Changes in Respiration of Seedling Roots Inoculated with Phytophthora cinnamomi

An increase in rate of respiration was recorded for intact roots of seven native Australian species 16 h after inoculation with Phytophthora cinnamomi. By 24 h the magnitude of the increase ranged from 2—159% above that of the uninoculated controls and was evidently not related to host susceptibility. A time sequence study of lesion extension and the associated increased respiration rates for both susceptible and tolerant eucalypts demonstrated a difference in response. The rate of respiration in the tolerant species increased 2 % and only at the site of inoculation, whereas in the susceptible species the respiration rate increased in a wave which began at the inoculation site and continued along the root with the advancing fungal invasion. Respiration rate only increased in regions of the root actually inhabited by the pathogen. The fungal contribution to the total respiration of infected roots was less than 1 % and was determined by measuring respiration of inoculated killed roots. Respiration rates were measured in the presence of potassium cyanide (KCN) and salicylhydroxamic acid (SHAM). Both KCN-sensitive and SHAM-sensitive respiration occurred in normal uninfected E. marginate seedlings. A large proportion of the increase in total respiration rate of infected seedlings compared with uninoculated controls was due to the alternate, SHAM-sensitive pathway. The physiological implications of these results are discussed.     

Survival of Phytophthora cinnamomi in Buried Eucalyptus Roots

Colonization and survival of Phytophthora cinnamomi in roots was tested in 3 months old, axenically grown seedlings of Eucalyptus maculata (field resistant) and E. sieberi (susceptible). The roots were inoculated, then one week later were excised and buried in three non-sterile, conducive soils; a lateritic gravel, an infertile duplex soil, a loamy sand as well as in a fertile, suppressive krasnozem. Pathogen viability, percentage root colonization and chlamydospore numbers were examined at matric potentials of ?1/3, ?5 and ?10 bar after periods of 10, 100 and 200 days at 21°C. At 10 days, survival was 100% in the form of mycelium and the only significant difference was between the two Eucalyptus species. At 100 days survival was solely due to chlamydospores, but the pathogen was viable in all inoculated roots and at each matric potential. At 200 days soils had dried to less than ?10 bars and the pathogen failed to survive. No significant differences were found between the two pathogen isolates but significant differences were obtained between the susceptible and field resistant Eucalyptus species. Pathogen viability, percentage root colonization and chlamydospore number were highly correlated with soil types and matric potential. These components declined with decreasing soil matric potential. The Krasnozem was only suppressive at relatively high soil matric potentials (?1/3 bar). At lower values (?5, ?10 bar) survival of the pathogen, chlamydospore numbers and percentage colonization of the roots in the Krasnozem were comparable with that of the 3 conducive soils tested. Chlamydospores were present, but in low numbers in roots buried in the suppressive soil at ?1/3 bar.     

Behaviour of Phytophthora cinnamomi Zoospores on Roots of Four Avocado Cultivars

Scanning electron microscopy was used to study the behaviour of Phytophthora cinnamomi zoospores on the roots of three tolerant avocado cultivars. Duke 7, G6 and Martin Grande, and a susceptible Edranol cultivar. Zoospores were attracted to the region of cell elongation and encysted on the roots of all cultivars studied. Adhesion of the zoospores appeared to be aided by root slime. Cysts usually produced one germ tube which penetrated the root directly, or formed an appressorium-like swelling before penetration occurred. Extensive growth of germ tubes occurred where zoospores germinated some distance behind the region of elongation. Cysts germinating behind this region often formed branched germ tubes and more than one appressorium-like swelling. There were no clear differences in the type of pre-penetration structures, formed by zoospore cysts, on the roots of the different avocado cultivars.     

Changes in Mineral Content of Eucalyptus marginata and E. calophylla Grown under Controlled Conditions and Inoculated with Phytophthora cinnamomi

Mineral concentrations in infected roots and shoots were compared with similar material, but pathogen free, for the susceptible Eucalyptus marginata and the field resistant E. calophylla. All plants were mycorrhiza-free, grown under controlled conditions and inoculated with an axenic suspension of zoospores of Phytophthora cinnamomi. Plant material was ashed 30 days after inoculation and analyzed in an external proton beam using PIXE and nuclear reaction analyis to detect differences in mineral concentrations. The mineral content of infected roots of E. marginata was below that of the uninfected roots for all elements detected except chlorine, chromium and rubidium. The reductions were significant for calcium and copper. No such reduction was found, in E. calophylla roots, but some changes were detected. Shoot: root ratios of E. marginata showed significant differences associated with infection in phosphorus, calcium, copper and zinc. Relatively large differences were also recorded for sulphur chlorine and potassium. Shoot: root ratios of infected E. calophylla showed fewer differences but contained only half the concentrations of sulphur and potassium present in the controls. The reduced mineral concentrations reflect the failure in conduction of water and minerals through the infected and susceptible root system.     

Characterization of Phytophthora cinnamomi populations from ornamental plants in South Carolina,USA

Abstract

Fifty-one isolates of Phytophthora cinnamomi isolated from ornamental plants in South Carolina, USA, between 1995 and 2000 were characterized by sporangium morphology, mating type, sensitivity to the fungicide mefenoxam, fatty acid methyl ester (FAME) profile analysis, and amplified fragment length polymorphism (AFLP) analysis. Sporangium shapes were predominantly ovoid to ellipsoid, and size averaged 65.5×40.3 μm (length×breadth) with average length/breadth ratio of 1.6. Forty-nine isolates were the A2 mating type with only two A1 isolates found. This is the first report of the A1 mating type of P. cinnamomi in South Carolina. All isolates were sensitive to mefenoxam and EC₅₀ values for all isolates were less than 0.2 μg ml^?1. FAMEs of each isolate were analysed by gas chromatography and revealed five major fatty acids: myristic (14:0), palmitic (16:0), linoleic (18:2ω6c), oleic (18:1ω9c), and eicosapentaenoic (20:5ω3c) acids. These five fatty acids accounted for more than 80% of FAME profiles. Cluster analysis of FAME profiles showed that individual isolates had unique pattern that could be divided into four major clusters. AFLP analysis based on 200 informative loci also separated isolates into four major clusters. A1 isolates were different from all A2 isolates. The percentage of polymorphic loci (10.5%) and Nei's gene diversity (0.0435) were much higher for the two A1 isolates than for any cluster of A2 isolates even though A2 isolates had more isolates within a cluster. A2 isolates exhibited relatively little genetic variation overall, which suggests that these isolates may have come from a common source.     

Soil Water Extraction, Measured by Computer-Assisted Tomography, in Seedling Lupinus angustifolius cv. Yandee when Healthy and Infected with Phytophthora cinnamomi

The water-mould fungus Phytophthora cinnamomi Rands causes drought-likesymptoms on many hosts, and yet the mechanisms by which infectionleads to wilting are not fully understood. This is the firststudy to describe in detail changes in soil water around theroot with infection. Computer-assisted tomography (CAT) wasused with Lupinus angustifolius L. cv. Yandee to examine drawdowns(removal of soil water) around a central root infected by P.cinnamomi in a white sand. No growth differences in roots or shoots were found betweenhealthy and diseased plants during the 8 d of the experiment.However,drawdowns failed at high levels of inoculum (8–16 /Pc-infectedmillet seeds/plant) by 8 d. Water contents in pots with uninfectedplants were in the range 0·09–0·12 cm³ watercm^–3 soil in the centre of the pot, while water contentsin pots with infected plants at 16 millet seeds applied werein the range of 0·16–0·19 cm³ water cm^–3soil in the centre of the pot. A higher transpirational demand produced lower soil water contentsnear the root but this effect was confounded with infection:disease was more pronounced with higher transpirational demand,and disease led to an increase in water content. Key words: Root disease, Phytophthora cinnamomi, water uptake, soil-root interface, computer-assisted tomography     

Immunological Discrimination of Phytophthora cinnamomi from other Phytophthorae Pathogenic on Chestnut

A polyclonal antiserum (A379) against water soluble proteins from Phytophthora cinnamomi mycelium was produced in rabbit. In ELISA, the 1 : 10 000 diluted antiserum revealed only Phytophthora isolates, not allowing a clear‐cut discrimination among congenerous species, in spite of a generally higher reactivity on P. cinnamomi proteins. The antiserum gave positive reactions in Western blot analyses against mycelial proteins from nine species of Phytophthora and Pythium sp. (grown on rich media), but not with Rhizoctonia solani, binucleate Rhizoctonia, Verticillium dahliae, Fusarium oxysporum and Cryphonectria parasitica. All Phytophthora species showed common epitopes on proteins of molecular masses 77, 66, 51 and 48 kDa. However, a species‐specific protein of 55 kDa was immunodecorated only in P. cinnamomi samples, thus allowing univocal identification of this species. When tested against total proteins from the same fungi grown on water, the antibody revealed diagnostic bands of 55 and 51 kDa in P. cinnamomi only. The antiserum is therefore suitable for the specific identification of P. cinnamomi emerging in distilled water from infected tissues of chestnut, blueberry and azalea.     

Phytophthora cinnamomi in Shanghai and its possible origin

Camellia leaves were most effective for recoveringPhytophthora cinnamomi from soils followed by azalea leaves and cedar needles. A total of 131 isolates ofP. cinnamomi was obtained from soils and roots collected in Shanghai. Among them 125 were A¹ and 6 were A² type. There was little variation in morphological and physiological characteristics among 82 isolates tested. It is suggested that the fungus may have been a recent settler in Shanghai.     

Physiological differences among isolates of Phytophthora cinnamomi.

Significant differences in amylase, beta-glucosidase, and phosphatase activities were observed among four Phytophthora cinnamomi isolates grown in nutrient-amended sterilized soil for 20 days. Amylase pH optima for the four isolates were within a relatively narrow range; at pH 5.5 each isolate was within 90% of its peak activity. Isolates SB-216-1, 1-281, and C-39 each exhibited maximal beta-glucosidase activity at pH 5.0 and maximal phosphatase activity at pH 5.0-5.5. Maximal activity for these two enzymes of isolate A-7725 occurred at pH 3.5. In timed experiments, isolates 1-281 and A-7725 exhibited greater amylase activities than did the other two isolates. For beta-glucosidase, greatest activity was observed for SB-216-1; ACTivity of 1-281 was intermediate and least activity was observed for isolates A-7725 and C-39. Isolates SB-216-1 and 1-281 exhibited greatest phosphatase activities; isolate C-39 was intermediate in activity, and A-7725 was least active. Results indicate that significant differences exist among the isolates tested and that these differences can be quantitatively measured by the methods described.     

Safflower Seedling a Selective Host to Discriminate Phytophthora melonis from Phytophthora drechsleri

Safflower seedlings were used to discriminate two morphologically similar species of Phytophthora namely P. melonis and P. drechsleri . All isolates of P. melonis from different sources could not infect safflower seedlings under high inoculum potential whereas all isolates of P. drechsleri from various hosts attacked safflower within a short period. Of 31 authentic Phytophthora species inoculated to safflower seedlings only seven species including Phytophthora asparagi , Phytophthora cactorum , Phytophthora cryptogea , Phytophthora drechsleri , Phytophthora erythroseptica , Phytophthora palmivora and Phytophthora quercina caused hypocotyls infection. All cucurbit isolates of Phytophthora from different parts of Iran and one from China could not infect safflower seedlings and were identified as P. melonis which had been confirmed previously by molecular analysis.     

Identification and characterization of superoxide dismutase in Phytophthora cinnamomi

Superoxide dismutase (SOD) activities of the oomycete Phytophthora cinnamomi were examined. Five polypeptides with manganese superoxide dismutase (MnSOD) activity were found in mycelium growing in liquid culture with relative molecular weights ranging from approximately 25 to 100 kDa. Comparison with characterized avocado SODs showed no evidence for the presence of either iron or copper/zinc SODs in P. cinnamomi. The level of activity of the MnSOD polypeptides decreased in the presence of avocado root or cell wall components. Growth of P. cinnamomi, measured as dry weight, increased when the mycelium was grown in the presence of superoxide anion (O₂ ^?), which was added exogenously. Our results suggest that the metabolism of O₂ ^? has an important role in the development of P. cinnamomi.     

Phytophthora cinnamomi Associated with Root and Crown Rot of Walnut in Turkey

Walnut decline caused by Phytophthora sp. occurred in an orchard in Sakarya province in Turkey. Affected young trees showed poor growth, leaf discolouration, root and crown rot and eventual death. A Phytophthora sp. isolated from necrotic taproots and crown tissues. The causal agent of the disease was identified as Phytophthora cinnamomi by morphological characteristics and comparing sequences of internal transcribed spacer (ITS) region. Upon conducting pathogenicity test, averaging 7.8‐cm‐long canker developed on basal stem within 2 weeks, while no cankers developed in the control plants.