Altered Phenotypic Response to Peronospora parasitica in Brassica juncea Seedlings Following Prior Inoculation with an Avirulent or Virulent Isolate of Albugo candida

A study was made of the biological interactions between an isolate of Peronospora parasitica compatible with Brassica juncea and two isolates of Albugo candida either incompatible or compatible with the host species. Prior inoculation with the incompatible isolate of A. candida induced resistance to subsequently inoculated P. parasitica. The degree of resistance was proportional to the zoosporangia concentration of the incompatible isolate and induced resistance was more marked in the cotyledon receiving the inducing inoculum compared to the opposite cotyledon and subsequently emerging true leaves that had not been pre-inoculated. Induction of resistance was also observed if the incompatible isolate of A. candida and P. parasitica were co-inoculated simultaneously. However, the effect was greater the longer the interval between inoculations, up to a period of 4 days. When the incompatible isolate of A. candida was inoculated 4h after P. parasitica, there was no marked effect on resistance to the latter. In contrast, prior inoculation with the compatible isolate of A. candida increased susceptibility to P. parasitica inoculated subsequently. However, pre- or co-inoculation with P. parasitica suppressed the development of the compatible isolate of A. candida. A spectrum of responses was observed when one cotyledon was inoculated simultaneously with both the incompatible and compatible isolates of A. candida and followed subsequently with P. parasitica after different time intervals. In such combinations, a transition was observed in the host response to P. parasitica from induced resistance/reduced susceptibility, which increased up to 24h following a simultaneous inoculation with incompatible + compatible isolates of A. candida to an almost neutral reaction after 72h to induced susceptibility after 96h. This range of altered responses appeared to reflect the outcome of the differing kinetics and counter-effects of resistance and susceptibility induction.     

Genetics of resistance to downy mildew in Brassica oleracea and breeding towards durable disease control for UK vegetable production

Downy mildew resistance was previously identified from screening a Brassica oleracea collection against two standard UK isolates of Hyaloperonospora parasitica. Sources of resistance were chosen from this material and developed further in this study by generating doubled haploid (DH) and inbred lines. Seedlings from the new lines were tested for resistance to a larger collection of H. parasitica isolates collected in 2001–2002 and 2007–2008 from the main broccoli and cauliflower production regions of the UK. Three lines (derived from borecole or summer cabbage) were broadly resistant to the pathogen isolates. Three of the remaining lines exhibited strong isolate‐specific resistance; several examples of weak or basal level of resistance to some isolates were observed. A new H. parasitica variant collected in 2008 was virulent in the broadly resistant lines, but was avirulent in a line with narrow specificity of resistance. The F₂ and BC₁ seedlings derived from outcrossing each of the three broadly resistant lines to susceptible broccoli and cauliflower lines segregated in a manner indicating that the resistance was controlled by a single dominant gene. No susceptibility was observed amongst F₂ seedlings derived from intercrossing the three resistant lines, indicating that they all share the same or closely linked broad‐spectrum resistance gene(s). DH lines were produced from F₁ plants, and resistant plants were further backcrossed to produce broccoli and cauliflower‐like lines that could be useful pre‐breeding material. A combination of resistance from lines with broad and narrow specificity is recommended for controlling downy mildew in UK brassica production.     

Spinach: better management of downy mildew and white rust through genomics

Spinach (Spinacia oleracea) has become an increasingly important vegetable crop in many parts of the world. Significant changes in production practices, particularly in the U.S. and E.U., have occurred in the past 10–15 years as a result of increased product demand. These changes likely increased the incidence and severity of downy mildew, caused by Peronospora farinosa f. sp. spinaciae. Recently, progress has been made to define the genetics of resistance to this pathogen and the closely related white rust pathogen, Albugo occidentalis. In this paper, we outline the genetic and genomic resources currently available for spinach, draw parallels between spinach diseases and more thoroughly characterized pathosystems, and describe efforts currently underway to develop new genetic and genomic tools to better understand downy mildew and white rust of spinach. Presently, many crucial tools and resources required to define the molecular underpinnings of disease are unavailable for either spinach or its pathogens. New resources and information for spinach genomics would provide a jumpstart for ongoing efforts to define (and deploy) genetic resistance against downy mildew and white rust.     

Leaf position,leaf age and plant age affect the expression of downy mildew resistance in <Emphasis Type="Italic">Brassica oleracea</Emphasis>

Downy mildew caused by the oomycete Hyaloperonospora parasitica (formerly Peronospora parasitica) is a worldwide foliar disease of Brassica vegetables, which may cause seedling loss in the nurseries and damage to adult plants in the field. Disease symptoms start from the lower leaves and progress upwards. Three experiments were conducted under controlled environment conditions, using inoculated leaf discs, to determine the influence of leaf position, plant age, and leaf age on the expression of resistance to downy mildew in various Brassica oleracea genotypes. The upper leaves were more resistant than the lower leaves when 7–19 week-old plants of broccoli and Tronchuda cabbage were tested. Broccoli lines ‘PCB21.32’ and ‘OL87123-2’ were fully susceptible at the cotyledon stage, showed a clear resistance increase from lower to upper leaves at 6 weeks and ‘PCB21.32’ was fully resistant 16 weeks after sowing. Immature leaves were more resistant than adjacent fully expanded mature leaves. Susceptibility increased with leaf age when the same leaf was tested at two to 4-week intervals. Leaf age and upper-leaf position on the stem had opposite effects on disease score, since younger leaves collected from lower positions and older leaves collected from upper positions tended to score similarly in compatible interactions. The progression of downy mildew from the base of the plant upwards on B. oleracea in the field could be due to differences in leaf resistance in addition to environmental variation. To maximise the expression of a compatible reaction in adult plants lower leaves of Brassica plants that are at least 12 weeks-old should be used.     

Fungicidal activity of N-(2-cyano-2-methoximinoacetyl)amino acids and their derivatives

More than 20 N-(2-cyano-2-methoximinoacetyl)amino acids and derivatives were synthesised and tested for antifungal activity against grape downy mildew, caused by Plasmopara viticola (de Bary) Berl. & de Toni, and rape downy mildew, Peronospora parasitica Fr. Two of the compounds containing a free carboxylic acid group, a moiety which has been shown to confer phloem mobility on compounds, showed high activity, especially against P. parasitica, in protectant tests. These results indicate that there is no incompatibility between the acid function and fungicidal activity. A number of the esters showed activity comparable with that of cymoxanil in the protectant tests, and the tert-butyl esters of the methionine derivative and its sulfone were more than ten times as active as the commercial compound.     

枯草芽胞杆菌HMB-20428与化学杀菌剂互作对葡萄霜霉病菌抑制作用和替代部分化学药剂减量用药应用

为明确生防菌剂枯草芽胞杆菌Bacillus subtilis HMB-20428与化学杀菌剂互作对葡萄霜霉病菌Plasmopara viticola的抑制作用,采用生物测定与田间药效试验研究其最佳用药时期,筛选最佳生化互作组合,结合空间竞争能力和诱导抗病性生理测定试验了解互作增效机制,并建立葡萄霜霉病的综合防控技术。结果显示,枯草芽胞杆菌HMB-20428预防葡萄霜霉病最佳时期为发病前1个月,连续喷雾用药3~4次,间隔期10~15 d。研发出协同增效生化互作组合生防菌剂HMB-20428+嘧菌酯,及其与硅氧烷化合物增效组合,防效分别为91.06%~98.92%和87.78%~92.04%。枯草芽胞杆菌HMB-20428与嘧菌酯互作可增强枯草芽胞杆菌HMB-20428定殖能力和植株抗病作用,且定殖能力和抗病机制作用的增强时间基本一致。以生防菌剂HMB-20428替代部分化学药剂减量用药流程的防效为91.40%。表明枯草芽胞杆菌HMB-20428与嘧菌酯互作可达到减少化学药剂用量的目的。     

Application timing of herbicides,glyphosate and atrazine,sway respective epidemics of foliar pathogens in herbicide-tolerant rapeseed

Rapeseed (Brassica napus) production in Australia relies heavily on triazine-or glyphosate-tolerant cultivars. For 14 triazine-tolerant cultivars, disease development of Neopseudocercosporella capsellae (white leaf spot), Alternaria brassicae and A. japonica (Alternaria leaf spot), and Hyaloperonospora brassicae (downy mildew) were all dependent upon herbicide application timing (p < 0.001), with significant differences between cultivars (p < 0.001) and a significant interaction (p < 0.001) between herbicide application timing and cultivars. Atrazine applied preinfection by N. capsellae, A. brassicae, or A. japonica enhanced disease incidence, severity, and leaf collapse, while atrazine application postinfection for these same pathogens reduced all three disease parameters. However, for H. brassicae, application of atrazine after, and especially prior to, infection resulted in lower disease incidence, severity, and leaf collapse. Application of glyphosate on five glyphosate-tolerant cultivars for N. capsellae resulted in significant differences (p < 0.05) between glyphosate application treatments, and between host cultivars in terms of incidence and consequent leaf collapse. For A. brassicae, A. japonica, and H. brassicae, glyphosate resulted in significant differences (p < 0.001) across application timings between cultivars, and a significant interaction (p < 0.001) between herbicide application timings and cultivars. Glyphosate applied on glyphosate-resistant rapeseed after, and especially prior to, attack by H. brassicae, reduced downy mildew. These are the first studies to highlight how the timing of application of triazine or glyphosate in relation to pathogen infection is critical to the susceptibility of rapeseed to white leaf spot, Alternaria leaf spot, and downy mildew. This new understanding offers fresh possibilities for improved management of these diseases in herbicide-tolerant rapeseed crops.     

Effect of the rust fungus Puccinia chondrillina TU 788 on plant size and plant size variability in Chondrilla juncea

Chondrilla juncea is one of the most serious introduced weed species in Australia. Successful biological control has been achieved by the release of the rust fungus Puccinia chondrillina. Although P. chondrillina is thought to reduce the competitive ability of C. juncea by reducing plant size, the precise mechanism by which the pathogen controls its host is poorly understood. In this experiment, we examined the effect of P. chondrillina TU 788 infection on individual plant size, and size variability, in two genotypes (one resistant, one susceptible) of C. juncea. Exposure to P. chondrillina significantly reduced the average size of susceptible C. juncea plants, a reduction apparent in both roots and shoots. Plant size variability of the susceptible C. juncea genotype was also reduced by P. chondrillina. Plant size, and plant size variability of the resistant genotype were unaffected by exposure to rust, or association with rust‐affected plants. The effect of P. chondrillina on plant size in rust‐susceptible C. juncea plants shows how effective bio‐control of this weed may occur in field populations.     

Silicon and increased electrical conductivity reduce downy mildew of soilless grown lettuce

Downy mildew of lettuce, caused by Bremia lactucae, is difficult to control in soilless systems by using conventional methods of disease management because few chemicals are registered, while resistant cultivars face the problem of resistance break down; therefore other methods for disease control need to be investigated. The effect of silicon salt as well as increased electrical conductivities against downy mildew was evaluated in four experiments carried out in hydroponically systems, using the cultivar of lettuce “Cobham Green”, known for its susceptibility to the pathogen. Silicon, as potassium silicate, was added at 100 mg l⁻¹ of nutrient solution at three levels of electrical conductivity: 1.5–1.6 mS cm⁻¹ (EC1), 3.0–3.5 mScm⁻¹ (EC2, 0.70 g l⁻¹ NaCl) and 4.0–4.5 mS cm⁻¹ (EC3, 0.95 g l⁻¹ NaCl) respectively. Lettuce plants, grown for 14–20 (trials 1 and 2) and 36–45 (trials 3 and 4) days in the different nutrient solutions tested, were inoculated with B. lactucae conidia with a maximum of two inoculations before final disease assessment carried out 14–21 days after the inoculation able to give symptoms. EC and potassium silicate significantly influenced downy mildew incidence and severity, while their interaction was not a significant factor. The addition to the standard nutrient solution (EC1) of potassium silicate resulted in a significant reduction of downy mildew severity in trials 1 and 2 where plants were artificially inoculated 15 and 20 days after transplanting. This efficacy was slight on plants grown for 36 and 45 days before inoculation in a soil drenched with EC1 amended with potassium silicate. EC2 gave a significantly similar downy mildew reduction than EC2 added with potassium silicate in trial 3. Plants grown for 36 and 45 days at the highest electrical conductivity (EC3) showed a significant reduction in severity of downy mildew compared with that observed at EC2 level. The best results, in terms of disease control, were given by the addition of potassium silicate to the EC3 solution. This combination also led to a significantly increased plant biomass. The possibility and benefits of applying potassium silicate and increased EC amendments in practice is discussed.     

Formation of satellite uredinia as an important trait related to grapevine colonization by Phakopsora meliosmae-myrianthae

Phakopsora meliosmae-myrianthae, the causal agent of Asian grapevine leaf rust, significantly reduces the photosynthetic efficiency of grapevine leaves in green symptomless tissues surrounding lesions. This study took a close look at grapevine leaf colonization kinetics by P. meliosmae-myrianthae and compared it to P. pachyrhizi–soybean and Uromyces appendiculatus–bean colonization. It is already known from the literature that soybean rust, similar to grapevine rust, has a negative effect on leaf photosynthesis greater than would be expected based on visual lesions. However, in contrast to soybean and grapevine rusts, the effect of bean rust on leaf photosynthesis is proportional to the diseased leaf area. Colonization progress was monitored by fungal biomass assessed via histological staining and quantitative polymerase chain reaction (qPCR). Individual lesions of P. meliosmae-myrianthae on grapevine, P. pachyrhizi on soybean and U. appendiculatus on common bean leaves were evaluated every 3–4 days, and the number of uredinia was counted. Staining showed that mycelial colonization did not extend beyond the lesion border. The number of P. pachyrhizi and P. meliosmae-myrianthae uredinia within the lesions increased over time (on average 14-fold), whereas the number of U. appendiculatus uredinia remained the same. These findings were corroborated by qPCR, which revealed a greater increase in fungal biomass for Phakopsora spp. than for U. appendiculatus until 12 days post-inoculation. The high number of satellite uredinia within lesions might be directly related to the impact of this pathogen in photosynthetic efficiency on symptomless areas of diseased grapevine leaves. This study identified accelerated formation of satellite uredinia as an important feature of grapevine colonization by P. meliosmae-myrianthae.     

Pathotypes and phylogenetic variation determine downy mildew epidemics in Brassica spp. in Australia

Isolates of Hyaloperonospora brassicae inoculated onto cotyledons of 28 diverse Brassicaceae genotypes, 13 from Brassica napus, two from B. juncea, five from B. oleracea, two from Eruca vesicaria, and one each from B. nigra, B. carinata, B. rapa, Crambe abyssinica, Raphanus sativus and R. raphanistrum, showed significant effects (P ≤ 0.001) of isolate, host and their interaction. Host responses ranged from no visible symptom or a hypersensitive response, to systemic spread and abundant pathogen sporulation. Isolates were generally most virulent on their host of origin. Using an octal classification, six host genotypes were identified as suitable host differentials to characterize pathotypes of H. brassicae and distinguished eight distinct pathotypes. There were fewer, but more virulent, pathotypes in 2015–2016 isolates than 2006–2008 pathogen populations, probably explaining the increase in severity of canola downy mildew over the past decade. Phylogenetic relationships determined across 20 H. brassicae isolates collected in 2006–2008 and 88 isolates collected in 2015–2016 showed seven distinct clades, with 70% of 2006–2008 isolates distributed within clade I (bootstrap value (BVs) of 100%) and the remaining 30% in clade V (BVs 83.3%). This is the first study to define phylogenetic relationships of H. brassicae isolates in Australia, setting a benchmark for understanding current and future genetic shifts within pathogen populations; it is also the first to use octal classification to characterize pathotypes of H. brassicae, providing a novel basis for standardizing phenotypic characterization and monitoring of pathotypes on B. napus and some crucifer species in Australia.     

Development of detection systems for the sporangia of <Emphasis Type="Italic">Peronospora destructor</Emphasis>

A monoclonal antibody that recognises components of the wall of sporangia of Peronospora destructor was raised. Tests using spores of higher fungi and other species of mildew demonstrated the specificity of the monoclonal. The antibody was used to develop lateral flow devices for sporangia of P. destructor. A competitive lateral flow format was developed which could detect onion downy mildew sporangia. Five-microliter gold anti-mouse IgM solution pre-mixed with 10 μl of P. destructor monoclonal antibody (EMA 242) proved the optimal concentration for detection of sporangia of P. destructor when applied to sample pads of lateral flow devices. Limits of approximately 500 sporangia of P. destructor could be detected by the absence of a test line on the lateral flow device within test samples. Using a scanning densitometer improved the sensitivity of detection. Further development and validation of the test is required if it is to be used for risk assessments of onion downy mildew in the field.     

Evidence that a Leaf-Disk Test Allows Assessment of Isolate-Specific Resistance in Brassica oleracea Crops Against Downy Mildew (Peronospora parasitica)

A rapid resistance/susceptibility test for Peronospora parasitica (downy mildew) was established by inoculating leaf-disks of four Brassica oleracea accessions. Several conditions were tested: disk disinfection or not, agar medium with or without nutrients and with 50 or 100 ppm of benzimidazole. Using disinfected disks placed on agar (no nutrient and benzimidazole at 50 or 100 ppm), the responses of leaf-disks to four isolates were similar to those obtained using the classical cotyledon test, whereas undesired contaminations occurred in all other conditions. The possible effect of the particular leaf used for obtaining the disks was also studied. In each incompatible interaction tested, disks were resistant whatever the leaf used. In compatible interactions, susceptible phenotypes were observed on disks derived from the six lowest leaves, but disks from upper leaves were resistant. The genetic basis of resistance in a F1 hybrid broccoli was assessed, by testing six isolates on an F2 population derived from this hybrid. The cotyledon test only allows inoculation of two isolates per seedling, whereas many isolates can be tested on each plant by using leaf-disks. The segregation of the resistance to each of the six isolates was analysed: two dominant genes (tightly linked) control resistance to all isolates (one to five isolates; the other to only one isolate).     

Activity of carboxylic acid amide (CAA) fungicides against Bremia lactucae

Four carboxylic acid amide (CAA) fungicides, mandipropamid (MPD), dimethomorph (DMM), benthiavalicarb (BENT) and iprovalicarb (IPRO) were examined for their effects on various developmental stages of Bremia lactucae, the causal agent of downy mildew in lettuce, in vitro and in planta. Spore germination in vitro or on leaf surfaces was inhibited by all CAA fungicides (technical or formulated). MPD was more effective in suppressing germination than DMM or BENT, whereas IPRO was least effective. CAA induced no disruption of F-actin microfilament organisation in germinating spores of B. lactucae. CAA applied to germinating spores in vitro prevented further extension of the germ tubes. When applied to germinated spores on the leaf surface they prevented penetration. Preventive application of CAA to intact plants inhibited infection. MPD was more effective in suppressing infection than DMM or BENT, whereas IPRO was least effective. Curative application was effective at ≤3 h post-inoculation (hpi) but not at ≥18 hpi. CAA (except IPRO) applied to upper leaf surfaces inhibited spore germination on the lower surface and hence reduced infection. CAA suppressed sporulation of B. lactucae on floating leaf discs and when sprayed onto infected plants two days before onset of sporulation. BENT and DMM were more effective in suppressing sporulation than MPD or IPRO. Epidemics of downy mildew in shade-house grown lettuce were suppressed by CAA. A single spray applied to five-leaf plants before transplanting controlled the disease for 50 days. The results suggest that CAA are effective inhibitors of spore germination and therefore should be used as preventive agents against downy mildew of lettuce caused by B. lactucae.     

Phenotypic and phylogenetic studies associated with the crucifer white leaf spot pathogen,Pseudocercosporella capsellae,in Western Australia

Pseudocercosporella capsellae (white leaf spot disease) is an important disease on crucifers. Fifty‐four single‐conidial isolates collected from Brassica juncea (Indian mustard), B. napus (oilseed rape), B. rapa (turnip), and Raphanus raphanistrum (wild radish) across Western Australia were investigated for differences in pathogenicity and virulence using cotyledon screening tests, genetic differences using internal transcribed spacer (ITS) sequencing and phylogenetic analysis, and growth rates on potato dextrose, V8 juice and malt extract agars. All isolates from the four crucifer hosts were pathogenic on the three test species: B. juncea, B. napus and R. raphanistrum, but showed differences in levels of virulence. Overall, isolates from B. juncea, B. napus and B. rapa showed greatest virulence on B. juncea, least on R. raphanistrum and intermediate virulence on B. napus. Isolates from R. raphanistrum showed greatest virulence on B. juncea, least on B. napus and intermediate virulence on R. raphanistrum. Growth and production of a purple‐pink pigment indicative of cercosporin was greatest on malt extract agar and cercosporin production on V8 juice agar was positively correlated with virulence of isolates on B. juncea and B. napus. ITS sequencing and phylogenetic analysis showed that isolates collected from B. napus, B. juncea and B. rapa, in general and with few exceptions, had a high degree of genetic similarity. In contrast, isolates from R. raphanistrum were clearly differentiated from isolate groups collected from Brassica hosts. Pseudocercosporella capsellae reference isolates from other countries generally grouped into a single separate cluster, highlighting the genetic distinctiveness of Western Australian isolates.     

Protection of Brassica seedlings against downy mildew and damping-off by seed treatment with CGA 245704, an activator of systemic acquired resistance

Control of seedling diseases is a major priority in many crop systems. Seed treatments that induce systemic resistance after seedling emergence may be an ideal way to provide protection against disease during the establishment of the crop. CGA 245704, a chemical activator of systemic acquired resistance, was tested as a seed treatment against two Brassica diseases with contrasting infection biologies, the airborne downy mildew pathogen, Peronospora parasitica, and the soilborne fungus, Rhizoctonia solani. Seeds of two Brassica spp. were either imbibed with various concentrations of the compound or imbibed and then dried. Both the imbibition treatment alone and the imbibition treatment followed by seed drying had a significant effect on the sporulation intensity of P. parasitica for all concentrations of the compound used, whereas the imbibition treatment provided some control of damping-off caused by R. solani, with the degree of control being highly dependent on the concentration applied to the seed. Seed treatment with the plant activator CGA 245704 might therefore simultaneously control several seedling diseases, thereby providing a novel option for management of these diseases. © 1998 SCI.     

Pathotypic diversity of Hyaloperonospora brassicae collected from Brassica oleracea

Downy mildew caused by Hyaloperonospora brassicae is an economically destructive disease of brassica crops in many growing regions throughout the world. Specialised pathogenicity of downy mildews from different Brassica species and closely related ornamental or wild relatives has been described from host range studies. Pathotypic variation amongst Hyaloperonospora brassicae isolates from Brassica oleracea has also been described; however, a standard set of B. oleracea lines that could enable reproducible classification of H. brassicae pathotypes was poorly developed. For this purpose, we examined the use of eight genetically refined host lines derived from our previous collaborative work on downy mildew resistance as a differential set to characterise pathotypes in the European population of H. brassicae. Interaction phenotypes for each combination of isolate and host line were assessed following drop inoculation of cotyledons and a spectrum of seven phenotypes was observed based on the level of sporulation on cotyledons and visible host responses. Two host lines were resistant or moderately resistant to the entire collection of isolates, and another was universally susceptible. Five lines showed differential responses to the H. brassicae isolates. A minimum of six pathotypes and five major effect resistance genes are proposed to explain all of the observed interaction phenotypes. The B. oleracea lines from this study can be useful for monitoring pathotype frequencies in H. brassicae populations in the same or other vegetable growing regions, and to assess the potential durability of disease control from different combinations of the predicted downy mildew resistance genes.     

Age-related resistance to Pseudomonas syringae pv. tomato is associated with the transition to flowering in Arabidopsis and is effective against Peronospora parasitica

As plants mature it has been observed that some become more resistant to normally virulent pathogens. The ability to manifest the Age-Related Resistance (ARR) response in Arabidopsis to Pseudomonas syringae pathovars tomato (Pst) coincided with the transition to flowering in plants both delayed and accelerated in the transition to flowering. ARR was also associated with a change in PR-1 gene expression, such that young plants expressed PR-1 abundantly at 3 days post inoculation (dpi) while mature plants expressed much less. The Arabidopsis ARR response requires SA accumulation via isochorismate synthase (ICS1) [24]. ICS1 was expressed one dpi with virulent and avirulent Pst in both young and mature plants. The ARR response was also effective versus avirulent Pst providing an additional 4-fold limitation in bacterial growth. Arabidopsis ARR was found to be ineffective against two necrotrophs, Erwinia carotovora subspecies carotovora (bacterium) and Botrytis cinerea (fungus) and one obligate biotroph, Erysiphe cichoracearum (fungus). However, mature wild type, SA-deficient sid2 and NahG plants supported little growth of the obligate biotrophic oomycete, Peronospora parasitica. Therefore ARR to P. parasitica appears to be SA-independent, however the level of ARR resistance was somewhat reduced in these mutants in some experiments. Thus, there may be numerous defence pathways that contribute to adult plant resistance in Arabidopsis.     

Disease cycle of Austropuccinia psidii on Eucalyptus globulus and Eucalyptus obliqua leaves of different rust response phenotypes

Myrtle rust poses a significant biosecurity threat to Australia with potential for long-term damaging impacts on native flora and plant industries. This study describes the disease cycle of Austropuccinia psidii, the myrtle rust pathogen, in Eucalyptus globulus and Eucalyptus obliqua, two commercially and ecologically important species from different subgenera of Eucalyptus. Ontogeny and morphology of infection structures of A. psidii on plants of both Eucalyptus species with different rust response phenotypes, i.e. completely resistant, hypersensitive and highly susceptible, were investigated. Plants were inoculated with single-uredinium-derived urediniospores and examined by scanning electron microscopy. No differences between rust response phenotypes were observed in germination of urediniospores, formation of appressoria or length of germ tubes. The growth of germ tubes had no affinity towards stomata of either species. Histological observations indicated direct penetration by infection pegs through the leaf cuticle and no penetration beyond the epidermis on rust-resistant E. obliqua. Eucalyptus obliqua plants that were identified as susceptible to A. psidii at 3- and 6-months-old showed no disease when reinoculated with A. psidii at 12-months-old; this indicated possible early acquisition of adult plant resistance to A. psidii in this species. In the susceptible phenotype of E. globulus rust inoculation led to rapid colonization of leaf parenchyma cells with the disease cycle completed within 10 days. These findings provide valuable insights into host–pathogen interactions in the Eucalyptus–A. psidii pathosystem, which might be useful for the development of effective rust control strategies across Eucalyptus subgenera.