Susceptibility evaluation of Picea abies and Cupressus lusitanica to the pine wood nematode (Bursaphelenchus xylophilus)

Pine wilt disease (PWD), recently introduced into Europe, is caused by the pine wood nematode (PWN) Bursaphelenchus xylophilus and is a devastating illness that affects mainly pine trees. It is known that the PWN is capable of infecting other conifers; however, there is currently no information on which other plant species may be susceptible to PWD. In this study, the potential susceptibility of two common species of European forests, Picea abies and Cupressus lusitanica, to PWN was assessed through the monitoring of visual external symptoms, dimension and localization of the nematode population in stems, quantification of total chlorophyll, total soluble phenolics and lignin, at 7, 14, 21 and 28 days after inoculation. The degree of susceptibility was established through the comparison of symptoms with Pinus pinaster, a well‐known PWN host. Furthermore, the stem ultrastructure of P. abies, C. lusitanica and Pn. pinaster was analysed by scanning electron microscopy. The results suggest that P. abies and C. lusitanica are resistant to PWN, and that lignin biosynthesis in these species is affected at an early stage of the infestation. Nevertheless, P. abies seems to be a compatible host that could act as a repository for PWN.     

Prepenetration stages in infection of clematis by Phoma clematidina

A detailed study of conidial germination, germ-tube growth and the formation of infection structures in Phoma clematidina , the causal agent of clematis wilt, is described for two clematis varieties differing in disease resistance. On both the resistant and susceptible varieties, the fungus entered leaves and stems by direct penetration of the cuticle, often, but not always, following the formation of infection structures. More germ tubes per conidium were formed on the susceptible host, but these germ tubes were on average shorter than on the resistant host. Although germ tubes regularly entered the plant via trichomes, stomata were not found to be sites of entry. Following penetration of the cuticle of resistant plants, germ-tube growth was sometimes restricted to the subcuticular region, and halo formation occurred at the sites where penetration was attempted. Subcuticular growth and halo formation were not observed on susceptible plants. These observations may partly explain the resistance of small-flowered clematis varieties to P. clematidina .     

Resistance of kiwifruit cultivars to ceratocystis wilt: An approach considering the genetic diversity and variation in aggressiveness of the pathogen

Kiwifruit (Actinidia spp.) is native to southern China, but was first cultivated in New Zealand and then spread worldwide. Emerging diseases such as ceratocystis wilt have attracted the attention of kiwifruit growers due to the great losses observed in southern Brazil. Effective control can be achieved by screening for resistance, but the genetic variability of the pathogen must be considered. Thus, this study aimed to assess the genetic diversity and variation in aggressiveness of Ceratocystis isolates from kiwifruit in southern Brazil and then evaluate the resistance of kiwifruit cultivars with the most aggressive isolates. A collection of 46 isolates were obtained from southern Brazil and 14 simple-sequence repeat (SSR) markers was successfully used for genotyping. Out of 14 markers, 13 were polymorphic and identified 26 genotypes. Fourteen distinct genotypes were tested on a susceptible cultivar to select the most aggressive ones. Finally, inoculation with an equal mixture of five of the most aggressive isolates was used to evaluate the resistance of seven kiwifruit cultivars: Red Arguta, Green Arguta, Allison, Chieftain, Hayward, Monty, and Tomury. Cultivars varied in levels of susceptibility, with disease severity ranging from 40% to 100%. Considering the length of stem lesions, Chieftain showed the lowest level of severity at 40%, while no wilt symptoms were observed at 45 days after inoculation. In addition to the seven cultivars, a half-sibling progeny with 618 plants of the rootstock cv. Bruno was also assessed, but only seven individuals were resistant. These seven plants can be cloned and used as resistant rootstocks in commercial orchards.     

Behavior and mutation of <Emphasis Type="Italic">Ralstonia solanacearum</Emphasis> in <Emphasis Type="Italic">Solanum toxicarium</Emphasis> grown in aseptic culture

To study the behavior and mutation of Ralstonia solanacearum in Solanum toxicarium, which is resistant to bacterial wilt, S. toxicarium was grown in aseptic culture and inoculated with R. solanacearum. Although 60%–80% of the inoculated plants were wilting after 2 to 3 days, most wilted plants had recovered by 20 days after inoculation. The pathogen was reisolated from over 98% of inoculated plant stems, but the percentage of recovery decreased the closer the isolation sites were toward the upper stem sections. Three colony types, characterized as fluidal white, nonfluidal red, and a mixture of fluidal white and nonfluidal red, were reisolated from the stems. Nonfluidal red colonies were less virulent on tomato plants than fluidal white colonies.     

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.     

Factors influencing Ralstonia pseudosolanacearum infection incidence and disease development in rose plants

Glasshouse experiments were conducted to study infection and disease development in rockwool-grown rose plants inoculated with Ralstonia pseudosolanacearum. A R. pseudosolanacearum strain isolated from rose plants was more aggressive than strains from anthurium or curcuma. The three rose cultivars tested, Avalanche, Red Naomi, and Armando, differed in susceptibility. At 20°C, the rose strain caused hardly any symptoms over a 6-week period, whereas at 28°C typical wilt symptoms were observed within 2 weeks after stem inoculation of Armando, the most susceptible cultivar. Inoculating roots with the rose strain resulted only in weak atypical symptoms. Nevertheless, inoculating roots of cv. Armando at a relatively low inoculum dose of 10⁴ cfu/ml led to high densities in the base of stems in one out of two experiments. R. pseudosolanacearum occasionally spread from stem inoculated plants with symptoms in rockwool slabs. This limited spread resulted in a low infection incidence, and only of plants directly adjacent to the plants with symptoms.     

Susceptibility of Commercial Tomato Cultivars to Bacterial Wilt in Hydroponic System

Commercially available tomato cultivars were hydroponically cultured for inoculation, with Ralstonia solanacearum (K-101), which causes bacterial wilt, by pouring an inoculum suspension into the nutrient solution. Cultivar susceptibility to the bacteria was evaluated, based on the highest percentage of wilting. Because the length of time for wilt appearance varied among cultivars, some cultivars appeared to be suppressive to the translocation and/or multiplication of the invading pathogen. Thus, this hydroponic inoculation system is effective for examining levels of susceptibility in tomato cultivars to bacterial wilt. Received 13 December 2000/ Accepted in revised form 27 March 2001     

Characterization of accessions and species of Macadamia to stem infection by Phytophthora cinnamomi

Phytophthora cinnamomi is a major pathogen in most macadamia plantations worldwide. Due to stem lesions, stem cankers and leaf defoliation, it results in loss of productivity and tree death. This study examined accessions of the four Macadamia species and their hybrids, produced via rooted stem cuttings or germinated seeds, for susceptibility to stem canker and necrotic lesions caused by P. cinnamomi. Plants were wound‐inoculated with agar containing P. cinnamomi. The symptoms produced in inoculated plants were used to characterize host susceptibility variation within and among the population. Lesion length and severity of stem canker were recorded. The four species and hybrids differed significantly in stem canker severity (P < 0.001) and lesion length (P = 0.04). Macadamia integrifolia and M. tetraphylla hybrids were the most susceptible. Macadamia integrifolia had the greatest stem canker severity and the most extensive lesions above and below the site of inoculation. Restricted lesion sizes were observed in M. ternifolia and M.  jansenii. The effects of basal stem diameter and the method of propagation either from cuttings or from seed were not significant. The genetic variation in the reaction of macadamia accessions to stem infection by P. cinnamomi is discussed.     

Immunity of strains of Agaricus bitorquis to mushroom virus disease

Agaricus bitorquis, a species originally found in nature, produces snowwhite, tasty and well storable fruiting bodies on the common mushroom substrate. Several strains of this species were tested for susceptibility to mushroom virus, viz, the original wild isolatesA. bitorquis Nos 1, 2, 4 and 6, and the selection and breeding products ‘Somycel 2.017’, Horst B30, Horst B6, Horst K26 and Horst K32. Inoculum consisted of either mycelium or spores from virus-infected mushrooms (A. bisporus), or both. Trays withA. bisporus were always inoculated simultaneously to check the infectivity of the inoculum. UnlikeA. bisporus, the strains ofA. bitorquis tested did not show symptoms of virus disease after inoculation. Their fruiting bodies did not present mushroom virus particles, either in extracted cell-free preparations, or in ultrathin sections of the tissue. Yields ofA. bitorquis were not decreased by inoculation. The efficacy of the inoculation technique is discussed. The recent commercial availability of immune mushroom strains was a relief to Dutch mushroom growers, who found the disease difficult to eradicate by sanitary measures only.     

Resistance to a highly aggressive isolate of Sclerotinia sclerotiorum in a Brassica napus diversity set

Sclerotinia stem rot (SSR) of oilseed rape (OSR, Brassica napus), caused by Sclerotinia sclerotiorum, is a serious problem in the UK and worldwide. As fungicide‐based control approaches are not always reliable, identifying host resistance is a desirable and sustainable approach to disease management. This research initially examined the aggressiveness of 18 Sclerotinia isolates (17 S. sclerotiorum, one S. subarctica) on cultivated representatives of B. rapa, B. oleracea and B. napus using a young plant test. Significant differences were observed between isolates and susceptibility of the brassica crop types, with B. rapa being the most susceptible. Sclerotinia sclerotiorum isolates from crop hosts were more aggressive than those from wild buttercup (Ranunculus acris). Sclerotinia sclerotiorum isolates P7 (pea) and DG4 (buttercup), identified as ‘aggressive’ and ‘weakly aggressive’, respectively, were used to screen 96 B. napus lines for SSR resistance in a young plant test. A subset of 20 lines was further evaluated using the same test and also in a stem inoculation test on flowering plants. A high level of SSR resistance was observed for five lines and, although there was some variability between tests, one winter OSR (line 3, Czech Republic) and one rape kale (line 83, UK) demonstrated consistent resistance. Additionally, one swede (line 69, Norway) showed an outstanding level of resistance in the stem test. Resistant lines also had fewer sclerotia forming in stems. New pre‐breeding material for the production of SSR resistant OSR cultivars relevant to conditions in the UK and Europe has therefore been identified.     

不同向日葵品种对黄萎病的抗性*

采用浸根接种法鉴定不同向日葵种质资源对黄萎病的抗性差异。结果表明：40份供试向日葵品种依据抗性划分标准共分为3类,其中中抗品种有5份如：‘118’、‘新葵杂5号’等;低抗品种有8份如‘767’、‘KJ003’等;其余27份供试材料均呈现感病;没有鉴定出免疫和高抗的向日葵品种。     

Production of cotton varieties with resistance to Fusarium wilt with special reference to Tanzania

Abstract

Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum (Atk.) Sny. & Hans. is an important disease of cotton in several developing countries where the use of resistant varieties continues to be the most practical method of control. The inheritance of resistance to wilt is complex, involving several major and minor genes. In order to identify resistant genotypes in segregating populations large numbers of plants must be screened using inoculation methods which minimise disease escape. Wilt incidence is increased when plants are attacked by the rootknot nematode (Meloidogyne acrita Chitwood, 1949) Esser et al. 1976. In areas where both organisms occur together, the wilt resistance‐potential of a variety cannot be realised unless it also has resistance to root‐knot. Problems of breeding for wilt resistance are discussed with reference to Tanzania, and methods are described for their application to a wilt‐resistance programme.     

Root and stem rot of chrysanthemum caused by five <Emphasis Type="Italic">Pythium</Emphasis> species in Japan

Root and stem rot with wilt of above ground parts of cultivated chrysanthemums was first found in Ibaraki, Toyama and Kagawa prefectures, Japan in 2002 and 2003. Pythium species were isolated from the diseased tissues and identified as P. dissotocum, P. oedochilum, P. sylvaticum, P. ultimum var. ultimum and asexual strains of P. helicoides based on their morphologies and sequences of rDNA-ITS region. All the Pythium species were strongly pathogenic to chrysanthemums in pot conditions and were reisolated from the inoculated plants. Because Pythium root and stem rot of chrysanthemum has never been reported in Japan, we propose that this is a new disease that can be caused by the five Pythium species.     

Biochemical responses to ultraviolet‐C radiation and methyl jasmonate in Pinus radiata seedlings that accompany induced resistance to Diplodia pinea

Irradiation of Pinus radiata seedlings with ultraviolet‐C (UV‐C) radiation or spray application of methyl jasmonate (MeJA) each resulted in induced resistance to subsequent wound inoculation with Diplodia pinea. Induced resistance was expressed by lower incidence of disease and by reduced size of stem lesions than in untreated seedlings. UV‐C was more effective than MeJA and the induced resistance was greatest in seedlings that were irradiated with UV‐C for 60 min, 1 week before pathogen inoculation. Induced resistance in the UV‐C treated stems was concomitant with increases in peroxidase (POX) and polyphenol oxidase (PPO) activity but showed no correlation with concentrations of α‐pinene and β‐pinene or total phenolics. Furthermore, POX induction in stem tissue was cumulative and was greatest after three repeat treatments with UV‐C at 6, 3 and 1 week before inoculation. In contrast to UV‐C, MeJA induced a significant increase in β‐pinene concentration in stem tissue but did not affect PPO activity. POX activity was induced by MeJA in stems, although to a lesser extent than by UV‐C, but was not affected in needles. This appears to be the first report demonstrating the use of UV‐C radiation to induce resistance to fungal infection in a coniferous species. The implications of the underlying differences between UV‐C‐ and MeJA‐mediated resistance to D. pinea are discussed.     

棉花凝集素的形成与抗枯萎病性关系的研究

 试验表明:棉花凝集素广泛存在于陆地棉、中棉、海岛棉中,种子内凝集素活性程度与品种的抗性呈正相关。棉花生长过程中,1-6片真叶期抗病种的根、茎、叶凝集素活性比感病种强,花铃期以后差异逐渐缩小;抗病种花粉凝集素的含量比感病种花粉高4倍以上,未受精胚珠差异不显著;种子形成的30天前抗、感品种的凝集素活性无差异,30天后抗病种凝集素活性逐渐增强,感病种逐渐减弱;棉株中部内围棉铃种子凝集素活性强且稳定。抗病吕种凝集素的粗提物对棉枯萎病菌孢子萌发有明显的抑制作用,该粗提物的血凝活性可被半乳糖解除。     

Evaluation of Induction of Systemic Resistance in Pepper Plants (Capsicum Annuum) to Phytophthora capsici Using Trichoderma harzianum and its Relation with Capsidiol Accumulation

The effect of pepper seed and root treatments with Trichoderma harzianum spores on necrosis caused in stems by Phytophthora capsici inoculation and on the course of capsidiol accumulation in the inoculated sites were studied. The results indicate that seed treatments significantly reduced stem necrosis, which fell by nearly a half compared with the values observed in plants grown from non-treated seeds. Necrosis was also reduced in plants whose roots were drenched with various doses of T. harzianum spores, although the extent of necrosis was not correlated with the dose used. Attempted isolation of P. capsici and T. harzianum from the zones immediately contiguous with the necrotic zones revealed the presence of the former but not of the latter, suggesting that there was no direct contact between them in the zones of isolation, which means that there was no competition for space. The percentage of P. capsici isolated 9 days after inoculation was greater in non-treated inoculated plants than in treated inoculated plants. These results suggest that T. harzianum, introduced into the subterranean part of the plant, induces a systemic defense response against P. capsici in the upper part of the plant. Analysis of capsidiol in the stems of treated inoculated plants by the end of the sixth day after inoculation, revealed that its concentration was more than seven-fold greater than in non-treated and inoculated plants, while after 9 days, the concentration of capsidiol decreased in the treated inoculated plants and increased in the non-treated inoculated plants. The high concentration of capsidiol detected in treated and inoculated stems after 6 days might be one of the contributing factors, but not necessarily the main factor, in delaying lesion development in the stems of pepper plants.     

Accumulation of phytoalexins in susceptible and resistant near-isogenic lines of tomato infected with Verticillium albo-atrum or Fusarium oxysporum f.sp. lycopersici

The time course of accumulation of two phytoalexins, the terpenoid rishitin and the polyacetylene cis-tetradeca-6-ene-1,3-diyne-5,8-diol, was determined in near-isogenic susceptible and resistant tomato lines inoculated with either Verticillium albo-atrum or Fusarium oxysporum f.sp. lycopersici.Cultivars containing the Ve gene for verticillium wilt resistance accumulated phytoalexins at a rate similar to that in susceptible plants following stem inoculation with V. albo-atrum. Higher amounts of phytoalexins were isolated from susceptible than from resistant plants at 11 days after inoculation. Inoculum concentrations of 10⁵, 10⁶, 10⁷ and 10⁸ conidia ml⁻¹ had no differential effect on phytoalexin accumulation at 3 days after inoculation. Also, no differences were observed between fungal growth in susceptible and resistant cultivars during that period.A cultivar containing the I-1 gene for fusarium wilt resistance contained more rishitin than did susceptible plants at 2 and 3 days after inoculation with 10⁷ conidia of F. oxysporum f.sp. lycopersici ml⁻¹, but at 7 and 11 days after inoculation more rishitin had accumulated in the susceptible plants.No difference was observed between the rate of accumulation of phytoalexin in stem segments from resistant and susceptible plants inoculated by vacuum-infiltration.To estimate the concentration of phytoalexins in the xylem fluid, sap was expressed from vascular tissue and amounts of phytoalexins were determined in the sap and in the expressed tissue. Less than 5% of the phytoalexins present in stem segments was recovered from the sap, indicating that their concentration in the xylem fluid may be relatively low.The role of phytoalexins in resistance to verticillium and fusarium wilt is discussed.     

Comparative effect of root-knot nematode on severity of verticillium and fusarium wilt in cotton

The effect of root-knot nematode (RKN) (Meloidogyne incognita) onVerticillium dahliae andFusarium oxysporum f.sp.vasinfectum in cotton (Gossypium hirsutum) was investigated. Two different inoculation methods were used, one in which inoculum was added to the soil, so that nematode and fungal inoculum were in close proximity; the other, inoculation into the stem, whereby the two inocula were spatially separated. Invasion of the roots by RKN enhanced disease severity, as measured by the height of vascular browning in the stem, following inoculation with either wilt pathogen. The effect of RKN on Fusarium wilt was more pronounced than that on Verticillium wilt. Nematode-enhanced infection byF. oxysporum is a well known effect but there are few reports of enhanced infection byVerticillium due to RKN. Relative resistance of a number of cotton cultivars to both wilt diseases, as measured by height of vascular browning, was similar to the known field performance of the cultivars. The use of vascular browning as an estimate of disease severity was therefore validated. http://www.phytoparasitica.org posting Feb. 3, 2003.     

Passive transport of microconidia of Fusarium oxysporum f. sp. dianthi in carnation after root inoculation

Root inoculation of susceptible carnations withFusarium oxysporum f. sp.dianthi induced characteristic unilateral wilt only if root woundings and use of a microconidial suspension had not been combined at the time of inoculation. The combination, however, induced atypical and sudden stem breaking soon followed by death. In all cases wilt was due to destruction of the xylem. Unilateral wilt appeared to follow sparse natural infection of single roots. Stem breaking was due to destruction of the vascular tissues all around the stem and is ascribed to multilateral infection caused by translocation of microconidia at inoculation through several wounded roots directly into the stem.Microconidia were carried passively 5–10(–10) cm into stems of susceptible and resistant carnations within 24 h both after immersing cut ends of the roots in a conidial suspension and after pouring a suspension on the soil. Passive spore transport is an inoculation artefact which may severely affect interpretation of experimental results; it seems to be unimportant in natural Fusarium wilt development in carnation.Samenvatting Inoculatie van de wortels van vatbare anjers metFusarium oxysporum f. sp.dianthi veroorzaakte de kenmerkende eenzijdige verwelking alleen als wortelbeschadigingen bij de inoculatie niet werden gecombineerd met het gebruik van een suspensie van microconidiën. Die combinatie veroorzaakte namelijk afwijkende symptomen waarbij de planten plotseling omknakten en vervolgens snel afstierven. De verwelking leek in alle gevallen veroorzaakt te worden door afbraak van het xyleem. Eenzijdige verwelking leek te volgen op spaarzame natuurlijke wortelinfecties. Bij omgeknakte planten bleek het vaatweefsel rondom in de stengel aangetast te zijn, hetgeen toegeschreven wordt aan infectie van verschillende kanten van de stengel als gevolg van passief transport van microconidiën bij de inoculatie door verscheidene beschadigde wortels direct de stengel in.Microconidiën werden binnen 24 uur 5–10(–30) cm de stengels van vatbare en resistente anjers ingezogen wanneer de wortels afgesneden en met het uiteinde in een sporensuspensie gehangen werden, maar ook wanneer de suspensie op de grond gegoten werd. Passief transport van sporen is een inoculatie-artefact dat echter belangrijke consequenties kan hebben voor de interpretatie van de resultaten van proeven. Bij de natuurlijke verspreiding vanF. oxysporum in anjers lijkt passief sporentransport van weinig belang.     

Evaluation of the susceptibility of olive cultivars to verticillium wilt1

In the present study, we evaluated the susceptibility of different commercial olive cultivars to verticillium wilt. Two Verticillium dahliae isolates, obtained from olive and artichoke, were used in pathogenicity tests. Two-year-old rooted cuttings were inoculated using either the root-dip or the stem-wounding method. The results were similar with both inoculation methods. Cvs Carolea and Cipressino proved to be moderately susceptible whereas Cassanese, Nocellara del Belice, Nocellara Etnea, Tonda Iblea and Uovo di Piccione were very susceptible. The response of cv. Coratina varied from susceptibility to moderate susceptibility.