Distribution of Phytophthora cinnamomi-Supprcssive Soil in Nature

Phytophthora cinnamomi-suppressivc soils were found to be widely distributed in nature. About 40 % of soil samples collected from locations with different vegetation, soil type or elevation throughout the island of Hawaii were suppressive to chlamydospore germination of Phytophthora cinnamomi. Soil samples collected from the same general areas varied greatly in degree of suppressive-ness to P. cinnamomi, ranging from conducive to strongly suppressive. Among the 155 soil samples tested, those with pH close to 4 or 8 tend to be more suppressive to P. cinnamomi than those with pH close to 6.     

Defining Plant Resistance to Phytophthora cinnamomi: A Standardized Approach to Assessment

Phytophthora cinnamomi is a soil‐borne plant pathogen that causes devastating disease in agricultural and natural systems worldwide. While a small number of species survive infection by the pathogen without producing disease symptoms, the nature of resistance, especially under controlled conditions, remains poorly understood. At present, there are no standardized criteria by which resistance or susceptibility to P. cinnamomi can be assessed, and we have used five parameters consisting of plant fresh weight, root growth, lesion length, relative chlorophyll content of leaves and pathogen colonization of roots to analyse responses to the pathogen. The parameters were tested using two plant species, Zea mays and Lupinus angustifolius, through a time course study of the interactions and resistance and susceptibility defined 7 days after inoculation. A scoring system was devised to enable differentiation of these responses. In the resistant interaction with Z. mays, there was no significant difference in fresh weight, root length and relative chlorophyll content in inoculated compared with control plants. Both lesion size and pathogen colonization of root tissues were limited to the site of inoculation. Following inoculation L. angustifolius showed a significant reduction in plant fresh weight and relative leaf chlorophyll content, cessation of root growth and increased lesion lengths and pathogen colonization. We propose that this technique provides a standardized method for plant–P. cinnamomi interactions that could be widely used to differentiate resistant from susceptible species.     

Containment and spot eradication of a highly destructive,invasive plant pathogen (<Emphasis Type="Italic">Phytophthora cinnamomi</Emphasis>) in natural ecosystems

The invasive plant pathogen Phytophthora cinnamomi (Stramenopila, Oomycota) has been introduced into 15 of the 25 global biodiversity hotspots, threatening susceptible rare flora and degrading plant communities with severe consequences for fauna. We developed protocols to contain or eradicate P. cinnamomi from spot infestations in threatened ecosystems based on two assumptions: in the absence of living hosts, P. cinnamomi is a weakly competitive saprotroph; and in the ecosystems we treated, the transmission of the pathogen occurs mainly by root-to-root contact. At two P. cinnamomi-infested sites differing in climate and vegetation types, we applied increasingly robust treatments including vegetation (host) destruction, fungicides, fumigation and physical root barriers. P. cinnamomi was not recovered at three assessments of treated plots 6–9 months after treatments. Given the high rates of recovery of P. cinnamomi from untreated infested soil and the sampling frequency, the probability of failing to detect P. cinnamomi in treated soil was <0.0003. The methods described have application in containing large infestations, eradicating small infestations and protecting remnant populations of threatened species.     

Effect of Brassica Biofumigant Amendments on Different Stages of the Life Cycle of Phytophthora cinnamomi

The oomycete plant pathogen Phytophthora cinnamomi causes a highly destructive root rot that affects numerous hosts. Integrated management strategies are needed to control P. cinnamomi in seminatural oak rangelands. We tested how biofumigation affects crucial stages of the pathogen's life cycle in vitro, in infested soils under laboratory conditions and in planta. Different genotypes of three potential biofumigant plant species (Brassica carinata, Brassica juncea, Brassica napus) were collected at different phenological stages, analysed for their glucosinolate contents, and subsequently tested. The most effective genotypes against mycelial growth and sporangial production were further tested on the viability of chlamydospores in artificially infested natural soils and in planta on Lupinus luteus, a host highly susceptible to P.cinnamomi. Brassica carinata and B. juncea genotypes inhibited mycelial growth, decreased sporangial production, and effectively inhibited the viability of chlamydospores in soil, but only B. carinata decreased disease symptoms in plants. Effective genotypes of Brassica had high levels of the glucosinolate sinigrin. Biofumigation with Brassica plants rich in sinigrin has potential to be a suitable tool for control of oak root disease caused by P. cinnamomi in Spanish oak rangeland ecosystems.     

Mechanisms of salt tolerance in transgenic Arabidopsis thaliana constitutively overexpressing the tomato 14-3-3 protein TFT7

A hydroponics experiment was conducted to investigate the effects of iron plaque on root surfaces with respect to selenite uptake and translocation within the seedlings of two cultivars of rice (Oryza sativa L. cv Xiushui48 and Bing9652). Different amounts of iron plaque were formed by adding 0, 10, 30, 50, 70 mg Fe l⁻¹ in the nutrient solution. After 24 h of growth, the amount of iron plaque was positively correlated with the Fe²⁺ addition to the nutrient solution. These concentrations of Fe, inducing plaque, had no significant effect on the shoot and root growth of rice plants in 50 μg Se l⁻¹ nutrient solution. The amount of Se accumulated in iron plaque was positively correlated to the amount of iron plaque. Increasing iron plaque decreased the selenium concentration in shoots and in roots. At the same time, the translocation of Se from roots to shoots was reduced with increasing amounts of iron plaque. At both the shorter and longer exposure times, the ratio of root- to-shoot selenium was higher than in the controls. More Se stayed in the roots at the longer exposure time than at the shorter time. The concentration of selenium in the xylem sap was sharply decreased with increasing amount of iron plaque on the rice roots. The DCB (dithionite-citrate-bicarbonate)-extracted Se was up to 89.9–91.1% of the total Se when roots with iron plaque (Fe 70) were incubated in 50 μg Se l⁻¹ solution for 30 min. This DCB-extracted Se, however, accounted for only 21.9–28.7% of total Se when roots with iron plaque were incubated in the same solution for 3 days. Se adsorbed in iron plaque can be desorbed by low-molecular-weight organic acids, similar to the desorption of Se from ferrihydrite. These results suggest that iron plaque might act as a ‘buffer’ for Se in the rhizosphere.     

Combined effects of soil properties and Phytophthora cinnamomi infections on Quercus ilex decline

Aims

The importance of soil properties as determinants of tree vitality and Phytophthora cinnamomi root infections was analysed.

Methods

The study comprised 96 declining stands in western Spain, where declining and non-declining holm oak (Quercus ilex L.) trees were sampled. Soil properties (soil depth, Ah horizon thickness, texture, pH, redox potential, soil bulk density and N-NH₄ ⁺ and N-NO₃ ^? concentrations) and P. cinnamomi infections were assessed.

Results

Tree mortality rates increased with low soil bulk densities, which were also associated with more P. cinnamomi-infected trees. Occurrence of infected trees was higher in fine textured soils and in thick Ah horizons. Fine textured soils favoured trees, but with the presence of P. cinnamomi their health status deteriorated. Soil under declining trees had higher N-NO₃ ^?/N-NH₄ ⁺ ratio values than under non-declining trees. Additional soil properties changes associated to grazing were not related to decline and P. cinnamomi infections.

Conclusions

The implications of P. cinnamomi in holm oak decline and the influence of soil properties as contributors to pathogen activity were demonstrated. Fine soil textures and thick Ah horizons, usually favourable for vigour and vitality of trees growing in the Mediterranean climate, were shown to be disadvantageous soil properties if P. cinnamomi was present. Fine soil textures and thick Ah horizons are frequently related with higher levels of soil moisture, which increase the inoculum of the pathogen and favours root infection. Grazing does not seem to be directly linked to Q. ilex health status or P. cinnamomi root rot.     

Current and projected global distribution of Phytophthora cinnamomi,one of the world's worst plant pathogens

Globally, Phytophthora cinnamomi is listed as one of the 100 worst invasive alien species and active management is required to reduce impact and prevent spread in both horticulture and natural ecosystems. Conversely, there are regions thought to be suitable for the pathogen where no disease is observed. We developed a climex model for the global distribution of P. cinnamomi based on the pathogen's response to temperature and moisture and by incorporating extensive empirical evidence on the presence and absence of the pathogen. The climex model captured areas of climatic suitability where P. cinnamomi occurs that is congruent with all available records. The model was validated by the collection of soil samples from asymptomatic vegetation in areas projected to be suitable by the model for which there were few records. DNA was extracted, and the presence or absence of P. cinnamomi was determined by high‐throughput sequencing (HTS). While not detected using traditional isolation methods, HTS detected P. cinnamomi at higher elevations in eastern Australia and central Tasmania as projected by the climex model. Further support for the climex model was obtained using the large data set from south‐west Australia where the proportion of positive records in an area is related to the Ecoclimatic Index value for the same area. We provide for the first time a comprehensive global map of the current P. cinnamomi distribution, an improved climex model of the distribution, and a projection to 2080 of the distribution with predicted climate change. This information provides the basis for more detailed regional‐scale modelling and supports risk assessment for governments to plan management of this important soil‐borne plant pathogen.     

Trunk injection of fosetyl‐aluminium controls the root disease caused by Phytophthora cinnamomi on Quercus ilex woodlands

In Spain, Quercus open woodlands are animal ranching systems of organic production seriously threatened by the exotic pathogen Phytophthora cinnamomi. The root disease it causes kills thousands of oaks annually. Effective disease management needs to integrate different techniques, and the use of a resistance inducer such as fosetyl‐Al can play a key role, because the use of potassium phosphite is prohibited in Spain. In a woodland where the pathogen recently arrived, 60 holm oaks in three different defoliation classes (asymptomatic, slight and moderate defoliation) were selected for trunk injection with pressurised capsules containing 4% of commercial fosetyl‐Al or water (controls). Holm oaks were checked periodically for defoliation and presence of the pathogen in roots and rhizosphere soil. Three years after treatments, defoliation was significantly lower in oaks treated with fosetyl‐Al, which even increased canopy cover, in comparison with control oaks, independent of the initial defoliation class considered. Chlamydospore density in rhizosphere soil, as well as the presence of the pathogen into the roots, was not significantly influenced by fosetyl‐Al treatments, although a trend to a lower presence of P. cinnamomi in roots was observed in treated oaks at every soil inoculum density detected. This study has shown that fosetyl‐Al, a phosphonate registered as a fungicide in the European Union, provides protection to holm oaks against P. cinnamomi, even exhibiting a therapeutic effect on pre‐existing infections. Consequently, this effective measure should be considered as part of the integrated approach to control this highly destructive pathogen in holm oak woodlands.     

Potential for dissemination of Phytophthora cinnamomi by feral pigs via ingestion of infected plant material

Feral pigs have long been implicated as potential vectors in the spread of the devastating plant pathogen Phytophthora cinnamomi due to their rooting and wallowing activities which may predispose them as vectors of infested soil. In this study, we aim to determine whether feral pigs have the potential to act as vectors of plant pathogens such as P. cinnamomi through their feeding activity. The typically omnivorous diet of feral pigs may also lead to the passage of P. cinnamomi infected plant material through their digestive system. This study investigates the potential for feral pigs to pass viable P. cinnamomi in their faeces following the ingestion of millet seeds, pine plugs and Banksia leptophilia roots inoculated with P. cinnamomi. Recovery rates of P. cinnamomi from the millet seeds, pine plugs and B. leptophilia roots following a single ingested bolus were 33.2, 94.9 and 10.4 %, respectively supported by quantitative PCR analysis. These results demonstrate that P. cinnamomi remain viable within infected plant material following passage through the pig digestive tract, although the digestive processes reduce the pathogen’s viability. An inverse relationship was observed between the viability of infected material and passage time, suggesting that partially digested plant material provides protection for P. cinnamomi against the adverse environmental conditions of the pig digestive tract. Phytophthora cinnamomi remained viable for up to 7 days in larger pieces of colonised woody plant material such as the pine plugs. A plant infection trial using passaged P. cinnamomi colonised pine plugs showed that even material that remained in the digestive tract for 7 days was capable of infecting and killing healthy plants, susceptible to P. cinnamomi. This study provides compelling evidence that feral pigs have the ability to transport viable P. cinnamomi in their digestive tract.     

Eastern gamagrass (Tripsacum dactyloides) root penetration into and chemical properties of claypan soils

Claypans restrict rooting depth and availability of moisture and nutrients to plants during periods of drought. Eastern gamagrass (Tripsacum dactyloides var. dactyloides [L.] L.) often remains green during summer droughts, while other plants turn brown. Questions arose whether eastern gamagrass roots had or could penetrate claypans to obtain needed moisture. Pits were dug (2 m deep) under eastern gamagrass plants that had been growing 50+ and 5+ years at two sites in Missouri. Clay contents were 30 to 50% in soil layers below 30 cm, and moisture was not limiting in these deep soil layers. Soil pH_Ca in the lower soil layers, except at 180 cm, was below 5.0, and in some cases near 4.0. Extractable Al was especially high in the 90 and 120 cm deep soil layers where pH was low. Extractable Ca, Mg, and K increased with soil depth. The eastern gamagrass roots effectively penetrated claypan soils. Root lengths and root weights were extensive to 180 cm depth, and decreased from the surface with soil depth. Roots of eastern gamagrass were aerenchymous (having cellular compartments which allow air movement) at all depths, were mycorrhizal to at least 150 cm depth, and had relatively high tolerance to acidic Al toxic Tatum subsoil (Typic Hapludult) and toxic levels of Al in nutrient solution. The eastern gamagrass roots also provided root channels through claypans, which could enable new eastern gamagrass or other plant roots to grow into deeper soil layers.     

Regulation of defence responses in avocado roots infected with Phytophthora cinnamomi (Rands)

Phytophthora cinnamomi occurs worldwide and has a host range in excess of 1,000 plant species. Avocados (Persea americana Mill) have been described as highly susceptible to this soil-borne pathogen. Here, the regulation of defence responses in avocado root seedlings inoculated with P. cinnamomi mycelia is described. A burst of reactive oxygen species (ROS) was observed 4 days after inoculation. The higher physiological concentration of H₂O₂ induced by P. cinnamomi on avocado roots had no effect on in vitro growth of the oomycete. Total phenols and epicathecin content showed a significant decrease, but lignin and pyocianidins exhibited no changes after inoculation. Also, increased nitric oxide (NO) production was observed 72 h after treatment. We studied the effects of one NO donor [sodium nitroprusside (SNP)], and one NO scavenger [2- to 4-carboxyphenyl-4,4,5,5-tetramethylimidazole-1-oxyl-3-oxide (CPTIO)] to determine the role of NO during root colonisation by P. cinnamomi mycelia. Pretreatment of the roots with CPTIO, but not with SNP, inhibited root colonisation suggesting an important role for NO production during the avocado–P. cinnamomi interaction. Our data suggest that although defence responses are activated in avocado roots in response to P. cinnamomi infection, these are not sufficient to avoid pathogen invasion.     

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     

Chemical,physical and biological control of carrot seedling diseases

In naturally infested soil containingPythium ultimum, P. acanthicum andPhytophthora megasperma, onlyP. ultimum was associated with root rot and damped-off seedlings. Damping-off was promoted by low soil temperatures and by flooding. Seedling stands were markedly reduced when seed was pre-incubated in soil at 12°C but not at 25°C or 35°C. Dusting carrot seed with metalaxyl significantly increased seedling stands in the field at rates from 1.5–6 g kg⁻¹ seed and in both flooded and unflooded, naturally infested soil at 3.15 g kg⁻¹. In greenhouse experiments using artifically infested soil,P. ultimum andP. paroecandrum caused damping-off of carrot seedlings andRhizoctonia solani reduced root and shoot weights.R. solani caused damping-off in nutrient-enriched soil.P. acanthicum andP. megasperma were not pathogenic to seedlings, although both fungi colonized roots. Soil populations of allPythium spp., particularlyP. ultimum, increased during growth of seedlings and population growth ofP. megasperma was promoted by periodic flooding. Infestation of soil withP. acanthicum did not reduce damping-off of carrot seedlings byP. ultimum orP. paroecandrum, but significantly increased root and shoot weights and decreased root colonization byR. solani P. acanthicum has potential as a biocontrol agent againstR. solani.     

Effects of extracts of Alnus glutinosa seeds on growth of Frankia strain ArI3 under static and fermentor culture conditions

The amount of root mortality caused by root pathogens such as Phytophthora nicotianae (syn. Phytophthora parasitica) has typically been inferred from the net change in root length density in sequential soil cores. Because such measurements give information only on net changes in root populations, the actual rate of root turnover is often underestimated. We used minirhizotrons to track the fate of a large number of individual fine roots of mature field-grown citrus trees over a 6-month period. This method enabled us to examine the effect of P. nicotianae population levels on fine-root mortality. Seasonal and genotypic variation in patterns of citrus fine root mortality were associated with variation in population levels of P. nicotianae. Fine root lifespans were shorter when populations of P. nicotianae were high. Fine roots of the Phytophthora-susceptible rootstock, rough lemon (Citrus jamibhiri), had shorter median lifespans and supported larger populations of P. nicotianae than the fine roots of the more tolerant rootstock, Volkamer lemon (Citrus volkameriana). Rates of root mortality were either relatively constant for roots of all ages, or increased with age; the latter pattern was most pronounced for Volkamer lemon roots. Differences in the age-dependence of root mortality may, therefore, play a role in genotypic differences in tolerance of Phytophthora root rot by these two rootstocks. H Lambers Section editor     

Differential plant response to inoculation with two VA mycorrhizal fungi isolated from a low-pH soil

Sericea lespedaza and ladino clover were inoculated withAcaulospora laevis andGigaspora margarita VA mycorrhizal fungi, both isolated from a local soil having a pH of 4.4. Plants were grown in a greenhouse in fumigated (methyl bromide) soil with four rates of applied P. This soil had a pH of 5.1 for theG. margarita experiment and a 5.8 for theA. laevis experiment. Neither plant species responded to theG. margarita isolate in terms of mycorrhizal infection of roots, top growth, or elemental uptake. TheA. laevis isolate caused increased growth of Ladino clover at the lowest rate of P application and increased growth of sericea lespedeza at the two lowest P application rates. Shoot tissue concentrations of P for both plant species were greater at the two lowest rates of P application when inoculated withA. laevis. Inoculation withA. laevis also resulted in different VAM fungal root colonization patterns between the two plant species as a function of P application rate. Roots of both plants had high infection rates (near 70%) for the two lowest P application rates but sericea lespedeza declined to 40 and 6% at the next two P application rates, respectively, whereas infection in Ladino clover was 74 and 41% at the next two P application rates, respectively. The results of this study support the concept that plant host-VAM fungal endophyte interactions are extremely variable and that characterization of individual combinations must be done if types and magnitudes of responses are to be defined.     

Interactions between the soilborne root pathogenPhytophthora nicotianae var.parasitica and the arbuscular mycorrhizal fungusGlomus mosseae in tomato plants

In order to study the influence of Arbuscular Mycorrhiza (AM) on the development of root rot infection, tomato plants were raised with or withoutGlomus mosseae and/orPhytophthora nicotianae var.parasitica in a sand culture system. All plants were fed with a nutrient solution containing one of two phosphorus (P) levels, 32µM (I P) or 96µM (II P), to test the consequence of enhanced P nutrition by the AM fungus on disease dynamics. Mycorrhizal plants had a similar development to that of control plants. Treatment withPhytophthora nicotianae var.parasitica resulted in a visible reduction in plant weight and in a widespread root necrosis in plants without mycorrhiza. The presence of the AM fungus decreased both weight reduction and root necrosis. The percentage reduction of adventitious root necrosis and of necrotic root apices ranged between 63 and 89% The enhancement of P nutrition increased plant development, but did not appreciably decrease disease spread. In our system, mycorrhiza increased plant resistance toP. nicotianae var.parasitica infection. Although a contribution of P nutrition by mycorrhiza cannot be excluded, other mechanisms appear to play a crucial role.     

Thermal sensitivity of Phytophthora cinnamomi and long-term effectiveness of soil solarisation to control avocado root rot

Root rot caused by the fungus Phytophthora cinnamomi is a major disease of avocados worldwide. Heat sensitivity of a collection of P. cinnamomi isolates was determined by exposing agar discs containing mycelium or mycelium plus chlamydospores at various temperatures for different periods. Long‐term effectiveness of soil solarisation to control Phytophthora root rot was evaluated in two field trials. In the first, soil disinfestation by solarisation was applied in 1990 to a naturally infested plot before planting avocado (Persea americana) and viñatigo (Persea indica) seedlings. In the second trial, established avocado trees were solarised for four consecutive summers (1996–1999). Results for heat sensitivity showed that fungal mycelium was inactivated after 1–2 h at 38°C. However, 1–2 h at 40°C was needed to kill all propagules when chlamydospores were present. Fungal growth inhibition after thermal treatments was related to levels of time and temperature, and detrimental effects occurred as consequence of sublethal thermal doses. Soil solarisation presented long‐term positive effects when applied as a preplanting treatment. Five years after solarisation, disease severity (0–5 scale where 0 = healthy and 5 = dead plant) of avocado and viñatigo planted in solarised soil was 2.03 and 0.71, respectively, compared with 4.65 and 4.84 in controls. Eleven years after solarisation, the percentage of dead plants in solarised soil was 73% for avocado and 43% for viñatigo but 100% in controls. In contrast, an insufficient level of control was observed in established orchards, probably because of the lower temperature reached during solarisation under the shade of tree canopy. In this situation, maximum temperatures at 5‐cm depth were 10–13.7°C lower than under solar‐heated mulch, only exceeding 40°C in 1997.     

Benzoic acid induces tolerance to biotic stress caused by Phytophthora cinnamomi in Banksia attenuata

Banksia attenuata plants were treated with soil drenches or foliar sprays of benzoic acid (BZA) to determine induced resistance to Phytophthora cinnamomi. Stems of B. attenuata were inoculated with the pathogen 1 week after treatment with BZA. Resistance was estimated by measuring P. cinnamomi lesions on stems. Treatment with 0.10 mM, 0.25 mM or 0.50 mM BZA caused a reduction in lesion size with 0.50 mM BZA applied as a soil drench being the most effective treatment at suppressing the development of lesions. This is the first report of BZA induced host resistance in any plant species to any pathogen.     

Relationship of alien species continues in a foreign land: The case of Phytophthora and Australian Banksia (Proteaceae) in South African Fynbos

Fungal invasions only recently started to receive more attention in invasion biology. This is largely attributed to little or non‐existent information about these inconspicuous organisms. Most invasion hypotheses focus on factors that increase invasion success; few try to explain why invasions fail. Here we hypothesize that a host–pathogen relationships can limit the invasiveness of an alien plant species in a novel range. To test this, we investigate whether the invasiveness of the Australian genus of Proteaceae, Banksia, in South Africa is determined by the alien and major invasive phytopathogen, Phytophthora cinnamomi. The presence of P. cinnamomi in Banksia root and soil was evaluated using morphological and molecular techniques. Isolates were cultured onto selective media and polymerize chain reactions and internal transcribing spacers were used for identification. Acetone leaf extracts of 11 Banksia spp. were screened for antimicrobial activity against P. cinnamomi, using the minimum inhibitory concentration assay. A total of 3840 Banksia individuals from seven localities were surveyed. Phytophthora  cinnamomi was consistently isolated from Banksia species root and soil samples. Out of the 12 Banksia species that were screened for antimicrobial activity, four introduced species, B. burdettii, B. coccinea, Banksia hookeriana, and B. prionotes and the invasive B. integrifolia and B. ericifolia exhibited relatively high antimicrobial activity against P. cinnamomi (strain 696/12). We show that the phytopathogen in the native range has similar impact in the novel range and in doing so may limit invasion success of Banksia species with low antimicrobial activity.     

Effect of different soil types,pH and phosphorus levels on the wheat seedling and the incidence ofPythium graminicolum

Summary In the present study, the effect of different types of soil, pH and phosphorus levels on root, shoot development and browning extent on shoot of wheat seedlings growth in soil inoculated withPythium graminicolum; was studied. Out of the four soil types the development of root and shoot was better in Domatta soil where as the disease development was more in black clay soil. The pH values below 5.5 and above 8.0 are favourable for the root and shoot development whereas these values are unfavourable for the development of the Pythium and consequently for infection by it. The percentage of browning of the collar region was more in soils near neutral (pH 6.5–7.1) and it was less in acidic (pH 5.0) or alkaline (pH 8.6) soils. The development of wheat roots and shoot in soil with 30 lb/acre phosphorus was good as compared to that in soil with P₀, P_15,45 and P₆₀ lb/acre in inoculated soil. The percentage of extent of browning on collar region was less at 15 lb and 30 lb/acre P, due to less development of the disease.