Susceptible Rootstocks Research Articles

Two distinct major QTL for resistance to fire blight co-localize on linkage group 12 in apple genotypes ‘Evereste’ andMalus floribundaclone 821

Fire blight, caused by the bacterium Erwinia amylovora, is one of the most destructive diseases of apple (Malus xdomestica) worldwide. No major, qualitative gene for resistance to this disease has been identified so far in apple. A quantitative trait locus (QTL) analysis was performed in two F1 progenies derived from two controled crosses: one between the susceptible rootstock cultivar 'MM106' and the resistant ornamental cultivar 'Evereste' and the other one between the moderately susceptible cultivar 'Golden Delicious' and the wild apple Malus floribunda clone 821, with unknown level of fire blight resistance. Both progenies were inoculated in the greenhouse with the same reference strain of E. amylovora. The length of stem necrosis was scored 7 and 14 days after inoculation. A strong QTL effect was identified in both 'Evereste' and M. floribunda 821 at a similar position on the distal region of linkage group 12 of the apple genome. From 50% to 70% of the phenotypic variation was explained by the QTL in 'Evereste' progeny according to the scored trait. More than 40% of the phenotypic variation was explained by the M. floribunda QTL in the second progeny. It was shown that 'Evereste' and M. floribunda 821 carried distinct QTL alleles at that genomic position. A small additional QTL was identified in 'Evereste' on linkage group 15, which explained about 6% of the phenotypic variation. Although it was not possible to confirm whether or not 'Evereste' and M. floribunda QTL belonged to the same locus or two distinct closely related loci, these QTL can be valuable targets in marker-assisted selection to obtain fire blight resistant apple cultivars and form a starting point for discovering the function of the genes controlling apple fire blight resistance.

Fire Blight Resistance of Budagovsky 9 Apple Rootstock.

Erwinia amylovora, the causal agent of fire blight, can cause a fatal infection of apple rootstocks known as rootstock blight. Budagovsky 9 (B.9) apple rootstock is reported to be highly susceptible when inoculated with E. amylovora, although results from multiple trials showed that B.9 is resistant to rootstock blight infection in field plantings. Conflicting results could stem from genetic variation in the B.9 population, appearing as phenotypic differences in rootstock material. However, genetic testing, using 23 microsatellite loci, confirmed the clonal uniformity of B.9 in commerce. Variation in growth habit between B.9 rootstocks originating from two nurseries also has been discounted as a source of disease resistance. Instead, results indicate a possible novel resistance phenotype in B.9 rootstock. B.9 rootstock was susceptible to leaf inoculation by E. amylovora, statistically similar to the susceptible rootstock Malling 9 (M.9). Conversely, inoculation assays targeting woody 4- to 5-year-old tissue revealed a high level of resistance in B.9, whereas M.9 remained susceptible. Although the mechanism by which B.9 gains resistance to E. amylovora is unknown, it is reminiscent of age-related resistance, due to an observed gain of resistance in woody rootstock tissue over succulent shoot tissue. Durable fire blight resistance correlated with tissue development could be a valuable tool for rootstock breeders.

Desenvolvimento de Pantoea agglomerans em diversas temperaturas, pH e concentrações de carboxi-metil-celulose e o seu impacto no controle de Rosellinia necatrix

Rosellinia necatrix Prill induz a podridão branca da raiz da macieira, doença que causa severa perda em pomares localizados no sul do Brasil. O manejo da doença é principalmente preventivo e inclui o uso de porta-enxertos resistentes e a fumigação do solo. A proteção das mudas de macieiras antes do plantio com um isolado antagonista de Pantoea agglomerans foi recentemente proposto para reduzir a incidência da doença. Os objetivos desta pesquisa foram caracterizar o relacionamento entre o patógeno e a bactéria antagonista; a produção de metabólitos biológicamente ativos pelo isolado bacteriano e a sua ação sobre o patógeno; o efeito do pH, da temperatura e de carboxi-metil-celulose (CMC) sobre o crescimento do antagonista e do patógeno, isolados ou não e no controle da doença. Os resultados demonstraram que o crescimento de P. agglomerans foi maior em pH 5,5 e 6,0 e nas temperaturas de 20 °C e 30 °C. O crescimento micelial de R. necatrix foi inibido em meio de cultura e previamente colonizado pelo antagonista e com CMC nas concentrações de 0,25 e 0,5 %. A proteção das macieiras da infecção por R. necatrix foi observada quando utilizadas as concentrações de 10(7); 10(8) e 10(9) cel/mL e nas diferentes concentrações de CMC. Maior desenvolvimento radicular das macieiras foi constatado em todas as concentrações de CMC quando usada a concentração de 10(9) cel/mL. A formulação de P. agglomerans com CMC tornará possível a incorporação desta estratégia de controle ao manejo integrado da podridão branca das raízes da macieira no Sul do Brasil.

Effect of Soybean Cyst Nematode on Fatty Acid Levels of Soybean Seed

Growing soybean cyst nematode (SCN, Heterodera glycines Ichinohe) resistant varieties in fields infested with SCN can improve yield, but the impact of SCN on seed quality of lines with modified seed composition is unknown. The objective of this study was to determine if SCN resistant or susceptible rootstock influenced the composition of seed of soybean lines developed for desirable elevated levels of unsaturated fatty acids (FA). Five soybean lines with modified or normal levels of FA were Y-grafted to generate a 5-×-5 combination of plants with a SCN susceptible (SCNS) or resistant (SCNR) rootstock having a self-grafted branch, a scion (alien) grafted branch, or non-grafted branch; control plants were not grafted. In 2004 and 2005 plants were transplanted into blocks of Asgrow 3302 (SCNR) or Asgrow 3701 (SCNS) verified to be infested with SCN. Seeds were harvested at physiologic maturity (R8) in 2004 and 2005 and analyzed for five unsaturated FA. Differences in FA levels between 2004 and 2005 were significant (P < 0.05), but there were no significant (P = 0.05) block × year effects. Significant differences were observed between the FA levels of seed harvested from non-grafted control plants and self-grafted or non-grafted branches of grafted plants within lines. Seed oleic acid of the mid-oleic line S03-1379-2 was significantly greater when grafted onto an SCNR line than when grafted onto SCNS lines. The low linolenate trait appeared to be insensitive to SCN presence as seed linolenic acid of the low linolenic line IA-3017 was not different when grafted onto SCNR or SCNS lines. SCN can negatively impact the expression of seed quality of soybean if grown where SCN infestations are evident. Accepted for publication 8 September 2008. Published 3 November 2008.

Phoma tracheiphila

<i>Phoma tracheiphila</i>

Inducible Proteins in Citrus Rootstocks with Different Tolerance Towards the Root Rot Pathogen Phytophthora palmivora

Activities of defence-related proteins (β-1,3-glucanases, chitinases and peroxidases) and concentrations of total soluble phenolics were measured in roots and leaves of non-infected and infected plants to investigate the response of different citrus rootstock genotypes to the root rot pathogen Phytophthora palmivora Butler. Infection with the pathogen increased concentrations of total proteins, total phenolics and β-1,3-glucanase activity in roots of all genotypes, and increases were associated with the extent of root mass reductions and thus susceptibility of the plants. Root chitinase and root peroxidase levels were slightly reduced or unaltered upon infection. β-1,3-Glucanase activity was also elevated in leaves of infected plants, but increases did not differ between tolerant and susceptible rootstocks. Effects of root infection on leaves were typically the reverse of effects on roots for chitinase- and peroxidase levels and more pronounced in susceptible rootstock genotypes. Although differences in enzyme expression were observed between susceptible and tolerant citrus seedlings, effects were usually associated with disease progression, and not with resistance to P. palmivora. It is suggested that increased activities of the proteins and soluble phenolics studied are not implicated in the primary defence to Phytophthora root diseases, but may contribute to the inhibition of the pathogen during infection in tolerant citrus.

Microbial community development in the rhizosphere of apple trees at a replant disease site

Apple replant disease (ARD) is a complex syndrome that affects young trees in replanted orchard sites causing necrotic lesions on feeder roots, stunted tree growth and reduced cumulative yields. Use of ARD-tolerant rootstocks is an emerging control strategy. We studied the bacterial, fungal, and oomycetes populations in the rhizosphere of five rootstock cultivars (M.7, M.26, G.16, G.30 and CG.6210) planted into the old tree row or grass lanes of a previous orchard in Ithaca, NY, to better understand the role of rhizosphere microbial communities in the prevalence and control of ARD. The possible involvement of antagonistic Pseudomonas species, Pythium spp., Phytophthora spp. and rhizosphere cyanide concentrations in ARD were also examined. The rootstocks M.7, M.26 and G.16 were susceptible to ARD, while G.30 and CG.6210 were more tolerant. Tree growth on the rootstocks M.7, M.26 and G.16 was reduced by 10% when planted in the old tree rows, but this did not significantly reduce yields in the first fruiting year. The susceptible rootstocks, M.7 and M.26, supported higher densities of culturable rhizosphere fungi and bacteria than G.16, G.30 and CG.6210. Over 2 years, microbial densities were highest in July, lower in May and lowest in September. The composition of bacterial and fungal communities in the rhizosphere was highly variable and changed over seasons and years, as assessed by terminal restriction fragment length polymorphism (T-RFLP) analyses. Initial differences in fungal rhizosphere communities between the two planting positions converged 2 years after the trees were replanted. In contrast, the bacterial rhizosphere community composition still differed significantly between the two planting positions 3 years after the orchard was replanted. The bacterial and fungal rhizosphere community compositions of susceptible rootstocks, M.7 and M.26, differed from those of the tolerant rootstocks, G.30 and CG.6210; G.16, differed from all the other rootstocks. The observed effects of rootstocks, planting positions and time on microbial community composition were small relative to the high variability observed overall. Pythium spp. and Phytophthora spp. infestations were high and similar for all rootstocks and planting positions. Neither potentially antagonistic Pseudomonas nor rhizosphere cyanide concentrations appeared to be involved in the ARD-complex at the studied site. Avoiding replanting into the old tree rows coupled with use of tolerant rootstocks appear to be the best strategies for reducing ARD in replanted orchards. Changes in rhizosphere microbial communities are among the many factors that contribute to improved tree growth when these management strategies are used.

Phytosiderophores released by graminaceous species promote 59Fe-uptake in citrus

Chlorosis-susceptible fruit trees growing on calcareous soils have been observed to recover in the presence of grass cover species. However, the physiological mechanisms behind this phenomenon are only scarcely understood. An investigation was carried out to verify whether citrus plants can use 59Fe solubilized from a sparingly soluble source by the phytosiderophores (PS) released from graminaceous species. Experiments were performed in hydroponics, using two citrus rootstocks differing in their sensitivity to Fe-deficiency in the field (Poncirus trifoliata × Citrus paradisi, citrumelo “Swingle”, highly susceptible, and Citrus aurantium L., moderately tolerant). Barley (Hordeum vulgare L., cv Europa) was used as a model species for PS-releasing graminaceous plants. Fe-deficient citrus plants increased 59Fe-uptake from 59Fe-hydroxide supplied inside a dialysis tube, when Fe-deficient barley plants or PS-containing barley root exudates were present in the uptake solution, this effect being particularly evident for the susceptible rootstock. 59Fe-uptake from 59Fe-hydroxide was also enhanced in Fe-deficient citrumelo “Swingle” in the presence of Fe-deficient Poa pratensis L. and Festuca rubra L., two perennial grasses normally grown in association with fruit trees; no effect was found when Fe-sufficient grasses were employed. The uptake of 59Fe by the susceptible citrus rootstock increased in proportion to the amount of 2′-deoxymugineic acid (DMA), the major PS released by Fe-deficient F. rubra, present in the uptake solution. The beneficial effect of F. rubra or P. pratensis was evident from the leaf re-greening observed when Fe-deficient citrumelo “Swingle” plants were grown in association with the grasses in pots filled with a calcareous soil. Leaf re-greening was not observed when citrumelo “Swingle” plants and yellow stripe 3 (ys3) maize (Zea mays L.) mutant plants, unable to release PS, were co-cultivated in pots filled with calcareous soil, unless exogenous PS were added to the soil. Results indicate that graminaceous cover species can improve the Fe-nutrition of fruit trees grown on calcareous soils by enhancing Fe-availability.

Field Trials of Plum Clones Transformed with the Plum pox virus Coat Protein (PPV-CP) Gene.

Transgenic clones C2, C3, C4, C5, C6, and PT-6, of plum (Prunus domestica L.) transformed with the coat protein (CP) gene of Plum pox virus (PPV), PT-23 transformed with marker genes only, and nontransgenic B70146 were evaluated for sharka resistance under high infection pressure in field trials in Poland and Spain. These sites differed in climatic conditions and virus isolates. Transgenic clone C5 showed high resistance to PPV at both sites. None of the C5 trees became naturally infected by aphids during seven (Spain) or eight (Poland) years of the test, although up to 100% of other plum trees (transgenic clones and nontransgenic control plants) grown in the same conditions showed disease symptoms and tested positively for PPV. Although highly resistant, C5 trees could be infected artificially by chip budding or via susceptible rootstock. Infected C5 trees showed only a few mild symptoms on single, isolated shoots, even up to 8 years post inoculation. These results clearly indicate the long-term nature and high level of resistance to PPV obtained through genetically engineered resistance.

Influence of Rootstocks on the Resistance of Grafted Cucumber (Cucumis sativus L.) Scions to Powdery Mildew (Podosphaera xanthii U. Braun &amp; N. Shishkoff)

Whether or not powdery mildew resistant and susceptible rootstock cultivars would impart these characteristics to the grafted cucumber scions was investigated. Two newly released pumpkin rootstock cultivars used for bloomless cucumber production, ‘Tokiwa Power Z’ and ‘White Power’, were found to be highly resistant to powdery mildew from the seedling stage (tested parts: cotyledons) to the adult stage (tenth true leaves) at cool to moderate temperatures. The accession PPMR-1, a blooming rootstock, showed moderate to high levels of resistance, especially during the adult stage. The conventional bloomless rootstock ‘Hikari Power Gold’ and the conventional blooming rootstock ‘Shin-tosa’ were found to be susceptible. The rootstocks had little to no influence on the resistance to powdery mildew of the grafted scions at younger stages, even though the levels of resistance of the rootstock cultivars differed considerably. Scions, grafted onto PPMR-1 or ‘Shin-tosa’ rootstocks, showed increasing powdery-mildew resistance or tolerance with growth (tested parts: fifth and seventeenth true leaves). The powdery-mildew-resistant ‘Tokiwa Power Z’ and ‘White Power’ did not impart high levels of resistance to scions, whereas ‘Hikari Power Gold’, which is used for bloomless cucumber production, reduced the resistance of scions to powdery mildew. These results indicate that rootstock cultivars, including certain pumpkin rootstocks, e.g., PPMR-1, can alter the powdery-mildew resistance or tolerance of cucumber scions at the adult stage.

First record of a Hop stunt viroid variant on Nagpur mandarin and Mosambi sweet orange trees on rough lemon and Rangpur lime rootstocks

Citrus rootstocks play a significant role in tree health and productivity. Viroids (subviral pathogens) pose a major threat to trees and susceptible rootstocks, as they bring about slow decline (Ahlawat, 1997). In a survey of citrus orchards in certain parts of Central India, symptoms of bark scaling, bark splitting and leaf yellowing were observed on Nagpur mandarin ( Citrus reticulata ) (seven plants) and Mosambi ( Citrus sinensis ) (10 plants) grafted on rough lemon and Rangpur lime rootstocks. Following grafting of scions from all these plants onto Etrog citron ( Citrus medica ), an indicator host for Citrus exocortis viroid (CEVd), an unusual symptom of reduced size and yellowing of newly emerging leaves developed. This prompted further investigation. Electrophoresis under denaturing conditions of total nucleic acid, extracted from the grafted Etrog citron plants with symptoms, indicated the presence of viroidlike low molecular weight (LMW) RNA in all samples. These LMW RNAs were used in RT-PCR using a set of citrus viroid-specific primers (Ramachandran et al ., 2003). Amplicons ≈ 300 bp were obtained in all samples with primers specific to Hop stunt viroid (HSVd), while no amplification was observed with CEVd-specific primers. Cloning and sequencing of these amplicons identified a viroid RNA (Accession No. AY237168) with 100% sequence similarity to HSVd-Ca905 (Accession No. AF131250; Reanwarakorn & Semancik, 1999) and HSVdycv (Accession No. AJ490824; Roy & Ramachandran, 2003). This constitutes the first isolation of an HSVd isolate (HSVd-RL) from citrus rootstocks showing bark scaling and splitting symptoms in Central India. This result suggests that HSVd is also a component in producing bark-scaling symptoms in rootstocks which were previously considered to be due to CEVd-infection alone. This report indicates the need for testing of citrus plants for HSVd in budwood certification and germplasm evaluation.

Analogues of virus resistance genes map to QTLs for resistance to sharka disease in Prunus davidiana

Plum pox virus (PPV), the causative agent of sharka disease in Prunoideae, is one of the most serious problems affecting stone fruit production in Europe and America. Resistance to PPV was previously described in a Prunus davidiana clone, P1908, and introduced into peach (Prunus persica) genotypes. Genetic resistance to PPV displays a complex pattern of quantitative inheritance. An analysis of quantitative trait loci (QTLs) for resistance was performed on an F1 interspecific peach population obtained from a cross between the susceptible nectarine cultivar Summergrand and P. davidiana. The hybrids were graft-inoculated with PPV in duplicate following a classical procedure. The incidence of infection was evaluated four times, over two vegetative cycles, by symptom observation and enzyme-linked immunoadsorbent assays (ELISA). Restriction of systemic downward movement of the PPV virus was also evaluated by testing the susceptible rootstocks. Using both analysis of variance and non-parametric tests, six genomic regions involved in PPV resistance were detected. Depending on the scoring data considered, between 22 and 51% of the phenotypic variance could be explained by the quantitative model. One QTL, located in the distal region of linkage group 1, maps in a genomic region that is syntenic to the location of a resistance gene previously identified in the apricot cv. Goldrich. Some QTLs appeared to be temporally specific, reflecting the environmental dependence of PPV-resistance scoring. Candidate gene fragments were amplified by PCR, isolated and mapped on the peach interspecific linkage map. We report here the co-localization of three analogues of virus resistance genes with two distinct genomic regions linked to PPV resistance in P. davidiana.

SEOPA-1 and GOLGI-2 Apricot Seedlings Are Resistant to Plum Pox Virus

SEOPA-1 and GOLGI-2 Apricot Seedlings Are Resistant to Plum Pox Virus

Evaluation of Soybean Resistance to Sclerotinia Stem Rot Using Reciprocal Grafting.

Sclerotinia stem rot of soybean is one of the major soybean diseases in the north central region of the United States. One disease management option is to plant cultivars that have resistance. Some sources of partial resistance have been identified, but information pertaining to the nature of resistance is limited. The objective of this study was to determine if the expression of resistance is dictated by shoots of resistant plants and if this can be altered by using resistant and susceptible soybean genotypes grafted in different shoot and rootstock combinations of self-, single-, or double-shoot grafts. After successful grafts were made, several experiments were conducted using different inoculation techniques and soybean genotypes. In one experiment, cotyledons were inoculated with a plug of fungal mycelium, plants were incubated in a mist chamber for 23 h, and plant survival was recorded over time. Based on seven grafting combinations of cross- and self-grafted plants using two soybean cultivars, grafts with NKS19-90 (partially resistant) as shoots had greater (P ≤ 0.05) plant survival at 3, 4, and 5 days after inoculation than the other graft combinations. In another experiment, a total of 17 graft combinations were generated using resistant plant introductions and two susceptible cultivars. Resistant self-grafts of the plant introductions had greater (P ≤ 0.05) plant survival (mean = 75%) than self-grafts of the susceptible cultivars (mean = 15%) at 5 days after inoculation. Inter-genotypic grafts with resistant shoots had greater (P ≤ 0.05) plant survival (mean = 65%) than those in reciprocal combinations (mean = 8%) 5 days after inoculation. A cut stem inoculation method was used to test graft combinations of one resistant and two susceptible cultivars. Grafts with susceptible shoots of cvs. Williams 82 and Asgrow 2242 had greater (P < 0.05) lesion lengths (mean = 13.2 cm) than shoots of NKS19-90 (mean = 9.2 cm) regardless of the rootstock 15 days after inoculation. In a double-graft experiment, shoots of both NKS19-90 and Williams 82 were grafted to either NKS19-90 or Williams 82 rootstocks. Regardless of the rootstock, the shoots of Williams 82 died while shoots of NKS19-90 survived. For all the experiments, resistance was greater when the grafted shoot came from a resistant source on a susceptible rootstock compared with the reciprocal combination regardless of the type of grafting technique or inoculation method.

Effect on resistance to Tylenchulus semipenetrans of hybrid citrus rootstocks subjected to continuous exposure to high population densities of the nematode

Experiments simulating interplanting of resistant rootstocks with susceptible rootstocks that maintain high population densities of Tylenchulus semipenetrans in field soil were carried out in microplots at two locations, and in an naturally infested orchard. Selections of Cleopatra mandarin (03) × Poncirus trifoliata (01) 03.01.5 and 03.01.13, Citrus volkameriana (23) × P. trifoliata 23.01.17, Troyer citrange (02) × Cleopatra mandarin 02.03.24, Troyer citrange × Common mandarin (04) 02.04.18, King mandarin (05) × P. trifoliata 05.01.7, and Carrizo citrange were exposed to continuous high population densities of a population of the Mediterranean biotype of T. semipenetrans. The selection 23.01.17 retained its resistance in the microplots and in the field (< 1.2% females and eggs per gram fibrous root of those on Carrizo citrange). The selection 03.01.5 also retained its resistance in the microplots at Moncada (< 0.5% females and eggs per gram fibrous root of those on Carrizo citrange) but numbers of females and eggs per gram fibrous root were 27% and 22% at Amposta, and 139% and 18% in the orchard of those on Carrizo citrange, respectively. The selection 05.01.7 supported equal number of females and 43% eggs per gram fibrous root of those on Carrizo citrange in the nematode-infested orchard. The remaining selections supported high populations of T. semipenetrans.

Resistance of Geneva and Other Apple Rootstocks to Erwinia amylovora.

When vigorously growing shoots of 49 different apple rootstocks grown in a greenhouse were inoculated with different strains of Erwinia amylovora, Budagovsky 9 (B.9), Ottawa 3, Malling 9, and Malling 26 were the most fire blight susceptible rootstocks and Geneva 11, Geneva 65, Geneva 16, Geneva 30, Pillnitzer Au51-11, Malling 7, and several breeding selections were the most resistant. Significant strain-rootstock interactions were observed in the amount of fire blight that resulted from inoculation. Field-grown fruiting 'Royal Gala' trees on Geneva 16 and Geneva 30 rootstocks were highly resistant to rootstock infection (no tree mortality) when trees sustained severe blossom infection with E. amylovora, compared with Malling 9 and Malling 26 rootstock clones, which were highly susceptible to infection (36 to 100% tree mortality). In contrast to potted own-rooted B.9 plants inoculated in a greenhouse, B.9 rootstocks of orchard trees appeared resistant to rootstock infection (0% tree mortality). Orchard trees on Geneva 11 were moderately resistant to rootstock infection (25% tree mortality). There was general agreement in the evaluation of resistance under orchard conditions when rootstock resistance was evaluated in relation to controlled blossom inoculation or to natural blossom infection.

A Cucumber Mosaic Virus (CMV) RNA 1 Transgene Mediates Suppression of the Homologous Viral RNA 1 Constitutively and Prevents CMV Entry into the Phloem

Resistance to Cucumber mosaic virus (CMV) in tobacco lines transformed with CMV RNA 1 is characterized by reduced virus accumulation in the inoculated leaf, with specific suppression of accumulation of the homologous viral RNA 1, and by the absence of systemic infection. We show that the suppression of viral RNA 1 occurs in protoplasts from resistant transgenic plants and therefore is not due to a host response activated by the cell-to-cell spread of virus. In contrast, suppression of Tobacco rattle virus vectors carrying CMV RNA 1 sequences did not occur in protoplasts from resistant plants. Furthermore, steady-state levels of transgene mRNA 1 were higher in resistant than in susceptible lines. Thus, the data indicate that sequence homology is not sufficient to induce suppression. Grafting experiments using transgenic resistant or susceptible rootstocks and scions demonstrated that the resistance mechanism exhibited an additional barrier to phloem entry, preventing CMV from moving a long distance in resistant plants. On the other hand, virus from susceptible rootstocks could systemically infect grafted resistant scions via the phloem. Analysis of viral RNA accumulation in the infected scions showed that the mechanism that suppresses the accumulation of viral RNA 1 at the single-cell level was overcome. The data indicate that this transgene-mediated systemic resistance probably is not based on a posttranscriptional gene-silencing mechanism.

Effects of bicarbonate and iron supply on Fe(III) reducing capacity of roots and leaf chlorosis of the susceptible peach rootstock “Nemaguard”

Peach varieties grafted on the rootstock “Nemaguard” (Prunus persica (L.) Batsch) usually show chlorosis when grown in calcareous soils. This chlorosis can be corrected by applying iron chelates to the soil. In this work we present results on the behavior of “Nemaguard” seedlings grown in nutrient solutions with different concentrations of bicarbonate and Fe as FeEDDHA. Chlorosis degree was increased by either increasing bicarbonate (0–15 mM) or decreasing Fe (100–0 μM). Shoot dry weight was decreased in chlorotic plants, but also in green plants grown with bicarbonate (15 mM) and high Fe (100 μM). This indicates that the growth decrease caused by bicarbonate is not only related to Fe deficiency chlorosis. When plants were grown with 10 mM bicarbonate and 2.5 μM Fe and showed clear chlorosis symptoms, they were transferred to a similar nutrient solution but with 100 μM Fe. After one week plant regreening was appreciable, in particular in the youngest leaves. The root Fe(III) reducing capacity was measured in a medium with absence of bicarbonate and in the presence of 10 mM MES, at pH 5.0, for 2h and 30 min. In the treatment with 10 mM bicarbonate and 2.5 μM Fe, roots had a very low reducing capacity and plants became chlorotic. However, these plants markedly increased their reducing capacity just one day after transferring them to a similar nutrient solution but with 100 μM Fe, despite they were still very chlorotic. These results show that bicarbonate inhibits the development of reducing capacity under low Fe conditions but not at higher Fe levels. It is assumed that under field conditions both the induction of chlorosis and the regreening process may be partly consequence of changes in the reducing capacity.

The potential of Lagenaria rootstock to confer resistance to the carmine spider mite, Tetranychus cinnabarinus (Acari: Tetranychidae) in Cucurbitaceae.

Antibiosis and resistance of six Cucurbita and two Lagenaria accessions to the carmine spider mite, Tetranychus cinnabarinus Boisduval, were evaluated in the laboratory. Significant differences among accessions were observed three days after the inoculation of detached leaf discs. The Lagenaria accessions, Slawi and Sus, proved to be the most resistant to mites, with average populations of mite eggs, 87 and 95%, respectively less than that of the susceptible C. pepo accession, Orangetti. The Cucurbita accessions, Tace, Brava, Tetsukabuto, Phoenix and TZ-148 had mite egg totals 4, 9, 13, 26 and 40%, respectively, less than those of accession Orangetti. The Sus accession of Lagenaria was resistant to T. cinnabarinus from the four-leaf stage until fruit set in laboratory and field tests. Grafting the susceptible Brava onto Sus rootstock increased the resistance of the scion to the same level as that of non-grafted Sus. Grafting the susceptible Cucumis melo Noy Yizre'el on resistant or susceptible rootstocks of Cucurbita and Lagenaria accessions did not affect its susceptibility to T. cinnabarinus. The results indicate that resistance to T. cinnabarinus can be transferred by grafting from Lagenaria stocks to Cucurbita scions but not in the opposite direction.

The effect of resistance of rootstocks to crown gall (Agrobacterium spp.) on the susceptibility of scions in grape vine cultivars

The effects of two grapevine rootstocks on susceptibility of grafted scions to crown gall (Agrobacterium vitis and Agrobacterium tumefaciens) was investigated in greenhouse and field experiments. The incidence of crown gall on a susceptible grape scion cultivar (Vitis vinifera, cv. Kiralyleanyka) was not affected by grafting it on a crown gall resistant rootstock (Vitis riparia, cv. Gloire) or on a susceptible rootstock (V. berlandieri × V. riparia, cv. Teleki 5C) when the inoculated vines were observed over a 2‐month period in the greenhouse. Weights of galls that were induced by six of eight strains of Agrobacterium (six A. vitis and two A. tumefaciens) were not significantly different, but A. tumefaciens strain 16/6 and A. vitis strain 2/3 produced significantly larger galls on Kiralyleanyka vines that were growing on their own roots or were grafted on 5C than on those grafted on Gloire. When the same vines were planted in the field and observed over a 4‐year period, no differences in incidence of crown gall were appa_rent until the third and fourth years, when vines grafted on Gloire had less crown gall than vines on 5C. By that time, the natural symptoms (galls along the trunk) of the disease appeared, coinciding with the decline of whole vines. In another field experiment, with naturally infected scions of V. vinifera, cv. Blau Frankisch (susceptible), crown gall was appa_rent on vines grafted on both rootstocks after 2 years, but in the fifth year 79% of the vines on 5C had crown gall whereas only 9% on Gloire showed symptoms. After the appearance of typical natural symptoms, vines showed reduced vigor with substantial loss of trunks and entire vines. During this period, many vines on 5C died as compared with only a few on Gloire. By the end of the sixth year, 13% of the originally planted vines on 5C and 82% on Gloire were alive, and although many pathogenic strains of Agrobacterium spp. were isolated from the roots of 5C, none was isolated from Gloire.