. Sugar, organic and amino acid concentrations of root tips were determined after two weeks of iron shortage by proton nuclear magnetic resonance spectroscopy of extracts. Complementary analyses of organic acids were performed by liquid chromatography coupled to mass spectrometry. The major soluble sugars found were glucose and sucrose. The major organic acids were malic and citric acids, and the major amino acid was asparagine. Iron deficiency increased root sucrose, total organic and amino acid concentrations and phosphoenolpyruvate carboxylase activity. After two weeks of iron deficiency, the malic, citric and succinic acid concentrations increased in the three rootstocks, although no significant differences were found among genotypes with different tolerance to iron chlorosis. The tolerant rootstock Adesoto showed higher total organic and amino acid concentrations. In contrast, the susceptible rootstock Barrier showed lower total amino acid concentration and phosphoenolpyruvate carboxylase activity values. These results suggest that the induction of this enzyme activity under iron deficiency, as previously shown in herbaceous plants, indicates the tolerance level of rootstocks to iron chlorosis. The analysis of other metabolic parameters, such as organic and amino acid concentrations, provides complementary information for selection of genotypes tolerant to iron chlorosis.     

Effect of NaCl and CaCl<Subscript>2</Subscript> on growth and contents of minerals,chlorophyll, proline and sugars in the apple rootstock M 4 cultured <Emphasis Type="Italic">in vitro</Emphasis>

The apple (Malus domestica Borkh) rootstock M 4 shoots were grown in vitro for 4 weeks on Murashige and Skoog (MS) medium containing three NaCl concentrations (35, 100 and 200 mM) in combination with two CaCl₂ concentrations (5 and 10 mM). Inclusion of 10 mM CaCl₂ in the medium, in the presence of 35 mM NaCl, significantly increased the number of shoots and the fresh mass compared to 5 mM CaCl₂. The number of shoots, length of shoots, and the fresh mass of cultures were very low in the presence of 100 and 200 mM NaCl, independently of CaCl₂ concentration of the medium. By increasing NaCl and CaCl₂ concentrations in the culture medium, contents of N, Na, Cl, proline and soluble sugars in plantlets increased, whereas K, Mg, B, Zn and chlorophyll content decreased in comparison to the control.     

Glutamine: A Suitable Nitrogen Source for Enhanced Shoot Multiplication in Cucumis sativus L.

Shoot tip explants of cucumber (Cucumis sativus L. cv. Poinsett 76) were cultured in vitro on Murashige-Skoog medium with L-glutamine, ammonium nitrate, adenine sulphate, asparagine, ammonium succinate, potassium nitrate and sodium nitrate as the nitrogen sources along with optimal concentration of 0.044 mM benzyladenine to study their effects on in vitro morphogenesis. The explants grown with 0.068 mM L-glutamine displayed the highest culture response (74.6 %) and greatest shoot number per explant (13.6) at the end of two subcultures. The explants cultured with other nitrogen sources resulted in low culture frequency and low number of shoots per explant accompanied by basal callusing and necrosis.     

Differential Response to Root-Knot Nematodes in Prunus Species and Correlative Genetic Implications

Responses of 17 Prunus rootstocks or accessions (11 from the subgenus Amygdalus and 6 from the subgenus Prunophora) were evaluated against 11 isolates of Meloidogyne spp. including one M. arenaria, four M. incognita, four M. javanica, one M. hispanica, and an unclassified population from Florida. Characterization of plant response to root-knot nematodes was based on a gall index rating. Numbers of females and juveniles plus eggs in the roots were determined for 10 of the rootstocks evaluated against one M. arenaria, one M. incognita, one M. javanica, and the Florida isolate. These 10 rootstocks plus Nemaguard and Nemared were retested by growing three different rootstock genotypes together in containers of soil infested individually with each of the above four isolates. Garfi and Garrigues almonds, GF.305 and Rutgers Red Leaf peaches, and the peach-almond GF.677 were susceptible to all isolates. Differences in resistance were detected among the other rootstocks of the subgenus Amygdalus. The peach-almond GF.557 and Summergrand peach were resistant to M. arenaria and M. incognita but susceptible to M. javanica and the Florida isolate. Nemaguard, Nemared, and its two hybrids G x N no. 15 and G x N no. 22 were resistant to all but the Florida isolate. In the subgenus Prunophora, Myrobalan plums P.1079, P.2175, P.2980, and P.2984; Marianna plum 29C; and P. insititia plum AD.101 were resistant to all isolates. Thus, two different genetic systems of RKN resistance were found in the subgenus Amygdalus: one system acting against M. arenaria and M. incognita, and another system also acting against M. javanica. Prunophora rootstocks bear a complete genetic system for resistance also acting against the Florida isolate. The hypotheses on the relationships between these systems and the corresponding putative genes of resistance are presented.     

Micropropagation of Sesbania sesban from the Mature Tree-Derived Explants

The nodal and internodal explants excised from the orthotropic shoots of Sesbania sesban var. bicolor elicited the development of shoots directly from the explants as well as via an intervening callus phase on Nitsch (N) medium. On benzyladenine (BA) supplemented media, the adventitious shoot buds developed involving a callus phase. An average of 8.9 ± 4.1 shoots developed per nodal explant on N medium containing 0.5 mg dm⁻³ BA in 95 % cultures, whereas 65 % cultures of internodal explants developed shoots with an average of 5.9 ± 3.6 shoots per explant on N medium supplemented with 1.0 mg dm⁻³ BA. On kinetin (Kn) supplemented medium shoots developed directly from the surface of both the explants at all the concentrations tried. Nodal explants on N medium supplemented with 1.5 mg dm⁻³ Kn developed an average of 12.5 ± 7.9 shoots per explant in 100 % cultures, while internodal explants induced an average of 11.6 ± 7.4 shoots per explant in 75 % explants at 0.5 mg dm⁻³ Kn. The in vitro regenerated shoots developed roots when implanted on N medium supplemented with 2 mg dm⁻³ indole-3-butyric acid (IBA), after 30 d of inoculation. The in vitro developed plantlets were initially acclimatized under controlled conditions for four months, prior to their transfer to the field. This revised version was published online in July 2006 with corrections to the Cover Date.     

A comparative histological study between normal and fasciated shoots of <Emphasis Type="Italic">Prunus avium</Emphasis> generated <Emphasis Type="Italic">in vitro</Emphasis>

A combination of Murashige and Skoogs medium and N⁶–benzyladenine (BA) at various concentrations (0, 0.1, 0.25, 0.5, 0.75, 1.0, 1.25 and 1.5 mg l^–1) was supplied to shoot tips from root cuttings of a 50-year-old wild-cherry tree (Prunus avium). The concentration of BA in the growing medium was a determining factor with respect to the number of proliferated shoots per explant in vitro.Normal and fasciated shoots were generated when BA was present at 0.5, 0.75, 1.0 and 1.25 mg l^–1 in the medium and the mean numbers of normal shoots per explant were 3.63, 5.37, 8.93 and 7.30 respectively, and those of the fasciated shoots per explant were 0.03, 0.1, 0.47 and 0.4 respectively. Anatomical analysis by confocal microscopy of sections of paraffin-embedded specimens revealed that the cell structure and organization of the cortex and vascular cylinder in the fasciated shoots was similar to that in normal shoots. However, the cross-sectional area of stem of the fasciations was apparently greater than that of the normal shoots. In particular, the volume of vascular tissues, of pith and of some individual parenchyma cells in the cortex and pith was apparently greater in fasciated shoots than in normal shoots. Increases in cytokinesis and morphogenetic activity, such as the development of callus-like regions and the formation of adventitious shoots, were observed in the cortex and pith throughout the fasciations. The fasciated shoots had numerous buds and initiating new shoots at their apices while normal shoots had a single dominant axial bud.     

Response to increasing rates of boron and NaCl on shoot proliferation and chemical composition of in vitro kiwifruit shoot cultures

The in vitro response of kiwifruit (Actinidia deliciosa) to increasing concentrations of boron (B) and NaCl in the culture medium was studied. Kiwifruit shoot cultures were grown in vitro for 12 weeks on an MS medium containing two B concentrations (0.1 and 2 mM) combined with five NaCl concentrations (0, 10, 20, 40 and 80 mM). Kiwifruit produced the longest shoots with 2 mM B when NaCl concentration was 0--20 mM. More shoots were produced with 2 mM B for all NaCl treatments. More shoots were produced with 2 mM B and 10 and 20 mM NaCl. High B concentrations in the culture medium significantly increased shoot proliferation. Explants exhibited a moderate chlorotic appearance with 40 mM NaCl and shoots died with 80 mM NaCl. With 2 mM B, the B concentration of explants was 5--9X greater for the various NaCl treatments compared to the control. Increasing the NaCl concentration from 10 to 80 mM, resulted in higher Na and Cl concentrations in explants for all B treatments, while K and Ca concentrations decreased. Phosphorus concentration in the explants was significantly increased by increasing the NaCl concentration reaching a maximum value at 80 mM NaCl for the two B concentrations.     

Improving micropropagation: effect of Azospirillum brasilense Sp245 on acclimatization of rootstocks of fruit tree

Abstract

The effect of Azospirillum brasilense Sp245 on the micropropagation of three fruit rootstocks: Mr.S 2/5 plum (Prunus cerasifera×P. spinosa), GF 677 hybrid (Prunus persica×P. amigdalus), and MM 106 apple (Northen Spy×M1) was assessed. Rooted shoots were treated with 3×10⁷ of Sp245 cells during transplantation from in vitro cultures to the acclimatization phase. After 60 days, growth parameters were positively affected by Sp245 inoculum. In the case of Mr.S 2/5, an increase in rootstock stem length and node number by 37% and 42%, respectively, compared to the control was noted. In the case of GF 677, the bacterial inoculum increased stem length and node number by up to the 75% and 65%, respectively, compared to the control. The inoculum did not exert on MM 106 for both parameters suggesting that the effects of Sp245 could depend on a specific clone-microbe association. In all cases, however, a higher vigor, consistent with a wider leaf area, was present in the inoculated plantlets demonstrating that the use of Azospirillum can significantly contribute to optimize plant performance during the phase of adaptation of plants to post-vitrum conditions.     

Design of tissue culture media for efficient Prunus rootstock micropropagation using artificial intelligence models

Establishing optimized protocols for micropropagation of some economical plants, such as Prunus sp., is still one of the most important challenges for in vitro plant culture researchers. As an example, micropropagation of GF677 hybrid rootstocks (peach × almond) are extremely dependent on the medium ingredients and a large undesirable proportion of GF677 shoots need to be discarded as a result of hyperhydricity and chlorosis. In this study, an artificial intelligence technique—specifically neurofuzzy logic—has been employed, as a modeling tool, to increase knowledge on the effect of 8 ion macronutrients (NH₄ ⁺, NO₃ ^?, Ca²⁺, K⁺, Mg²⁺, SO₄ ^2?, PO₄ ^2? and Na⁺; as inputs) on three growth parameters (outputs): total number of shoots per explant, healthy number of shoots per explant, and their bud number. The model delivered new insights, by three sets of IF–THEN rules, pinpointing the key role of NO₃ ^? and their interactions (NO₃ ^? × Ca²⁺ and NO₃ ^? × Ca²⁺ × K⁺) on all growth parameters measured. All growth parameters showed a high correlation ratio between experimental and predicted values being 77.48, 91.78 and 90.78 for total shoots, healthy number and bud number, respectively. Regression coefficients higher than 77 % together with statistical significant ANOVA (p < 0.01) indicated good performance of neurofuzzy logic models. Moreover, The model also can be used for inferring the best combination of ion concentrations to obtain high quality GF677 micropropagated shoots. In conclusion, we assess the utility of neurofuzzy logic technology in modeling complex databases, identifying new complex interactions among macronutrients, and inferring new results and valuable knowledge, which can be applied to design new plant tissue culture media and improve plant micropropagation.     

Clonal micropropagation of <Emphasis Type="Italic">Crataeva adansonii</Emphasis> (DC.) Prodr.: A multipurpose tree

Summary A method of micropropagation through multiple shoot formation from axillary buds of mature tree and rootstock growths of Crataeva adansonii (DC.) Prodr. (a multipurpose tree) has been developed. Factors affecting multiplication rate included season, age of explant source, explant type, type of bud, position of bud on the foliage twig, type of medium, various additives, and explant implantation on the medium. The maximum number of buds was produced from the sixth to 10th axillary buds taken from foliage twigs of 40–50-d-old rootstock growth in the months of October to December. At this time of the year the contamination was minimum. Optimum response was recorded on Murashige and Skoog (MS) medium supplemented with 6-benzyladenine (BA; 3 mgl⁻¹, 13.3μM) and 1-naphthaleneacetic acid (NAA; 0.05 mgl⁻¹, 0.27 μM) after 21 d of culture. Shoot buds were further multiplied and maintained on BA (1–1.5 mgl⁻¹, 4.4–6.7 μM) while shoots elongated on BA (0.1–0.5 mgl⁻¹, 0.44–2.2 μM) supplemented medium. The number of shoots was further multiplied by using nodal segments of in vitro-regenerated shoots as microcuttings and repeated subculturing of stumps after excising the microshoots. In vitro rooting on growth regulator-free MS medium was possible with 70% of microshoots after 4 wk. From one nodal segment 150 plantlets were produced within 14 wk.     

<Emphasis Type="Italic">In vitro</Emphasis> propagation of northern red oak (<Emphasis Type="Italic">Quercus rubra</Emphasis> L.)

In vitro propagation of northern red oak (Quercus rubra) shoots was successful from cotyledonary node explants excised from 8-wk-old in vitro grown seedlings. Initially, four shoots per explant were obtained on Murashige and Skoog (MS) medium supplemented with 4.4 μM 6-benzylaminopurine (BA), 0.45 μM thidiazuron (TDZ), and 500 mg l⁻¹ casein hydrolysate (CH) with a regeneration frequency of 64.7% after 3 wk. Subculturing explants (after harvesting shoots) to fresh treatment medium significantly increased shoot bud regeneration (16.6 buds per explant), but the buds failed to develop into shoots. A higher percentage (73.3%) of the explants regenerated four shoots per explant on woody plant medium (WPM) supplemented with 4.4 μM BA, 0.29 μM gibberellic acid (GA₃), and 500 mg l⁻¹ CH after 3 wk. Explants subcultured to fresh treatment medium after harvesting shoots significantly increased shoot regeneration (16 shoots per explant). Shoot elongation was achieved (4 cm) when shoots were excised and cultured on WPM supplemented with 0.44 μM BA and 0.29 μM GA₃. In vitro regenerated shoots were rooted on WPM supplemented with 4.9 μM indole-3-butyric acid. A higher percentage regeneration response and shoot numbers per explant were recorded on WPM supplemented with BA and GA₃, than on MS medium containing BA and TDZ. Lower concentrations of BA and GA₃ were required for shoot elongation and prevention of shoot tip necrosis. Each cotyledonary node yielded approximately 20 shoots within 12 wk. Rooted plantlets were successfully acclimatized.     

Micropropagation of Sesbania rostrata from the Cotyledonary Node

Multiple shoots were induced from the cotyledonary nodes derived from seedling of Sesbania rostrata on Nitsch (1969; N) medium supplemented with various concentrations of benzyladenine (BA). 1 mg dm⁻³ BA proved to be the best, eliciting 5.8 ± 1.0 shoots per explant in 100 % cultures. The elongation of shoots was best at 2.0 mg dm⁻³ BA. The shoot proliferation capacity increased to 7.5 shoots per explant following transfer of explants to the fresh shoot multiplication medium (MS + 1.0 mg dm⁻³ BA), after an initial incubation of 30 d. To further enhance number of shoots per explant an alternative strategy of cultivation of mother explant on fresh shoot multiplication medium after excision of shoots was adopted. Following the repeated harvesting of shoots an average of 33 shoots per explant could be obtained. The in vitro regenerated shoots produced roots when transferred to half-strength MS medium supplemented with 3 % sucrose and 1 mg dm⁻³ IBA. The developed plantlets were planted in the soil and transferred to the field after an acclimatization period of 3 – 4 months. These plants produced flowers and fruits in the field and exhibited the development of prominent and more organized stem nodules as compared to the in vivo raised plants of the same age. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of ethylene on in vitro rooting of GF 677 (Prunus persica × Prunus amygdalus) hybrid peach rootstock

Summary Treatments differing from each other for the type of tube closure (i.e., cotton plug for free gas exchange, airtight rubber cap, and rubber cap with ethysorb) and/or rooting culture medium (i.e., enriched or not by 25 to 100 μM acetylsalicylic acid) were compared for their effects on gaseous composition of the culture atmosphere and microcutting rooting of the GF 677 (Prunus persica × Prunus amygdalus) hybrid. Rubber capping, which leads to rapid ethylene accumulation inside tubes, strongly reduced rooting time and in some cases enhanced final rooting percentage over that of cotton plugs. Ethysorb almost completely absorbed ethylene produced by shoots, which showed lower rooting percentages within 9 d than microcuttings cultured in the absence of ethysorb. In contrast, no significant difference in rooting was found between the two treatments after 14 d. Carbon dioxide concentration was similar in all treatments within 5 to 9 d and seemed to be ineffective for rooting. The influence of acetylsalicylic acid on rooting was unclear. Root number and length were not significantly influenced by the treatments. These results demonstrate that the use of airtight closures, leading to rapid ethylene accumulation, can reduce time of rooting expression for GF 677 microcuttings. However, free gas exchange towards the end of the rooting period (from Day 9 to Day 14) is advisable to prevent leaf yellowing. No significant difference in plantlet survival and growth after transfer ex vitro was found among treatments.     

Differences in Strain Virulence of the European Stone Fruit Yellows Phytoplasma and Susceptibility of Stone Fruit Trees on Various Rootstocks to this Pathogen

Twenty strains of the European stone fruit yellows (ESFY) phytoplasma showed great differences in virulence when examined by graft inoculation of trees on peach, peach hybrid GF 677 and P. 'Marianna' GF 8/1 rootstocks. The most virulent strains killed all trees on peach rootstocks whereas the mild strains did not cause mortality but induced only mild foliar symptoms and slightly reduced vigour. Virulence often depended on the pathogen–scion combination and was in several cases most severe when the scion consisted of the original host of the pathogen. To examine resistance in stone fruits, trees on a total of 23 rootstocks were inoculated with the ESFY strains. Trees on the Prunus domestica stocks Ackermann's, Brompton and P 1275 and on Prunus cerasifera stock Myrabi were little affected. Slightly more damage occurred in trees on rootstocks GF 677, GF 8–1, and the P. insititia stocks St Julien A and St Julien GF 655/2. Ishtara, P. cerasifera stock Myrobalan, and peach rootstocks Higama and GF 305 were shown to be moderately susceptible and a high susceptibility was found in trees on peach rootstocks Montclar, peach seedling, Rutgers Red Leaf, and Rubira, on apricot seedlings and St Julien 2. Of flowering cherry trees on various rootstocks, the least susceptible were those on Gisela 3 and F 12/1 whereas Gisela 1, Weihroot 158 and Gisela 5 were more affected. Phytoplasmas were detected by either DAPI (4'-6-diamidino-2-phenylindole) staining or polymerase chain reaction in all rootstocks and scions tested. However, detection frequency and phytoplasma concentrations were usually lower in the more tolerant hosts than in susceptible genotypes.