Phytophthora Spp Research Articles

The oxidative burst protects plants against pathogen attack: Mechanism and role as an emergency signal for plant bio-defence — a review

Various aspects, mechanisms and functions of the oxidative burst with generation of O 2 − superoxide anions in plant cells, which is stimulated by active defence-inducing agents such as fungal infection or elicitor treatment, were reviewed mainly on the basis of experimental evidence obtained in a system of Solanaceae plants and Phytophthora spp. The oxidative burst may be due to an O 2 −-generating NADPH oxidase in the plasma membrane, which is activated with combinations of cytosolic proteins, Ca 2+, calmodulin and protein kinase, following stimulation by elicitor molecules. The oxidative burst may play the role of an internal emergency signal for induction of the metabolic cascade for active defence.

Susceptibility of low chill peach rootstocks to Phytophthora spp

Peach seedlings (Prunus persica L. Batsch cv. Ansbacher, Boyles, Clarke's Shanghai, Neilson (Fred Hill), Okinawa, O'Meara, Richens Nos 1,2 and 3, Tomm's Early and Tomm's Shanghai), which are commonly used as rootstocks for low chill peaches in coastal New South Wales, were all susceptible to Phytophthora cinnamomi and P. cambivora and to a lesser extent P. parasitica, P. citricola, P. cryptogea, and P. megasperma, when tested by stem inoculations in the glasshouse. Myrobalan H29C plum (P. cerasifera Ehrh.) cuttings showed more resistance than peach seedlings. Variety of seedling peach was highly significant in stem inoculation experiments, but the performance of each varied between experiments. Root inoculations in aerated nutrient solution showed all rootstocks were susceptible to P. cinnamomi, but Neilson (Fred Hill) peach was more tolerant than other seedling peaches and less tolerant than Myrobalan H29C plum. No recommendation could be made on a rootstock for low chill peaches more tolerant of root and collar rots and waterlogging.

Effects of Month of Inoculation on Severity of Disease Caused byPhytophthoraspp. in Apple Root Crowns and Excised Shoots

Effects of month of inoculation on severity of disease caused by Phytophthora spp. in apple roostock EMLA.106 were studied in excised shoots in vitro and in intact root crowns in an orchard. In both orchard and in vitro assays, 25 successive sets of host tissue were inoculated at monthly intervals with mycelial disks from V8 agar cultures of Phytophthora spp. Crown rot severity was measured in orchard trees as area of bark necrosis after 14 days of incubation, and canker severity was measured in excised shoots as length of bark necrosis after 5 days of incubation. In both assays, P. cactorum and P. cambivora caused very little necrosis in apple incubated during dormancy (December through February) and growth resumption (March). The amount of necrosis in root crowns and excised shoots was much greater following inoculations in late spring (May). Maximum necrosis and subsequent decline in disease development during incubation periods occurred I to 3 months later in orchard tree inoculations (August through October) than in excised shoot inoculations (May through August). Crown rot severity after 2 weeks of incubation was not a reliable predictor of disease severity after 13 months of incubation ; relatively mild crown rot caused by P. cactorum during and immediately following tree dormancy occasionally continued to develop, resulting in relatively severe crown rot on trees 13 months after inoculation. Under orchard conditions in California, apple root-stock EMLA.106 is apparently susceptible to development of relatively severe crown rot for a longer period of the year than would be expected from short-term indications of susceptibility in excised shoots in vitro.

Behaviour of zoospore cysts of the mycoparasite Pythium oligandrum in relation to their potential for biocontrol of plant pathogens

Several in vitro features of zoospore cysts of Pythium oligandrum suggest their potential value for biocontrol, in place of oospores which germinate slowly and erratically. Cysts germinated rapidly (20–30 min) in water at 20–25 °C. They also germinated, but more slowly, at water potentials down to −6·6 MPa (PEG 8000 solutions) or −2·65 MPa (mannitol solutions), whereas mycelia did not grow at −2·1 MPa. Cysts could be stored for at least 3 wk in PEG at 4–7°, and germinated when the PEG was diluted and temperature was raised to 20°. Germinating cysts rapidly (<1 h) parasitized hyphae of Fusarium oxysporum and F. culmorum in the absence of exogenous nutrients. When cysts of a metalaxyl-tolerant mutant of P. oligandrum were added to moist, unsterile soil at 20°, the fungus was recovered at one-third of the original level at 28 d, and was still detected at 80 d. Cysts and motile zoospores were studied for mycoparasitic activity on water-agar films in vitro . Cysts germinated readily on colonies of F. oxysporum and Pythium aphanidermatum , and in the presence of spores of Idriella bolleyi , but poorly on colonies of F. culmorum , which seemed to produce an inhibitor. Motile zoospores were not attracted to hyphae but if they encysted on hyphae they showed significant germ-tube emergence towards the host; pre-encysted spores germinated with random orientation. A higher incidence of parasitism from motile spores than from cysts indicated a need for anchorage (adhesion during encystment) in the conditions of the assays, implicating mechanical pressure in mycoparasitism. Sporelings needed to grow some distance (about 30 m) before they parasitized efficiently; this might involve transition from a germ-tube to a vegetative hypha, because it was similar to the maximum growth (26–27 m) from cysts at water potentials that inhibited hyphal growth. These findings extend previous work on zoospore behaviour in Pythium and Phytophthora spp.

Ultrastructural changes in roots of peppers resistant and susceptible to Phytophthora capsici

The infection by Phytophthora capsici Leonian causes foliar blight and crown and root rot of pepper plants. The ultrastructural examination of resistant and susceptible host-pathogen interactions was conducted in the pepper cultivars Ince Sivri-35, PM217, and PM702=CM 334 following inoculation with Phytophthora capsici. Responses were characterized and compared with healthy non-inoculated controls at 2,4 and 6 days after inoculation. Numerous ultrastructural studies have been made of the interaction of various host plants Phytophthora spp. No ultrastructural studies comparing the resistant and susceptible reactions of pepper cultivars to P. capsici. have been reported. It is important to examine the ultrastructural changes in inoculated and infected tissue of resistant and susceptible pepper cultivars to P. capsici. Information reported here characterizes the processes of pathogen containment in resistant interactions and compares these with the processes occuring in susceptible interactions.After 2 days in susceptible interactions, the pathogen grew intercellularly in roots, whereas in resistant interactions only a few intercellular hyphae were observed penetrating the host cells and forming haustoria.

Variation in enzyme activities in leaves and cell suspensions as markers of incompatibility in different Phytophthora-pepper interactions

Activities of a pool of enzymes were compared in leaves and cell suspensions of two near-isogenic lines of pepper, susceptible and resistant to Phytophthora capsici , inoculated with the pathogen and with P. nicotianae var. parasitica. Higher constitutive chitinase, β-1,3-glucanase and peroxidase activities were detected in uninoculated leaves and cell cultures of the resistant line. Post-infectional variations in leaves were related to symptom severity rather than varietal or non-host resistance to the Phytophthora spp. In cell suspensions, inoculation with both fungi induced an early and transient increase of phenylalanine ammonia-lyase (PAL) and β- d-glucosidase activities in the resistant line only; cell cultures, however, did not discriminate between P. capsici and P. parasitica. PAL and β- d-glucosidase variations appear to be suitable post-infectional markers of varietal resistance to P. capsici in pepper cell cultures but not in leaves.

Relative Resistance of Eighteen Selections ofMalusspp. to Three Species ofPhytophthora

Eighteen selections from 13 species and hybrids of Malus (M. baccata. M. brevipes. M. coronaria. M. domestica, M. fusca, M. halliana, M. ioensis, M. × magdeburgensis, M. mandshurica, M. × platycarpa, M. prunifolia, M. sargentii, and Malus sp.) were evaluated in a greenhouse for resistance to root and crown rots caused by Phytophthora cactorum, P. cambivora, and P. cryptogea. Seven- to nine-week-old seedlings were transplanted into noninfested soil or soil artificially infested with P. cactorum, P. cryptogea, or P. cambivora. The soil was flooded once every 2 wk for 48 h, and plants were evaluated for severity of disease 3 mo after transplanting. Seedlings of domestic apple (M. domestica, (used as a standard for comparison in this study) were among the most susceptible of the Malus spp. to crown and root rot caused by P. cactorum. P. cryptogea, and P. cambivora. The relative resistance of other selections of Malus spp. varied according to species of Phytophthora and whether crown or root rot variables were used to assess resistance. Among 10 selections of Malus spp. that were evaluated along with two selections of M. domestica for resistance to P. cactorum, P. cryptogea, and P. cambivora, six selections were relatively resistant compared to M. domestica to root and crown rot caused by P. cactorum, but only M. halliana, M. × magdeburgensis, and M. sargentii were relatively resistant to root and crown rot caused by all three fungi. Species of Malus can vary greatly in resistance to species of Phytophthora, and, among Malus spp., assessments of relative resistance to P. cactorum are not necessarily extendible to other Phytophthora spp

Antagonism of Chaetomium globosum, Gliocladium virens and Trichoderma viride to four soil‐borne Phytophthora species

Abstract Chaetomium globosum, Gliocladium virens and Trichoderma viride, isolated from soil samples from the Kivu‐region of Zaire, are antagonistic and mycoparasitic to four soil‐borne Phytophthora spp. Radial growth of Phytophthora cinnamomi, P. cactorum, P. fragariae or P. nicotianae mycelia was inhibited in all antagonist‐pathogen combinations; mycelial mats of Phytophthora colonies were overgrown by the antagonists and hyphae lysed. Samples of mycoparasited mycelium, plated on benlateamended PDA to inhibit the development of antagonists, showed severely reduced vitality after 77 days of dual culture.

Diseases of Pines and Eucalypts in South Africa Associated withPythiumandPhytophthoraSpecies

SYNOPSIS This review covers diseases of Pinus and Eucalyptus spp. associated with Pythium and Phytophthora spp. The relative importance of these fungi is discussed and their host range is described. It is shown that the most important species are Phytophthora cinnamomi and to a lesser extent Phytophthora cryptogea. Diseases associated with Pythium and Phytophthora spp. in South Africa have been inadequately studied in the past. This review emphasises the occurrence and importance of Pythium and Phytophthora in forestry in South Africa and attempts to place it in perspective.

896 PB508 A NOVEL METHOD OF AUXIN APPLICATION STIMULATES ADVENTITIOUS ROOT FORMATION IN WALNUT

The use of auxin-impregnated toothpicks stimulated adventitious root formation in genotypes of Juglans `Paradox' that had been backcrossed to J. regia. These genotypes were selected as potential rootstocks because of improved tolerance to cherry leaf roll virus and Phytophthora spp. Other auxin applications including quick dips and talc formulations had little or no effect. The use of toothpicks lowered the concentration of IBA necessary for root initiation compared to previously reported results using quick dips. Toothpicks were inserted transversely into holes drilled 1 to 2 cm above the base of cuttings. Callus and roots always formed at the location of the toothpicks rather than at the base of the cutting. Roots were formed using this method in simple layering, hardwood, and semi-hardwood cuttings. Of all the cuttings that rooted, 90% rooted with toothpicks whereas only 10% rooted using a quick dip. This method may have potential for increasing the efficiency of rooting other difficult-to-root plants.

Elicitor-induced chemiluminescence in cell suspension cultures of tomato, sweet pepper and tobacco plants and its inhibition by suppressors from Phytophthora spp

The hyphal wall components (HWC) from Phytopthora capsici, P. infestans and P. nicotianae var. nicotinae, which caused the hypersensitive cell death of suspension-cultured cells of sweet pepper ( Capsicum frutescenceL.), tomato ( Lycopersicon esculentum Mill.) and tobacco ( Nicotiana tabacumL.), enhanced the luminol-mediated chemiluminescence in suspensio-cultured cells of each plant within several minutes after treatment with HWC in a species-nonspecific manner. HWC-induced chemiluminescence in suspension-cultured cells was enhanced by incubation with superoxide dismututase but was significanyly inhibited by pretreatment with catalase or, the O 2 −scavenger, Tiron. Similarly, Tirom or water-soluble glucans (WSG) from respective species of Phytophthora suppressed both the HWC-induced chemilunescence and hypersensitive cell death specific only to its own host. These results suggested that Phytophthora spp. may produce suppressor-gluncans which inhibit the activation of O 2 − generation responsible for triggering hypersensitive cell death in the respective Solanaceous host plants and permit the establishment of a compatible interaction.

Transformation of four pathogenic Phytophthora spp by microprojectile bombardment on intact mycelia

Phytophthora capsici, P. citricola, P. cinnamomi and P. citrophthora were transformed without the removal of cell walls by particle acceleration with plasmids containing the beta-glucuronidase gene and hygromycin B resistance. Transformants were detected by histochemical and fluorometric beta-glucuronidase assays and confirmed by Southern-blot hybridization. It was found that the promoter of a plant virus is functional in Phytophthora. In addition, a method was designed to visually identify homogeneous transformed colonies, derived from zoospores of transformed multinucleated Phytophthora mycelia, based on blue color development on plates containing X-Gluc.

Comparison of ELISA Techniques and Standard Isolation Methods forPhytophthoraDetection in Citrus Orchards in Florida and California

Commercial enzyme-linked immunosorbent assay (ELISA) kits were evaluated for detection of Phytophthora spp. in fibrous roots and soil in citrus orchards in Florida and California, and compared to root rot evaluations, percent infection in root pieces, and dilution plating of soil on selective media. In Florida, root ELISA values with kits D and E were correlated with percent root infection and propagule densities of Phytophthora parasitica on an orchard basis. In California, root ELISA values with kit D were correlated with propagule densities of Phytophthora citrophthora on an orchard basis. Kit D was more effective than kit E for the detection of Phytophthora spp, in fibrous roots because of high background values with kit E

Effect of seed coating protectants on iron deficiency chlorosis and sorghum plant growth

Abstract Chemical coatings on seed are commonly used in the grain sorghum seed industry to protect plants against herbicide damage and to inoculate young plants against certain disease. In this study, grain sorghum grown from seed safened with CGA‐92194 ([(l,3‐dioxolo‐2‐yl‐methoxy)‐imino] benzene‐acetonitrile} against injury from the herbicide, metolachlor, was studied for seedling vigor, seedling emergence, plant growth, and Fe‐deficiency chlorosis. Other seed was additionally treated with a systemic fungicide [(Metalaxyl:N‐(2,6‐dimethylphenyl)‐N‐(methoxyacetyl) alaminemethyl ester)] for control of systemic downy mildew and diseases caused by soilborne Phythium and Phytophthora spp. The experiment was conducted on Fe‐sufficient and Fe‐deficient soils. CGA‐92194 safened seed showed a slight reduction in stand on Goliad sandy loam soils (Fe‐sufficient), while the addition of the fungicide together with the herbicide safener reduced plant emergence by 39%. Sorghum emergence was generally not affected by see...

Influence of Fusarium solani on citrus root rot caused by Phytophthora parasitica and Phytophthora citrophthora

Interactions between Fusarium solani and Phytophthora parasitica or F. solani and P. citrophthora influenced the development of root rot of citrus but depended on the temporal order of inoculation with F. solani or the two Phytophthora spp. Inoculation of citrus with either Fusarium solani and Phytophthora parasitica or Phytophthora citrophthora increased root rot compared to inoculation with P. parasitica or P. citrophthora alone when plants were inoculated with Phytophthora by dipping their roots in zoospore suspensions and subsequently transplanted into soil infested with F. solani. However, root rot was not increased by simultaneous co-inoculation of P. parasitica and F. solani or when plants were inoculated with F. solani first. Root rot was not increased when heat-stressed or non-stressed plants were inoculated with P. parasitica 30 days after transplanting into soil infested with F. solani. In most but not all experiments, F. solani alone reduced growth of tops or roots a small but significant amount. Co-inoculation of citrus by root-dipping into zoospore suspensions of P. parasitica and transplanting into soil infested with F. solani reduced feeder root length by 62% and root weight by 61% but did not significantly reduce the percentage of living roots when compared to inoculation with P. parasitica alone. When citrus roots were immersed in zoospore suspensions of P. citrophthora and transplanted into soil infested with F. solani, feeder root length was reduced by 68%, but feeder root weight and the percentage of living roots were not significantly reduced when compared to plants inoculated with P. citrophthora alone. Propagule densities of both P. parasitica and P. citrophthora in the rhizosphere of plants inoculated by root-immersion and then transplanting into soil infested with F. solani were not significantly different than propagule densities from plants transplanted into non-infested soil. Propagule densities of P. parasitica were suppressed an average of 41% when citrus was inoculated with P. parasitica 30 days after transplanting into soil infested with F. solani and by 41% when citrus was co-inoculated by transplanting into soil infested with both F. solani and P. parasitica.

Effects of Phenylamide Pesticides on the GSH-Conjugation System of Phytophthora spp. Fungi

Abstract Phytophthora cactorum, Ph. cambivora, Ph. cinnamomi, Ph. citricola, Ph. cryptogea, Ph. drechsleri, Ph. infestans, Ph. megasperma, Ph. parasitica, and Ph. syringae contain GSH-conjugation systems as indicated by the presence of active GST enzyme in addition to GSH. Basal levels of both GSH and GST in the thalli of Phytophthora strains studied did not correlate with either fungal sensitivity to phenylamides (acetochlor, butachlor, metolachlor, dimethachlor, propachlor, ofurace, CGA-29212, metalaxyl, RE-26745, benalaxyl, furalaxyl, LAB-149202) or their acquired resistance to metalaxyl. Thalli of Phytophthora strains from axenic cultures ex­ posed to sublethal concentrations of the above pesticides contained significantly higher levels of both GSH and GST than the untreated controls. This response was independent of the sen­sitivity and tolerance of the strains to phenylamides. When the responses of Phytophthora strains to phenylamide chemicals were compared by means of principal component analysis, four independent components were detected account­ ing for 88% of the total variation. Biological properties (basal and induced levels of GST and GSH, growth intensity, degree of acquired resistance to metalaxyl, sensitivity to propiconazole and to cis-and trans-tridemorph) of the strains contributed differently to this variation. It was concluded that, in contrast to plants, sensitivity or tolerance of Phytophthora species to phenylamide pesticides is not regulated by the efficiency of the GSH-conjugation system. In addition, our data clearly indicate that the acetanilide pesticides have multiple sites of action in the Phytophthora genus.

Tandem arrangement of tRNA Asp-encoding genes in Phytophthora spp

We have cloned a region of repetitive DNA from the phytopathogenic fungus, Phytophthora parasitica. The cloned region consists of 17 highly homologous units arranged in tandem. The consensus sequence is 562 bp long and carries the information for a tRNA Asp. All sequence motifs required for efficient RNA polymerase HI transcription are present, and the tRNA derived from the nucleotide sequence is able to form a complete cloverleaf structure with high homology to previously characterized tRNA Asp molecules. The isolated tRNA Asp gene cluster is located at a distance of 20 kb from the TRP1 gene of P. parasitica. It comprises about 0.1 % of the total genomic DNA. Similar clusters were detected in four other Phytophthora species.

Effects of sterile and non-sterile leachates extracted from composted eucalyptus bark and pine-bark container media on Phytophthora spp

Production of sporangia by Phytophthora cinnamomi. Phytophthora cryptogea, Phytophthora citricola, Phytophthora drechsleri and Phytophthora nicotianae var. nicotianae on Miracloth discs was compared in filter-sterilized and non-sterile lcachates extracted from a pine-bark based container medium (NM) and a composted Eucalyptus-bark medium (CEB). Sporangial production of all species was suppressed in non-sterile CEB leachate; whereas, in sterile CEB leachate it was stimulated in comparison to the NM leachate. P. drechsleri and P. cryptogea appeared to require the presence of certain microorganisms to induce zoospore release. Chlamydosporc production by P. cinnamomi was suppressed in CEB lcachates. On cornmcal and water agar mixed with filtcr-stcrilized or non-sterile leachate from NM and CEB. mycelial growth varied with leachate. agar type and Phytuphthora species. Isolates within P. drechsleri and P. cinnamomi responded differently to the extracts.

How container media and matric potential affect the production of sporangia, oospores and chlamydospores by three phytophthora species

Miracloth discs containing Phytophthora cinnamomi, Phytophthora cilricola and Phytophthura drechsleri were exposed to matrie potentials ( ψ m) between 0 and 10 kPa. in a composted Eucalyptus-bark mix (CEB) which was apparently suppressive to disease and a nurseryman's mix (NM) which was apparently conducive to it. The CEB and NM stimulated P. cinnamomi and P. cirechsleri to produce sporangia. The CEB suppressed indirect germination of both species. P. cilricola required an exposure period of 10 days before sporangia formed, which were produced in significantly higher amounts at −5 and −7.5kPa in the NM. P. cinnamomi and P. drechsleri produced more sporangia as ψ m decreased. Chlatnydosporcs produced by P. cinnamomi were lysed in both media at 0 and −1.0 kPa, but at matric potentials below −2.5 kPa they were lysed only in the CEB. Oospores of P. citricola were produced only up to 7 days in both media at ψ m below −2.5 kPa. Bacteria and actinomycetc filaments were closely associated with all propagules of the three Phytophthora spp in the CEB. They were most evident at matric potentials < −2.5 kPa.

Role of Phytophthora spp. in citrus decline in Apulia and Basilicata, Italy1

The results of field surveys carried out in citrus groves of Apulia and Basilicata in Italy to clarify the etiology of a decline condition associated with root and foot damage of citrus are reported. Field observations and laboratory tests have shown that the most frequent causes of citrus decline are root and foot infections by Phytophthora spp. The species isolated were P. nicotiunae var parasiticu and P. citrophthora; the former was the species most often isolated from both the soil and the feeder roots. Less frequently, ArmiNuria sp., frost damage and rodents were identified as additional causes ofdecline. Finally the role ofcultural factors in the decline caused by Phytophthora spp. is discussed.