Polygalacturonase-inhibiting protein (PGIP) from Japanese pear: possible involvement in resistance against scab

Venturia nashicola is the causal agent of scab, a fungal disease affecting Asian pears. The Japanese pear cv. ‘Kousui’ is highly susceptible to the race 1 of this fungus whereas the cv. ‘Kinchaku’ and the non-host European pear cv. ‘Flemish Beauty’ are resistant. The aim of this work is to investigate the role of polygalacturonase-inhibiting proteins (PGIPs) of pear during the interactions with V. nashicola leading to susceptibility or resistance. PGIP protein was detected from immature fruit of Kousui and Kinchaku. It showed a molecular mass of 42 kDa that shifted to 35 kDa after chemical deglycosylation. The gene pgip was amplified by PCR using genomic DNA and/or cDNA from young leaves of Kousui, Kinchaku, and European pear cvs. Flemish Beauty, ‘Bartlett’, and an Asian wild pear strain ‘Mamenashi 12’, then sequenced after sub-cloning. Some conserved variations were identified in the sequence indicating that gene family also exists in pgip of Japanese pear and confirmed by Southern blot analysis. The expression of PGIP was studied in scab-inoculated leaves of the susceptible Kousui and the resistant Kinchaku and Flemish Beauty. pgip Gene and its encoding protein were highly and rapidly activated in these resistant plants. In addition, PGIP extracts derived from Kinchaku and Flemish Beauty partially inhibited the activity of polygalacturonase (PG) from V. nashicola suggesting a possible role of PGIP in limiting fungal growth frequently observed in these resistant cultivars.     

Ultrastructural study on scab resistance expressed in epidermal pectin layers of pear leaves

Proliferation and collapse of subcuticular hyphae of Venturia nashicola race 1 were studied ultrastructurally, after inoculation of susceptible Japanese pear cv. Kousui, resistant Japanese pear cv. Kinchaku, resistant Asian pear strain Mamenashi 12 and nonhost European pear cv. Flemish Beauty leaves, to understand the nature of the resistance mechanism. After cuticle penetration by the pathogen, the hyphae were observed at lower frequency in epidermal pectin layers and middle lamellae of leaves of the three resistant plants than in those of susceptible ones. This result suggested that fungal growth was suppressed in the incompatible interaction between pear and V. nashicola race 1. In the pectin layers of all inoculated plants, some hyphae had modifications such as breaks in the plasmalemma with plasmolysis, necrotic cytoplasm and degraded cell walls. More hyphae had collapsed in the leaves of the three resistant plants than in those of the susceptible cv. Kousui. In collapsed hyphae, the polymerized cell walls broke into numerous fibrous and amorphous pieces, showing that the scab resistance might be associated with cell wall-degrading enzymes from pear plants.     

Enhancement of Postharvest Disease Resistance in Ya Li Pear (Pyrus bretschneideri) Fruit by Salicylic Acid Sprays on the Trees during Fruit Growth

The Ya Li pear (Pyrus bretschneideri) trees were sprayed three times with 2.5 mM salicylic acid (SA) around 30, 60 and 90 days after full flowering. The fruit were harvested at commercial maturity (about 120 days after full flowering), inoculated with Penicillium expansum, and incubated at 20 °C, 95–100% RH. The results showed that resistance to the pathogen of the mature pear fruit was remarkably enhanced by the SA sprays. Disease incidence in the SA-treated fruit was 58.0% or 26.5%, and lesion diameter on SA-treated fruit was 58.4% or 29.0% lower than that in/on fruit without SA treatment (control) on day 12 or 17 after incubation, respectively. The SA spray applied to the trees around 30 days after full flowering notably enhanced accumulation of hydrogen peroxide in the young fruit. Meanwhile, activities of defense enzymes, including peroxidase, phenylalanine ammonia-lyase (PAL), chitinase or β-1,3-glucanase in the young fruit from SA-treated trees was 29.5%, 60.0%, 24.4% or 35.7% higher than that in the control fruit 4 days after the SA spraying. Furthermore, after harvest, activities of PAL, chitinase and β-1,3-glucanase were still significantly higher in the mature pear fruit from the trees sprayed three times with SA than those of the control fruit. Activities of the antioxidant enzymes including catalase and ascorbate peroxidase in the young fruit were significantly reduced by SA spraying. However, the activity of another antioxidant enzyme, glutathione reductase in the young fruit was significantly enhanced by SA spraying. These results suggest that enzymes exerting their functions in different ways may be coordinately regulated by SA in the pear fruit. Our study indicates that treatment of SA sprays on the trees may provide further protection against postharvest disease of Ya Li pear fruit in practice and could be used as an alternative and economical approach to reduce application of chemical fungicides.     

Analysis of head and leaf reaction towards <Emphasis Type="Italic">Microdochium nivale</Emphasis>

This research examined the variation in the response of eight commercial wheat cultivars to Microdochium nivale isolates using both in vivo FHB tests (AUDPC and RHW measurements) and in vitro detached leaf assays (LGR). Irrespective of fungal variety, the two Italian cvs Fortore and Norba exhibited the greatest amount of visual disease symptoms (mean AUDPC=2.2 and 2.3, respectively), being significantly more susceptible than the other six cultivars (AUDPC 1.24) (P < 0.05). Irrespective of fungal variety, the Italian cv. Norba and the Irish cv. Falstaff were more susceptible than the other cultivars (except Fatima 2) in terms of RHW (P < 0.05), while the cvs Fortore, GK Othalom and Consort were more resistant than the other five cultivars (P < 0.05). In the detached leaf assay, the Hungarian cv. GK Othalom and the Italian cv. Norba were more susceptible (mean LGR=0.79 and 0.81 mm day^–1, respectively) to M. nivalethan the other six cultivars (mean LGR=0.51–0.72) (P < 0.05). Analysis of the relationship between head and leaf reaction to M. nivaleinfection revealed no significant correlation.     

Molecular cloning and expression analysis of genes related to phosphatidic acid synthesis in Japanese pear leaves inoculated with <Emphasis Type="Italic">Venturia nashicola</Emphasis>

Phosphatidic acid, which can be generated by the action of phospholipase D (PLD) and by the combined action of phospholipase C (PLC) and diacylglycerol kinase (DGK), is a plant defense signal. To identify the role of the PLD, PLC, and DGK genes encoding these enzymes in pear resistance to the scab fungus Venturia nashicola, we report the cloning of these genes and analysis of their expression in inoculated pear leaves. The results showed that PLD mRNA increased in the leaves of the susceptible pear cv. Kousui 1 day after inoculation (dai), then decreased to the basal level. In resistant pear cv. Kinchaku, PLD mRNA did not change significantly except for a small increase at 2 and 3 dai. However, the level of PLD mRNA in Kinchaku was higher than in Kousui 2 dai. The level of PLC mRNA significantly increased in cvs. Kousui and Kinchaku by 1 dai, peaked at day 2, then decreased to the basal level. Thus, the gene expression pattern did not differ between the two cultivars. PLC gene expression was also induced by environmental stress. The DGK gene seemed to be constitutively expressed and was not induced by inoculation in either the susceptible or resistant cultivars. Together, this data showed that both PLD and PLC are possibly involved in a defense response to V. nashicola in Japanese pear leaves, and the PLD gene seems to play a more important role than does the PLC gene.     

Resistance of wheat to Mycosphaerella graminicola involves early and late peaks of gene expression

Large-scale cDNA-AFLP profiling identified numerous genes with increased expression during the resistance response of wheat to the Septoria tritici blotch fungus, Mycosphaerella graminicola. To test whether these genes were associated with resistance responses, primers were designed for the 14 that were most strongly up-regulated, and their levels of expression were measured at 12 time points from 0 to 27 days after inoculation (DAI) in two resistant and two susceptible cultivars of wheat by real-time quantitative polymerase chain reaction. None of these genes was expressed constitutively in the resistant wheat cultivars. Instead, infection of wheat by M. graminicola induced changes in expression of each gene in both resistant and susceptible cultivars over time. The four genes chitinase, phenylalanine ammonia lyase, pathogenesis-related protein PR-1, and peroxidase were induced from about 10- to 60-fold at early stages (3 h–1 DAI) during the incompatible interactions but were not expressed at later time points. Nine other genes (ATPase, brassinosteroid-6-oxidase, peptidylprolyl isomerase, peroxidase 2, 40S ribosomal protein, ADP-glucose pyrophosphorylase, putative protease inhibitor, methionine sulfoxide reductase, and an RNase S-like protein precursor) had bimodal patterns with both early (1–3 DAI) and late (12–24 DAI) peaks of expression in at least one of the resistant cultivars, but low if any induction in the two susceptible cultivars. The remaining gene (a serine carboxypeptidase) had a trimodal pattern of expression in the resistant cultivar Tadinia. These results indicate that the resistance response of wheat to M. graminicola is not completed during the first 24 h after contact with the pathogen, as thought previously, but instead can extend into the period from 18 to 24 DAI when fungal growth increases dramatically in compatible interactions. Many of these genes have a possible function in signal transduction or possibly as regulatory elements. Expression of the PR-1 gene at 12 h after inoculation was much higher in resistant compared to susceptible recombinant-inbred lines (RILs) segregating for the Stb4 and Stb8 genes for resistance. Therefore, analysis of gene expression could provide a faster method for separating resistant from susceptible lines in research programs. Significant differential expression patterns of the defense-related genes between the resistant and susceptible wheat cultivars and RILs after inoculation with M. graminicola suggest that these genes may play a major role in the resistance mechanisms of wheat.     

Increased virulence of Globodera pallida during repeated rearing on different resistant potato cultivars explained by a simple model

Selection for virulence of Globodera pallida on potato cultivars was studied for four generations under controlled conditions. The reproduction rate (P_f/P_i) of a mixed Pa2/3 population increased by a factor of 61 during rearing on the partially resistant potato cv. Darwina compared to rearing on the susceptible cv. Irene. This was a result of selection for virulence on cv. Darwina, and achieving the Hardy–Weinberg equilibrium on cv. Irene. Increased virulence also significantly raised the reproduction rate on several other Solanum genotypes. These changes could be explained reasonably well by the monogenic inheritance of a virulence factor breaking the Grp1 locus. The virulence changes were probably mainly evoked by this gene only, inherited from S. vernei 1-3 or S. vernei 24/20. The Grp1 locus has probably provided the differential S. vernei hybrid (VTⁿ)² 62-33-3 with its resistance to the Pa2 group and not to the Pa3 group. Alternation of cultivars did not halt selection if the cultivars highly differentiated between the Pa2 and Pa3 populations. Only when alternation was with cultivars that harboured a different resistance gene against Pa3 was selection for virulence delayed. Differences in virulence levels (i.e. reproduction rates) within the nematode population determined the rate of selection, not the resistance level itself. Selection of a Pa3 population for three generations on cv. Karakter not only increased the reproduction rate on cv. Karakter itself by a factor 4.2, but also raised the reproduction on other potato genotypes. A simple monogenic model could explain these changes in virulence.     

A rapid technique for screening banana cultivars for resistance to Xanthomonas wilt

The banana Xanthomonas wilt disease (BXW) has threatened the livelihood of millions of farmers in East Africa. Use of resistant varieties is the most cost-effective method of managing this bacterial disease. A reliable and rapid screening method is needed to select resistant banana varieties. An in vitro screening method was developed for early evaluation of Xanthomonas wilt resistance using small tissue culture-grown plantlets. Eight cultivars of banana were screened with sixteen isolates of Xanthomonas campestris pv. musacearum using this method. There were significant differences (P < 0.0001) in susceptibility among the various banana cultivars tested, whereas no significant difference (P = 0.92) in pathogenicity was observed between the pathogen isolates. The cv. Pisang Awak (Kayinja) was found to be highly susceptible and Musa balbisiana resistant. Nakitembe was found to be moderately resistant while cvs Mpologoma, Mbwazirume, Sukali Ndiizi, FHIA-17 and FHIA-25 were susceptible. The susceptibility of these cultivars was further tested in vivo by artificial inoculation of potted plants with similar results. This study shows that an in vitro screening test can serve as a convenient, cheap and rapid screening technique to discriminate BXW-resistant from BXW-susceptible banana cultivars.     

Novel methodologies in screening and selecting olive varieties and root-stocks for resistance to <Emphasis Type="Italic">Verticillium dahliae</Emphasis>

An innovative inoculation process, involving the drilling of a trunk hole in 3 year-old olive trees and injecting a dense conidial suspension of Verticillium dahliae, was developed to study differentiation in foliar symptom expression between olive cultivars tolerant or susceptible to the pathogen. It was demonstrated that V. dahliae conidia could be translocated and colonize the xylem at the same distance above and below the point of trunk injection in both cultivars. However, the pathogen could be subsequently isolated at statistically significant percentages in susceptible cv. Amphissis compared to the tolerant cv. Kalamon, indicating operation of resistance mechanisms in the vascular phase of the disease. Consequently symptom development in the susceptible cultivar was at least sixfold more intensive compared to the tolerant cultivar, 6–11 months after trunk inoculation. Perennial olive orchard experiments, aimed at selecting Verticillium-resistant root-stocks, were conducted by applying the novel method in 2–3 year-old root-stock suckers of Amphissis olive trees and in the tolerant cvs Lianolia of Corfu and Koroneiki. It was indicated that potentially resistant root-stocks could be obtained following the trunk drilling technique. Resistance differentiation between cvs Amphissis and Kalamon was further verified through root inoculation by various V. dahliae microsclerotial concentrations and demonstrated that the trunk drilling inoculation procedure is equally efficient in resistance evaluation of olives to Verticillium wilt. The trunk inoculation procedure could be useful in selecting and screening root-stocks for resistance to V. dahliae and other vascular pathogens and could elucidate resistance mechanisms in woody plants against vascular wilt diseases.     

Investigation of rice blast development in susceptible and resistant rice cultivars using a gfp‐expressing Magnaporthe oryzae isolate

In this study, an isolate of Magnaporthe oryzae expressing the green fluorescent protein gene (gfp) was used to monitor early events in the interaction of M. oryzae with resistant rice cultivars harbouring a blast resistance (R) gene. In the resistant cultivars Saber and TeQing (Pib gene), M. oryzae spores germinated normally on the leaf surface but produced morphologically abnormal germ tubes. Germling growth and development were markedly and adversely affected in leaves of these resistant cultivars. Penetration of host cells was never seen, supporting the idea that disruption of germling development on the leaf surface might be one of the resistance mechanisms associated with Pib function. Thus, this particular R gene appeared to function in the absence of host penetration by the fungal pathogen. Confocal laser scanning microscopy of M. oryzae‐infected susceptible rice cultivars showed the dimorphic growth pattern that is typically observed during the biotrophic and necrotrophic stages of leaf colonization in susceptible cultivars. The suitability of the gfp‐expressing M. oryzae isolate for further research on R‐gene function and identification of resistant genotypes in rice germplasm collections is discussed.     

The role of syringic acid in the interaction between oil palm and Ganoderma boninense,the causal agent of basal stem rot

Novel inoculation and assessment methods for Ganoderma boninense infection of oil palm are reported. The involvement of phenolic acids in the interaction was examined. HPLC was used to monitor changes in the concentrations of three specific phenolics: syringic acid (SA), caffeic acid and 4‐hydroxybenzoic acid, identified as the main compounds that accumulated. The work reported here focuses on SA, the most antifungal of the molecules detected. The oil palm cv. AVROS, reported by local planters to be less susceptible than others, showed higher accumulation of SA than cvs Ekona and Calabar. Accumulation was promoted by addition of chitosan to the plant growing medium. By the end of the time‐course, the concentration of SA decreased in the oil palm tissues inoculated with G. boninense, suggesting possible metabolism by the pathogen. This loss was, however, not detected in tissues treated with chitosan alone and was greatly reduced when G. boninense was combined with this polymer. In vitro studies on antifungal activity of SA were done using concentrations ranging from 50 to 110 μg mL^?1, those typically recorded in oil palm roots. SA was found to be antifungal (EC₅₀ 90–100 μg mL^?1). The concentration of SA detected in root tissues, especially in the presence of chitosan, could inhibit growth of G. boninense. The pathogen was shown to degrade SA in vitro. However, at the highest concentration tested, metabolism was greatly delayed, only occurring after a lag phase in pathogen growth. Accumulation of phenolic acids, especially SA, may prove a useful trait in breeding resistant oil palm cultivars.     

Infection Behavior of Venturia nashicola, the Cause of Scab on Asian Pears

ABSTRACT The infection of Japanese pear by Venturia nashicola, the cause of scab on Asian pears (Japanese pear, Pyrus pylifolia var. culta; Chinese pear, P. ussuriensis), was examined using light and electron microscopy to determine the mechanism of resistance in pears. Early stages of infection were similar on the susceptible cv. Kosui, the resistant cv. Kinchaku, and the nonhost European pear (P. communis) cv. Flemish Beauty. V. nashicola penetrated only the cuticle layer on pear leaves and formed subcuticular hyphae on all three cultivars. Hyphae were localized in the pectin layer of pear leaves and never penetrated into the cytoplasm of epidermal cells. This restriction of fungal growth suggested that pectinases released by infection hyphae or subcuticular hyphae may be important in infection. Subcuticular hyphae were modified ultrastructurally in the pectin layer of resistant pear cultivars accompanied by fungal cell death. In contrast, fungal cells appeared intact in susceptible pear cultivars, suggesting the existence of resistance mechanisms.     

Resistance to <Emphasis Type="Italic">Rice yellow mottle virus</Emphasis> in rice germplasm in Madagascar

Rice yellow mottle virus (RYMV) is a recent and major threat to rice production in Madagascar. A large scale screening of resistance to RYMV in rice germplasm in Madagascar was conducted by visual symptom scoring and virus-assessment through ELISA tests. The response to virus inoculation of 503 local or introduced rice accessions was assessed. Most of them were susceptible to RYMV. A few cultivars expressed partial resistance at a level similar to the partially resistant Oryza sativa japonica cv. Azucena. Only one O. sativa cultivar expressed high resistance characterised by a lack of symptoms and an undetectable level of virus. It was a Malagasy traditional indica cultivar, named Bekarosaka, which originated from the northwest of the country. It was selected by farmers for its field resistance to RYMV. The response of cv. Bekarosaka to four pathotypes of RYMV was similar to that of cv. Gigante, the only other highly resistant indica cultivar. The sequence of the middle domain of the eIF(iso)4G, the genetic determinant of Rymv1 resistance on chromosome 4, of cv. Bekarosaka was similar to that of cv. Gigante. Subsequently, cvs Bekarosaka and Gigante probably carried the same resistance allele Rymv1-2. Rymv1-2 resistance was efficient against isolates of the major strains of RYMV, but was readily overcome by a pathotype from the northwest of Madagascar.     

Salicylic acid alleviates growth inhibition and oxidative stress caused by zucchini yellow mosaic virus infection in Cucurbita pepo leaves

The effects of zucchini yellow mosaic virus (ZYMV) infection and pretreatments with salicylic acid (SA) on biomass accumulation of pumpkin (Cucurbita pepo cv. Eskandarani) were investigated. The response of photosynthesis, transpiration and the activities of antioxidant enzymes in leaves was also considered. Significant reductions in growth parameters (i.e. leaf area, biomass and shoot height), photosynthesis and chlorophyll a and b content were detected in ZYMV-infected leaves in comparison to healthy controls. Antioxidant enzyme activities were increased up to 3-fold for peroxidase (POD), 2-fold for ascorbate peroxidase (APX) and catalase (CAT) activities and 1.3-fold for SOD activity by virus infection. ZYMV infection also caused increases in H₂O₂ and malondialdehyde (MDA) contents. These results suggest that ZYMV infection causes oxidative stress in pumpkin leaves leading to the development of epidemiological symptoms. Interestingly, spraying pumpkin leaves with SA led to recovery from the undesirable effects of ZYMV infection. Leaves treated with 100 μM SA three days before inoculation had the appearance of healthy leaves. No distinct disease symptoms were observed on the leaves treated with 100 μM SA followed by inoculation with ZYMV. In non-infected plants, SA application increased activities of POD and superoxide dismutase (SOD) and inhibited APX and CAT activities.In contrast, SA treatment followed by ZYMV inoculation stimulated SOD activity and inhibited activities of POD, APX and CAT. In addition, MDA displayed an inverse relation, indicating inhibition of lipid peroxidation in cells under SA treatment. It is suggested that the role of SA in inducing plant defense mechanisms against ZYMV infection might have occurred through the SA-antioxidant system. Such interference might occur through inhibition or activation of some antioxidant enzymes, reduction of lipid peroxidation and induction of H₂O₂ accumulation following SA application.     

Traditional and commercial apple and pear cultivars as sources of resistance to fireblight1

In Hungary, fireblight research programmes were initiated on pear in 1999 and on apple in 2000, with the aim of evaluating the susceptibility or resistance of commercial cultivars. Sources of resistance for future breeding were also sought among traditional apple cultivars collected from Ukraine and pear cultivars in the Hungarian gene bank (Szigetcsép). Experiments were done under secure conditions. Inocula were mixtures of characteristic Erwinia amylovora isolates from pear and apple in Hungary. Host responses (symptom development, disease severity and multiplication rate of bacterial cells in host tissues) were assessed on shoots, flowers and fruits. About 30 pear and 30 apple cultivars, and 35 apple hybrids, were tested and grouped into four categories for pear and three for apple. Of the pear cultivars tested, 50% were susceptible, 30% moderately susceptible and only 10% of low susceptibility. Different plant organs occasionally displayed different responses. Members of the last two groups might serve as useful candidates for growing under IPM conditions. Among the traditional Hungarian varieties tested, we found high resistance in ‘Sikulai’ and ‘Szemes alma’, which could be used as sources of fireblight resistance in breeding programmes and also grown in organic orchards. Furthermore, among the offspring of the apple ‘Prima’ (scab‐resistant), we have found highly resistant lines.     

Interactions Between <Emphasis Type="Italic">Barley yellow dwarf virus</Emphasis> and <Emphasis Type="Italic">Fusarium</Emphasis> spp. Affecting Development of Fusarium Head Blight of Wheat

Interactions between Barley yellow dwarf virus (BYDV) and Fusarium species causing Fusarium head blight (FHB) in winter wheat cvs Agent (susceptible to FHB) and Petrus (moderately resistant to FHB) were studied over three years (2001–2003) in outdoor pot experiments. FHB developed more rapidly in cv. Agent than in cv. Petrus. The spread of FHB was greater in BYDV-infected plants than in BYDV-free plants. Thousand grain weight (TGW) was reduced more in Fusarium-infected heads of cv. Agent than in cv. Petrus. A highly significant negative correlation was found between disease index and TGW in cv. Agent (r = −0.916), while in cv. Petrus the correlation was less significant (r = −0.765). Virus infection reduced TGW in cv. Petrus more than in cv. Agent. In plants with both infections, TGW reductions in cv. Petrus corresponded to those of BYDV infection, and in cv. Agent TGW was more diminished than in BYDV infection. Effects of different treatments determined over three years on ergosterol contents in grain were generally similar to effects on disease indices. Grain weight per ear and ear weight of the different treatments of both cultivars largely corresponded with the TGW results. Deoxynivalenol (DON) content in grain of cv. Agent infected with Fusarium spp. was 11–25 times higher compared to the corresponding treatments in cv. Petrus. The DON content in grain of plants of the two cultivars infected with both pathogens was higher than that of plants infected only with Fusarium over the three years.     

Genetic resistance derived from Solanum pimpinellifolium AAU2019 is monogenic recessive to tomato leaf curl virus

Tomato leaf curl disease is a severe threat to tomato production. Yield losses are generally high in the absence of effective management strategies. The disease is caused by tomato leaf curl virus (ToLCV) and is transmitted by a whitefly vector that is challenging to control. Resistance to ToLCV is absent from most cultivated tomato gene pools, although the use of resistant cultivars would provide a better control option than minimizing the vector population. Unfortunately, resistance sources based on field screening break down when virus pressure is severe. Our previous screening and virus testing of 40 tomato genotypes led to the identification of a highly resistant genotype, Solanum pimpinellifolium AAU2019, as a new source of resistance. In this study, we investigated the inheritance and genetics of resistance to ToLCV in the cross of Pusa Ruby × S. pimpinellifolium AAU2019 in F₂ and BC₁P_s populations, revealing a monogenic recessive gene (best-fit ratios of resistance: susceptible to be 1:3 and 0:1) responsible for ToLCV resistance in S. pimpinellifolium AAU2019. Hence, S. pimpinellifolium AAU2019 could be considered as a potential donor parent in breeding programmes to develop tomato cultivars with resistance to ToLCV.     

Symptom development, pathogen isolation and Real-Time QPCR quantification as factors for evaluating the resistance of olive cultivars to Verticillium pathotypes

Verticillium wilt is the most serious olive disease in the Mediterranean countries and worldwide. The most effective control strategy is the use of resistant cultivars. However, limited information is available about the level and source of resistance in most of the olive cultivars and there are no published data using microsclerotia, the resting structures of Verticillium dahliae, as the infective inoculum. In the present study, we correlated symptomatology and the presence of the fungus along with the DNA relative amount (molecules μl⁻¹) of a defoliating (D) and a non-defoliating (ND) V. dahliae strain in the susceptible cv. Amfissis and the tolerant cvs Kalamon and Koroneiki, as quantified by the Real-Time QPCR technology. The viability of the pathogen in the plant tissues was confirmed by isolating the fungus on PDA plates, while symptom assessment proved the correlation between the DNA relative amount of V. dahliae in plant tissues and cultivar susceptibility. It was further demonstrated that the D and ND strains were present at a significantly higher level in cv. Amfissis than in cvs Kalamon and Koroneiki. It was finally observed that the relative amount of the pathogen in roots was lower than in stems and shoots and declined in plant tissues over time. These data constitute a valuable contribution in evaluating resistance of olive cultivars or olive root-stocks to V. dahliae pathotypes.     

Involvement of jasmonic acid signalling in bacterial wilt disease resistance induced by biocontrol agent Pythium oligandrum in tomato

When the biocontrol agent Pythium oligandrum (PO) colonizes the rhizosphere, it suppresses bacterial wilt disease in tomato (Solanum lycopersicum cv. Micro‐Tom) caused by Ralstonia solanacearum, and a homogenate of its mycelia exhibits elicitor activity, inducing an ethylene (ET)‐dependent defence response in Micro‐Tom. Since salicylic acid (SA) and jasmonic acid (JA) play an important role in plant defence responses to pathogens, the involvement of SA‐ and JA‐dependent signal transduction pathways in resistance to R. solanacearum was investigated in tomato roots treated with a mycelial homogenate of PO. Bacterial wilt disease was also suppressed in tomato cv. Moneymaker treated with the PO homogenate. However, the SA‐inducible PR‐1(P6) gene was not up‐regulated in either Micro‐Tom or Moneymaker. SA did not accumulate in homogenate‐treated roots in comparison with distilled water‐treated controls, even 24 h after inoculation. Induced resistance against R. solanacearum was not compromised in SA‐non‐accumulating NahG transgenic plants treated with the PO homogenate. On the other hand, the expression of the JA‐responsive gene for the basic PR‐6 protein was induced in both tomato cultivars treated with the PO homogenate. Furthermore, quantitative disease assays showed that the induced resistance against R. solanacearum was compromized in PO homogenate‐treated jai1‐1 mutant plants defective in JA signalling. These results indicated that the JA‐dependent signalling pathway is required for PO‐induced resistance against R. solanacearum in tomato.