Indirect Germination Research Articles

Chitosan induces tomato basal resistance against Phytophthora nicotianae and inhibits pathogen development

Chitosan is a polymer of glucosamine with antimicrobial properties and an elicitor of plant defence responses. In this study, different chitosan concentrations were assessed for impact on the life cycle of Phytophthora nicotianae Breda de Haan, an oomycete pathogen of tomato and tobacco crops. Reduced mycelial growth was observed when chitosan was added to solid media, and this inhibitory activity was amplified with increasing chitosan concentrations from 0.5 g L−1 to 2.5 g L−1. Colony morphology and hyphal diameter were unaffected by the polymer; however, the number of oospores (sexual reproductive structures) was reduced at the lowest chitosan concentration (0.5 g L−1) tested. Mycelia plugs imbibed with 0.5 g L−1 chitosan were prone to increased sporangium formation, but the effect was subdued with higher chitosan concentrations. The length/width ratio of the sporangia did not change in the presence of the polymer. Indirect germination of the sporangia was particularly sensitive to this compound, and 1.5 g L−1 completely abolished this process. The zoospores treated with chitosan formed cysts that did not germinate and were unable to infect tomato plant roots. Chitosan was also found to prime tomato plantlets for P. nicotianae resistance; a foliar application of 0.1 and 1 g L−1 of chitosan 72 h before inoculation induced host resistance, which was concurrent with increased activity of phenylpropanoid pathway markers. The results presented herein show the potential of chitosan as a tool to control P. nicotianae inoculum and increase tomato crop resistance against this pathogen.

Phytophthora infestans Sporangia Produced in Culture and on Tomato Leaflet Lesions Show Marked Differences in Indirect Germination Rates, Aggressiveness, and Global Transcription Profiles.

Sporangia of Phytophthora infestans from pure cultures on agar plates are typically used in lab studies, whereas sporangia from leaflet lesions drive natural infections and epidemics. Multiple assays were performed to determine if sporangia from these two sources are equivalent. Sporangia from plate cultures showed much lower rates of indirect germination and produced much less disease in field and moist-chamber tests. This difference in aggressiveness was observed whether the sporangia had been previously incubated at 4°C (to induce indirect germination) or at 21°C (to prevent indirect germination). Furthermore, lesions caused by sporangia from plates produced much less sporulation. RNA-Seq analysis revealed that thousands of the >17,000 P. infestans genes with a RPKM (reads per kilobase of exon model per million mapped reads) >1 were differentially expressed in sporangia obtained from plate cultures of two independent field isolates compared with sporangia of those isolates from leaflet lesions. Among the significant differentially expressed genes (DEGs), putative RxLR effectors were overrepresented, with almost half of the 355 effectors with RPKM >1 being up- or downregulated. DEGs of both isolates include nine flagellar-associated genes, and all were down-regulated in plate sporangia. Ten elicitin genes were also detected as DEGs in both isolates, and nine (including INF1) were up-regulated in plate sporangia. These results corroborate previous observations that sporangia produced from plates and leaflets sometimes yield different experimental results and suggest hypotheses for potential mechanisms. We caution that use of plate sporangia in assays may not always produce results reflective of natural infections and epidemics.

Cataloguing variability in Phytophthora infestans with respect to ploidy status and response of different polyploids to temperature

Late blight, caused by the oomycete Phytophthora infestans, had been, and continues to be the most dreaded disease of potato and tomato world over. In the present study, P. infestans isolates collected from Indian hills, sub-tropical plains and plateau regions were analyzed for their ploidy status and results revealed that P. infestans population in the country are polyploid consisting of diploids, triploids and tetraploids. Diploids were predominant in all the locations while tetraploids were altogether missing from north-eastern and southern hills while in plateau only diploids were observed. Response of different polyploids to temperature was studied and results revealed that mycelial growth was observed in the range of 15–28 °C with the optimum temperatures being 15–20 °C. Indirect germination was highest at 4 °C, with germination decreasing drastically with increase in temperature while direct germination was fairly good in all polyploids in the range of 8–22 °C, though differed significantly, thereafter it declined drastically. Temperature differentially affected incubation period (IP), latent period (LP), lesion area (LA) and sporulation. In general, IP decreased with increase in temperature and shortest IP for all polyploids occurred at 24 °C. Lesion area increased with increase in temperature in all polyploids up to 20 °C and thereafter it decreased. At 28 °C, diploids and triploids could not establish infection while tetraploids could do after 72 h. No sporulation was observed for any of the polyploid at 10 and 28 °C while diploids and triploids did not sporulate at 24 °C. In general diploids had highest sporulation than tetraploids. Diploids were predominant at all locations and were more aggressive that could be one of the reasons for their dominance in the country.

Disease-Suppressive Vermicompost Induces a Shift in Germination Mode of Pythium aphanidermatum Zoosporangia.

Compost amendments to soils can minimize losses from soilborne plant pathogens, yet the mechanisms by which this occurs have not been well elucidated. In the present study, developmental responses of Pythium aphanidermatum zoosporangia to vermicomposts were observed to better understand how suppression of Pythium seedling disease is expressed. Mature zoosporangia were exposed to vermicompost extracts (VCEs) and monitored using time-lapse photomicroscopy. Sterile and nonsterile VCEs inhibited indirect germination and viable zoospore production whereas zoosporangia germinated directly in VCE to produce germ tubes. Additional treatments were tested to determine factors that promote direct over indirect germination. The pH (5 to 9 at 0.001 M) and ionic strength (0.1 to 0.0001 at pH 6) of potassium phosphate buffer did not alter zoosporogenesis compared with sterile water. Decreasing osmotic potentials in glucose and sucrose from -248 to -2,712 kPa or in polyethylene glycol 8000 from -0.335 to -105 kPa led to a decrease in indirect germination with a corresponding increase in direct germination. Significant levels of seed infection were observed within 1 h of exposure to zoospores (produced in sterile water) or to germ tubes (produced in sucrose solution). Our data demonstrate that VCEs suppress zoosporogenesis and stimulate direct germination; however, this did not result in the suppression of germ tube growth and seed infection.

Phenotypic Characterization of Recent Clonal Lineages of Phytophthora infestans in the United States.

Phytophthora infestans, the causal agent of late blight disease, has been reported in the United States and Canada since the mid-nineteenth century. Due to the lack of or very limited sexual reproduction, the populations of P. infestans in the United States are primarily reproducing asexually and, thus, show a simple genetic structure. The emergence of new clonal lineages of P. infestans (US-22, US-23, and US-24) responsible for the late blight epidemics in the northeastern region of the United States in the summers of 2009 and 2010 stimulated an investigation into phenotypic traits associated with these genotypes. Mating type, differences in sensitivity to mefenoxam, differences in pathogenicity on potato and tomato, and differences in rate of germination were studied for clonal lineages US-8, US-22, US-23, and US-24. Both A1 and A2 mating types were detected. Lineages US-22, US-23, and US-24 were generally sensitive to mefenoxam while US-8 was resistant. US-8 and US-24 were primarily pathogenic on potato while US-22 and US-23 were pathogenic on both potato and tomato. Indirect germination was favored at lower temperatures (5 and 10°C) whereas direct germination, though uncommon, was favored at higher temperatures (20 and 25°C). Sporangia of US-24 released zoospores more rapidly than did sporangia of US-22 and US-23. The association of characteristic phenotypic traits with genotype enables the prediction of phenotypic traits from rapid genotypic analyses for improved disease management.

Effects of Temperature on Events in the Infection Cycle of Two Clonal Lineages of Phytophthora infestans Causing Late Blight on Tomato and Potato in Brazil.

In Brazil, US-1 and BR-1 clonal lineages of Phytophthora infestans are widely distributed throughout the major growing areas of tomato and potato, respectively. Quantitative information regarding the effects of temperature (10, 15, 22, and 27°C) on direct and indirect sporangia germination, incubation period (IP), latent period (LP), lesion area (LA), and sporulation (SP), as well as combined temperature and leaf wetness duration periods (6, 12, 18, and 24 h) on the number of lesions (NL) was obtained under controlled conditions on either detached leaflets or whole plants. The percentage of indirect germination was higher for BR-1 isolates than for US-1. The percentage of direct germination was higher for US-1 than for BR-1. The shortest IP and LP were recorded at 22°C for both lineages: 69.3 h and 93.3 h for US-1 isolates on detached tomato leaflets, and 44.0 h and 68 h for BR-1 isolates on detached potato leaflets, respectively. US-1 isolates did not sporulate at 10°C, and BR-1 isolates did not sporulate at 27°C. Isolates of both lineages induced the largest LA at 22°C. The NL was highest at 15°C for US-1 isolates on whole tomato plants, and at 10°C for BR-1 isolates on whole potato plants. The differential effects of temperature on US-1 and BR-1 suggest that current decision support systems initially developed for controlling US-1 in Brazil may now be inaccurate for controlling BR-1, and management strategies should be properly validated before being used.

Control of late blight in organic potato production: evaluation of copper-free preparations under field, growth chamber and laboratory conditions

In order to replace copper fungicides in organic potato production, 53 copper-free preparations (CFPs) based on natural compounds, including plant extracts and microorganisms, and five copper preparations were evaluated for their potential to control Phytophthora infestans, the pathogen that causes late blight of potatoes. In in vitro assays, 30% of the CFPs inhibited indirect germination of sporangia, 26% mycelial growth and in growth chamber experiments, 21% efficiently reduced foliar blight of tomato plants. In micro-plot field trials with applications twice a week, the copper preparations were the most effective and reduced foliar blight by 99%. Of the CFPs tested, Oekofluid P, Mycosin and other sulphuric clays, and C-2000 reduced late blight the most, from 63% to 37%. In small-plot trials in 2001, 2002 and 2004, 27 CFPs with different formulations and four copper preparations were examined. In 2004, copper preparations at full and reduced rates and sulphuric clays were applied either weekly or according to the decision support system Bio-PhytoPRE. With Bio-PhytoPRE, copper preparations reduced foliar blight of potatoes by 23–77% and increased tuber yield by 2–28%, depending on the copper rate applied and year. With CFPs, maximal efficacy was 17% and no effect on tuber yield was observed. In vitro and in vivo trials showed that the rainfastness and the persistence of CFPs was low compared with copper preparations. This indicates that the failure of CFPs under field conditions is probably due to a lack of stability under prevailing environmental conditions and not to a lack of efficacy. Until stable formulations for CFPs are developed, an optimised and restricted use of copper fungicides using a decision support system could help to control late blight in organic potato production and to reduce copper input into the environment.

Antagonistic effects of metabolites of Pseudomonas fluorescens strains on the different growth phases of Phytophthora capsici, foot rot pathogen of black pepper (Piper nigrum L.)

Foot rot of black pepper caused by Phytophthora capsici causes heavy crop losses. Ecofriendly biological control was given high priority in the Integrated Disease Management programs (IDM). Strains of P. fluorescens which were antagonistic to P. capsici, the foot rot pathogen in black pepper produced inhibitory metabolites. Up to 72% inhibition of mycelial growth of P. capsici was observed in dual culture experiments. The metabolites of P. fluorescens strain, IISR-6 completely inhibited the sporangial production the most explosive phase in P. capsici. The indirect germination of sporangia (release of zoospores) also was inhibited considerably. The percent inhibition varied from 89 – 98%. The germination of zoospores of P. capsici was negatively influenced by the metabolites of P. fluorescens. Up to 90% inhibition of germination was noticed. Partial characterization of the metabolites let to the detection of two antibiotics – Pyoluteorin and pyrrolnitrin. The volatile metabolites of P. fluorescens also inhibited the different phases in life cycle of P. capsici, particularly mycelial growth and production of sporangia. HCN was detected in the volatile metabolites of the bacterial strains in varying quantities. The results are discussed in relation to the mechanism of disease suppression.

Effect of surface wettability on germination and gene expression in Cronartium quercuum f. sp. fusiforme basidiospores

Cronartium quercuum f. sp . fusiforme is an obligate pathogen of pine and oak. Its basidiospores are specifically adapted to recognize and establish infections on the pine host. Depending on environmental cues, the basidiospores can germinate directly, which typically leads to infection of pine, or indirectly, which usually results in formation of secondary basidiospores. We investigated how changes in surface wettability, or hydrophilicity, affect basidiospore germination. When we decreased surface wettability, the direct germination and total germination (direct+indirect) frequencies increased. Additionally, there was a critical threshold between 42% and 54% wettability, at which the basidiospores switched from direct to indirect germination. We conclude that the germination type of C. q. fusiforme basidiospores is influenced by the hydrophilicity of the surface upon which they land. To gain insight into gene expression during basidiospore germination, we made two suppression subtraction hybridization (SSH) libraries enriched for genes expressed during each type of germination. A total of 172 unique gene sequences were recovered from the two expression libraries. Annotation of these libraries indicates that they include several clones that may encode rust-specific or basidiomycete-specific functions.

Floristic Composition of Vegetation and the Soil Seed Bank in Different Types of Dunes of Kerqin Steppe

The floristic composition of the vegetation and the soil seed bank in different types of dunes of Kerqin steppe were compared. The aim of this study was to examine the influence of desertification on the floristic composition of the vegetation and soil seed bank. An indirect germination method was used to study the seed bank. Thirty species of the vegetation and 25 of the seed bank were identified, 23 species being common to both. Differences between four types of dunes (mobile sand dune, MSD; semifixed sand dune, SSD; fixed sand dune, FSD; interdunal lowland, ILD) in floristic composition of the vegetation and the soil seed bank were examined using nonparametric methods. In the four types of dune, species composition varied significantly except between site MSD and site SSD as well as between site FSD and site ILD in aboveground vegetation. As for soil seed bank, the number of species and seeds differed significantly only between site FSD and site ILD. Species diversity revealed that species numbers increased in the sequence of MSD, SSD, FSD and ILD. The dominant species of aboveground vegetation in site MSD and site SSD were Agriophyllum squarrosum and Setaria viridis, respectively. While in both site FSD and site ILD, the dominant was Eragrostis pilosa. Of total germinated seed, the most abundant species in site MSD were Agriophyllum squarrosum, but the dominant species was Eragrostis pilosa in the other three sites. In each site, more than half of the species recorded in aboveground vegetation were found in the seed bank, even more in the seed bank of site FSD and site ILD. In site FSD and site ILD, high correspondence was observed between the species composition of the aboveground vegetation and of the associated soil seed bank. Spearman's rank correlation coefficients were 0.295, 0.046, 0.704, and 0.612 for site MSD, site SSD, site FSD and site ILD, respectively. These results indicated that desertification development decreased correlation between the seed bank and vegetation in Kerqin Steppe.

Antifungal Activity of the Novel Fungicide Cyazofamid against Phytophthora infestans and Other Plant Pathogenic Fungi in Vitro

Antifungal activity of the novel fungicide cyazofamid (ISO proposed common name), 4-chloro-2-cyano-N,N-dimethyl-5-p-tolylimidazole-1-sulfonamide, was investigated. Cyazofamid proved highly selective to fungi classified as Oomycetes in mycelial growth inhibition tests. The EC50 values of cyazofamid against Phytophthora infestans, Phytophthora sojae, Pythium aphanidermatum, Pythium paddicum, Pythium sylvaticum, and Aphanomyces cochlioides were between 0.008 and 0.2 μg/ml. On the other hand, cyazofamid did not inhibit mycelial growth of fungi classified as Ascomycetes, Basidiomycetes, or Deuteromycetes. Cyazofamid strongly inhibited all stages in the life cycle of P. infestans. Minimum inhibitory concentrations (over 90% inhibition) against indirect germination of zoosporangia (zoospore release), zoospore motility, cystospore germination, and oospore formation were 0.1–0.5, 0.005, 0.05, and 0.01 μg/ml, respectively. Cyazofamid at 0.1 μg/ml exhibited complete fungicidal activity on zoospore release by P. infestans 60 min after treatment. When 20 μM ATP was applied to a zoosporangia suspension before addition of 0.05–0.1 μg/ml cyazofamid, inhibition of zoospore release was reduced. These results suggest that the mode of action of cyazofamid is connected to impairment of the ATP energy generation system.

Floristic composition of established vegetation and the soil seed bank in pasture communities under different traditional management regimes

The floristic composition of the established vegetation and the soil seed bank in seven herbaceous communities under different traditional management regimes were compared in the Valdeón valley (Picos de Europa, Spain), a valley chosen for study because of its high diversity and non-dominance of any single species which have become increasingly rare in Western Europe as a result of changes in agriculture practices. Although the biological richness in this valley is well know, few studies have been made of the effects of different traditional management regimes on the floristic soil seed bank composition of these grasslands. The aim of this study was to examine the influence of traditional management regimes on the floristic composition of the established vegetation and soil seed bank determining which species groups are most represented in each case to increase scientific information about these communities that every day are more scarce in Western Europe because of intensive management (generally due to fertiliser application to increase production) and abandonment of the traditional methods of management with a profound modification of the agrarian structure. An indirect germination method was used to study the seed bank. Soil samples were put into a germination chamber. 119 species of the vegetation and 104 of the seed bank were identified, 54 species being common to both. Results were analysed using Correspondence Analysis (CA), samples and species on the above-ground flora being ordered in the first two axes in well-defined groups: limestone and siliceous pastures, formerly-cultivated land and hay meadow. In the case of the below-ground seed bank, the manure used as fertiliser in some communities affected principally the concentration of therophytes in the soil and defined the different groups. Sampling time, management and differences between the persistence of seeds of different species in the soil seed bank were responsible for the large differences between the composition of the vegetation and seed bank. Ruderal species, species with persistent seeds and therophytes, which were better represented in the seed bank were responsible for the low correlation between seed bank and vegetation established.

Suppressed germination and early death of Phytophthora infestans sporangia caused by pectin, inorganic phosphate, ion chelators and calcium-modulating treatments

In laboratory experiments, chemical treatments were applied to sporangia of Phytophthora infestans incubated at 12 °C, conducive to cytoplasmic cleavage and release of zoospores (indirect germination), and at 20 °C, conducive to germination by hyphal outgrowth (direct germination). Both types of germination were suppressed by applying increasing concentrations (1–5 mM) of CaCl2 or MgCl2, or by low concentrations of pectin, inorganic phosphate, chelators (EGTA, BAPTA), calcium channel-blockers (lanthanum, gadolinium, verapamil) or compounds that interfere with intracellular calcium-mediated processes (trifluoperazine, caffeine). The suppression by some treatments was partly overcome by adding Ca2+ or Mg2+ in the early stages of incubation, but Ca2+ was usually more effective than Mg2+, and suppression at 20 °C was more easily overcome than at 12 °C. Pectin (0.1%) or BAPTA (5 mM) caused rapid death of sporangia at both 12 ° and 20 °, whereas EGTA (5 mM) or Na2HPO4 (5 mM) caused rapid death only at 12 °. The findings indicate that germinability and viability of P. infestans sporangia are strongly affected by the external availability of Ca2+ or other divalent cations, especially during zoosporogenesis.

Nuclear Behavior during Basidiospore Germination in Cronartium quercuum f. sp. fusiforme

Nuclear behavior during basidiospore germination in Cronartium quercuum f. sp. fusiforme was examined on glass slides and host seedlings using 4, 6-diamidino-2-phenylindole staining. Mononucleate basidiospores of Cronartium quercuum f. sp. fusiforme normally were produced following meiosis in the teliospore. However, a subsequent mitotic division often occurred within each basidiospore resulting in a short-lived binucleate condition. Within 1 h after spores were released from the basidium, one of the two nuclei in most basidiospores began to degenerate. Ninety-five percent of basidiospores directly cast from telia onto seedlings of Pinus taeda germinated directly forming thin germ tubes. More than 93% of germ tubes were mononucleate. On glass slides, however, 79% of basidiospores germinated indirectly, forming a secondary basidiospore. During indirect germination, degeneration of the second nucleus occurred quickly without exception. No differences in nuclear behavior were found between direct and indirect germination on host plants or on glass slides. However, nuclear movement from basidiospores into germ tubes was faster on the pine seedlings than on glass slides.

Effect of Some Inorganic Salts and Hydrogen Ion Concentration on Indirect Germination of the Sporangia of Phytophthora infestclns.

When the sporangia of Phytophthora infestans washed twice in distilled water were tested for indirect germination at 14°C, they did not germinate well (9% in 3hr) in distilled water, while in the tap water of Hokkaido Natl. Agric. Exp. Stn., they germinated well (78% in 3hr). Since some inorganic ions in the tap water appeared effective, various concentrations (0.01-10mM) of the salts, CaCl2, Ca(NO3)2, MgCl2, MgSO4, KCl, K2SO4, NaCl and Na2SO4, were evaluated for the effectiveness on indirect germination. Sporangial suspensions were preincubated for 22hr at 22°C for sporangial maturation and postincubated at 14°C for indirect germination. The most effective solution was 0.3mM CaCl2 (71% germination in 1hr), followed by 0.3mM MgSO4 (65%). Potassium and sodium salts were not very effective (<23%). At the cation concentration of 10mM all the salts except Na2SO4 inhibited indirect germination (0-5%). Hydrogen ion concentration markedly affected indirect germination, optimum at pH 8.0 (90% germination at 0.1mM CaCl2) but completely inhibitory at pH 4.5 (0%). Considering the results and the cations contained in the tap water, a synthetic salts solution favorable for indirect germination was devised: 0.2mM CaCl2, 0.05mM MgSO4, 0.05mM KH2PO4, 0.5mM NaHCO3, 0.01mM Fe-EDTA-Na. Using this solution the effect of sporangial concentration on indirect germination was also investigated. Sporangial suspensions below 2.5×104 sporangia/ml were favorable for indirect germination (≥97% in 2hr) but those above 105 were not.

Maturation of Sporangia of Phytophthora infestans Affecting the Rapidity of Indirect Germination.

The rapidity of indirect germination of the sporangia of Phytophthora infestans was investigated. Sporangial suspensions were prepared from the infected potato tuber slices or disks cultured for various periods at different temperatures. When the suspensions were incubated at 14°C, optimum for indirect germination, sporangia from older cultures could germinate more quickly than those from younger cultures. Sporangia immediately after production could germinate only slowly, while almost all the sporangia aged by preincubation for 6hr or more at 22°C could germinate quickly within 1hr when postincubated at 14°C. Therefore, it was considered that sporangial maturation by aging was essential to acquire the ability of indirect germination, and that matured sporangia could germinate quickly within about 1hr at 14°C. Time required for sporangial maturation was remarkably affected by the air temperature during sporangial production and also by the incubation temperature after preparation of sporangial suspension. The shortest maturation time, about 6hr, was found between 18 and 22°C. Temperatures below 15°C remarkably slowed maturation, and temperatures of 26°C and above inhibited it. Matured sporangia quickly lost the ability of quick germination when the suspensions were incubated at 26°C or above.

異なる温度条件下で形成されたジャガイモ疫病菌遊走子のうの間接発芽上限温度の変化

異なる温度条件下で形成されたジャガイモ疫病菌遊走子のうの間接発芽上限温度の変化

Effect of Water Temperature on Direct Germination of the Sporangia of Phytophthora infestans.

Effect of water temperature on conversion of the germinability from indirect to direct type of the sporangia of Phytophthora infestans was investigated. The sporangia produced on inoculated potato tuber slices were suspended in the tap water of Hokkaido Natl. Agric. Exp. Stn. which was very favorable for both indirect and direct germination. When the suspensions were incubated at the temperatures of 30-36°C, the ability of indirect germination was lost and the sporangia were converted to directly germinative type. The higher the water temperature was, the conversion occurred the more rapidly. This was also true between 20 and 30°C. By incubating the suspensions for 16-24hr at 33°C, almost all (>95%) sporangia could be converted to directly germinative type. By using such sporangia, effect of water temperature on the proportion and rapidity of direct germination was also investigated. The comparatively low temperatures of 10-17°C were optimum to achieve the highest proportion of direct germination (>95% in 48hr), while the comparatively high temperatures of 22-24°C were optimum to achieve the largest rapidity of direct germination, where nearly maximum proportions (about 60%) were attained in 24hr. Both the extreme temperatures of 3 and 28°C were inhibitory to direct germination.

Effect of root temperature, plant age, frequency and duration of flooding and inoculum placement and concentration on susceptibility of asparagus to Phytophthora rot

Abstract The conditions and recommended procedure to maximise disease severity for greenhouse screening of asparagus seedlings for resistance to Phytophthora rot are described. The optimum temperature for disease development on asparagus seedlings inoculated with Phytophthora megasperma var. sojae was 15°C and 18–24°C for P. cryptogea. The same temperatures were also optimal for indirect germination of sporangia of the two species. Young plants (20–29 weeks old) were more susceptible to Phytophthora root and crown rot than older plants (37–54 weeks old). V-8 vermiculite inoculum at a concentration of 1 : 100 (v:v) resulted in a minimum number of seedlings escaping infection and, when placed 60–80 mm below the seed also minimised pre-flooding root rot and damping off. Maximum disease severity was attained in the shortest time when seedlings were flooded three times for 48 h each time, but comparable disease severity resulted from flooding four times for 8, 24, or 48 h.

Sporangial responses do not reflect microbial suppression of Phytophthora drechsleri in composted eucalyptus bark mix

When morphology and germination behaviour of sporangia produced by Phytophthora drechsleri were examined in the presence of 262 isolates of fungi and 64 isolates of actinomycetes in sterile, irradiated Eucalyptus bark compost (CEB), seventeen response groups could be distinguished. The sporangial responses considered were combinations of direct and indirect germination, inviable maturenormal and deformed-small sporangia. Each sporangial response group was associated with a range of percentage pathogen survival or infectivtty values. There were no obvious relationships between sporangial response groups and pathogen survival or infectivity on Waratah ( Telopea speciossima). Few actinomycete or fungal intcractants reduced pathogen survival. However. 72 and 60.9% of the fungal and actinomycete populations, respectively, reduced percentage infectivity to ⩽75%. 173 (66%) fungal and 49 (76.5%) actinomycete intcractants caused disease suppression. The results suggest that most of the fungal and actinomycete interactants in the CEB did not suppress the pathogen, but did suppress disease and that P. drechsleri possesses some competitive saprophytic ability.