Severity Of Stem Canker Research Articles

DURIAN STEM CANKER STATUS AND RELATIVE SOIL PROPERTIES IN PENINSULAR MALAYSIA

The arising stem canker issues can be devastating when new durian orchards are developed without proper disease management strategies. Hence, this study aims to obtain the current durian stem canker incident rates in Peninsular Malaysia including soil properties. 15 plots (2,500 m2 per plot) of durian orchards were evaluated and composite soil samples were analysed. Results showed that the greatest disease incidence (100%) was in old orchards (> 10 years), with a severity of 63% whereas young orchards (< 10 years) are at risk of severe stem canker with 71% of incidence and 43% of severity. There was a negative correlation (r = –0.220) between the incidence of stem canker and soil pH which indicated that durian trees grown in acidic soil (pH < 5.0) were more prone to disease dispersion in the orchards. Moreover, severely infected trees were observed more in durian orchards with a higher proportion of finer sand particles in the soil (DI/ r = 0.199 & DSI/r = 0.206) compared to coarse particles of clay and silt. Our work will play a crucial role in advancing real-time monitoring of durian stem cankers and enabling the prediction of potential disease outbreaks for the implementation of preventive measures.

Characterization and report of Ectophoma multirostrata, causing stem canker disease in highbush blueberry in Iran

AbstractDuring a disease survey in greenhouses of blueberry seedlings in Mohammad Shahr City, Alborz Province, Iran, in the autumn 2021, severe stem canker symptoms were observed in the cv. Emerald of highbush blueberry (Vaccinium corymbosum). The disease incidence in the affected greenhouses was more than 30%–40%. Diseased branches and stems were sampled and the associated fungus was isolated on potato dextrose agar. Based on the fungal morphological features of the colony, three isolates among which the representative isolate of UT2020 were identified as Ectophoma multirostrata. Phylogenetic analysis of the concatenated ITS‐rDNA and beta‐tubulin (tub2) gene sequence data confirmed the morphological identification of the fungal isolates. E. multirostrata differs from closely related species by having several ostiolar necks, textura angularis wall in pycnidia and oblong to ellipsoidal conidia. The pathogenicity of the isolate UT2020 in the stems of highbush blueberry cv. Emerald was confirmed by our artificial wound inoculation method. After 14 days, the same stem canker symptoms were developed on the inoculated stems, and the same fungal isolate was re‐isolated from the newly developed symptoms, fulfilling the Koch's postulates. This is the first report of E. multirostrata causing a stem canker disease in highbush blueberries. Also, Vaccinium corymbosum is reported as a new host (matrix nova) for E. multirostrata worldwide.

Influence of Elevated Temperatures on Resistance Against Phoma Stem Canker in Oilseed Rape.

Cultivar resistance is an important tool in controlling pathogen-related diseases in agricultural crops. As temperatures increase due to global warming, temperature-resilient disease resistance will play an important role in crop protection. However, the mechanisms behind the temperature-sensitivity of the disease resistance response are poorly understood in crop species and little is known about the effect of elevated temperatures on quantitative disease resistance. Here, we investigated the effect of temperature increase on the quantitative resistance of Brassica napus against Leptosphaeria maculans. Field experiments and controlled environment inoculation assays were done to determine the influence of temperature on R gene-mediated and quantitative resistance against L. maculans; of specific interest was the impact of high summer temperatures on the severity of phoma stem canker. Field experiments were run for three consecutive growing seasons at various sites in England and France using twelve winter oilseed rape breeding lines or cultivars with or without R genes and/or quantitative resistance. Stem inoculation assays were done under controlled environment conditions with four cultivars/breeding lines, using avirulent and virulent L. maculans isolates, to determine if an increase in ambient temperature reduces the efficacy of the resistance. High maximum June temperature was found to be related to phoma stem canker severity. No temperature effect on stem canker severity was found for the cultivar ES Astrid (with only quantitative resistance with no known R genes). However, in the controlled environmental conditions, the cultivar ES Astrid had significantly smaller amounts of necrotic tissue at 20°C than at 25°C. This suggests that, under a sustained temperature of 25°C, the efficacy of quantitative resistance is reduced. Findings from this study show that temperature-resilient quantitative resistance is currently available in some oilseed cultivars and that efficacy of quantitative resistance is maintained at increased temperature but not when these elevated temperatures are sustained for a long period.

Increase of coffee tree susceptibility to stem canker disease mediated by glyphosate application

Stem canker is a major disease of coffee in Indonesia, caused by Fusarium solani. Previous studies showed a correlation between stem canker incidence and glyphosate application. The study aimed to determine the effect of glyphosate application on coffee stem canker and the underlying physiological mechanisms. The research used glyphosate treatments with the concentrations of 0, 4, 8, and 16 ml.L−1. In vitro experiments were carried out by measuring the growth of F. solani colony after glyphosate treatment. In planta, tests were conducted by artificial inoculation on glyphosate-treated coffee seedlings. The severity of stem canker was assessed. In a separate test, coffee seedlings were treated with glyphosate without pathogen inoculation. In the second experiment, physiological change was determined by measuring photosynthetic pigments, gas-exchange properties, lipid peroxidation, and activity of ascorbate peroxidase (APX) and catalase (CAT) enzymes. In vitro tests showed that glyphosate had antifungal activity against F. solani. In planta, the test showed that glyphosate treatment caused higher susceptibility to stem canker in coffee seedlings, indicated by the increase of canker wound depth. Glyphosate treatment on coffee decreased the chlorophyll, carotene contents, and reduced photosynthesis rate, caused oxidative stress depicted by higher lipid peroxidation and increased enzyme APX and CAT activity.

Effects of cultivar resistance and fungicide application on stem canker of oilseed rape (Brassica napus) and potential interseasonal transmission of Leptosphaeria spp. inoculum

AbstractPhoma stem canker is a damaging disease of oilseed rape (Brassica napus) that causes annual yield losses to UK oilseed rape growers worth approximately £100 million, despite the use of fungicides. In the UK, oilseed rape is sown in August/September and harvested in the following July. The disease epidemics are initiated by ascospores released from Leptosphaeria spp. pseudothecia (ascocarps) on stem stubble in the autumn/winter. Control of this disease is reliant on the use of cultivars with “field resistance” and azole fungicides. This study investigated the effects of cultivar resistance and application of the fungicide prothioconazole on the severity of stem canker before harvest and the subsequent production of pseudothecia on the infected stubble under natural conditions in the 2017/2018, 2018/2019, and 2019/2020 cropping seasons. The application of prothioconazole and cultivar resistance decreased the severity of phoma stem canker before harvest, and the subsequent production of Leptosphaeria spp. pseudothecia on stubble in terms of pseudothecial density. Results showed that stems with less severe stem cankers produced fewer mature pseudothecia of Leptosphaeria spp. on the infected stubble. This investigation suggests that the most sustainable and effective integrated control strategy for phoma stem canker in seasons with low quantities of inoculum is to use cultivars with medium or good field resistance and apply only one spray of prothioconazole when required.

Besides stem canker severity, oilseed rape host genotype matters for the production of Leptosphaeria maculans fruit bodies

For fungal cyclic epidemics on annual crops, the pathogen carry-over is an important but poorly documented step. Plant resistance affects the pathogen development within the epidemics but we lack data on the inter-annual transmission of inoculum. For Leptosphaeria maculans on 15 oilseed rape genotypes in field during 4 growing seasons, stem canker severity was visually scored at harvest. The number of fruit bodies produced on incubated stubble was quantified using an automated image analysis framework. Our results confirm that fruit body production increases with disease severity and is significantly affected by host genotype and nitrogen supply. Tracking individual stems through incubation, we confirm for the first time that the oilseed rape genotype has a direct effect on inoculum production, not only disease severity. This major effect of genotype on inoculum carry-over should be taken into account in models of varietal deployment strategies.

Quantitative resistance linked to late effectors.

Quantitative resistance linked to late effectors.

First Report of Stem Canker of Almond Trees Caused by Fusarium solani in Greece.

Almond (Prunus dulcis) is an important crop for Greece grown on 15.130 ha in 2019. In September 2019, a severe stem canker disease was observed in 6-year-old trees of cv Marta grafted on the rootstock 'F675C14', in a new almond grove of cvs Marta, Soleta, Antonela, Belona and Laurete, in Vlachiana, Heraklion, Crete, Greece. Only cv Marta trees were affected. Diseased trees exhibited cankers on trunks and branches with pale yellow to red-colored gum excreting from cankers, yellowing, leaf fall, twig and branch dieback, bark and wood tissue discoloration. Severely affected trees were killed. A Fusarium-like fungus was consistently isolated from symptomatic wood tissue previously surface-disinfested with 95% ethanol, on acidified potato dextrose agar (APDA). Emerging colonies were transferred to new PDA and the growth rate of the fungus was 7.86 mm/day at 24 °C in the dark. The abundant aerial mycelium was initially white, turning into pale orange in the centre after 7 days of growth on PDA. Microscopic observations revealed hyaline conidiophores measuring 26.74 ± 20.44 μm in length, developing microconidia 5.00 to 9.50 × 2.50 to 4.75 μm (average 6.64 × 3.50 μm) and macroconidia 10.00 to 23.25 × 3.75 to 5.50 μm (average 16.42 × 4.50 μm) in size. DNA from one representative single-spore isolate (code KOUB.AM.VR1) was extracted and the internal transcribed spacer region (ITS) of ribosomal DNA and translation elongation factor 1-alpha (EF 1-a) genes were amplified using the primer pairs ITS1/ITS4 (White et al. 1990) and EF1-F/EF2-R (O'Donnell et al. 1998), respectively. The PCR products were sequenced and deposited in GenBank (accession Nos. MW547397 and MW554492). Based on morphological characteristics (Leslie and Summerell 2006) and a BLAST search with 100.00% and 99.38% identity to published F. solani ITS and EF 1-a sequences in GenBank (KX034335.1, DQ247636.1) the fungus was identified as F. solani. Eight 3-year-old almond trees of cv. Marta were artificially inoculated in March 2020 by making a 6.0-mm-diameter hole into the trunk, inserting a 6-mm-diameter mycelial disc taken from a 10-day-old PDA culture, sealing the hole with cellophane membrane and covering with adhesive paper tape. Another eight trees of the same cultivar were mock-inoculated with sterilized PDA discs and served as controls. Potted trees were kept under ambient conditions. One month post inoculation, yellow gum was evident excreting around the inoculation point in F. solani-treated trees but not in the controls. Seven months post inoculation, longitudinal and transverse sections of inoculated trunks revealed internal and external symptoms similar to those observed under natural infection conditions and F. solani was steadily re-isolated from symptomatic wood tissue and identified by colony morphology. Neither symptoms nor positive isolations were observed in control trunks. Pathogenicity tests were repeated twice. Fusarium solani has been reported as the causal agent of stem canker or wood decay diseases in several woody hosts including bitternut hickory, black walnut, mulberry and pistachio trees (Crespo et al. 2019; Markakis et al. 2017; Park and Juzwik 2012; Tisserat 1987). To the best of our knowledge, this is the first worldwide report of stem canker caused by F. solani on almond tree. This disease could potentially be an increasing problem in almond growing areas and result in severe crop losses. Hence, effective management practices should be investigated and applied.

Inter‐individual genetic variation in the temperature response of Leptosphaeria species pathogenic on oilseed rape

AbstractIt is important to understand the likely response of plant pathogens to increased temperatures due to anthropogenic climate change. This includes evolutionary change due to selection on genetically based variation in growth rate with temperature. We attempted to quantify this in two ways. First, radial mycelial growth rates in agar culture were determined for a collection of 44 English isolates of Leptosphaeria maculans and 17 isolates of L. biglobosa, at 14 temperatures. For L. maculans the genotypic variances in four parameters were measured: minimum temperature allowing growth, optimum temperature, growth rate at the optimum temperature, and growth rate at the highest usable temperature, 31.8°C. The standard deviations were 0.068°C, 1.28°C, 0.21 mm/day, and 0.31 mm⋅day−1⋅°C−1, respectively. For L. biglobosa, these figures were, respectively: immeasurably small, 1.31°C, 0.053 mm/day, and 0.53 mm⋅day−1⋅°C−1. In addition, the incidence and severity of phoma stem canker in planta over a natural growing cycle at four temperatures (16, 20, 24, and 28°C) around the average culture optimum were determined. There was no correlation between in vitro and in planta growth, and the decrease in pathogen measures either side of the optimum temperature was much less for in planta growth than for in vitro growth. We conclude that both pathogens have the capacity to evolve to adapt to changes in environmental conditions, but that predictions of the effect of this adaptation, or estimates of heritability in natural conditions, cannot be made from measurements in vitro.

Effects of Crop Rotation and Biocontrol Amendments on Rhizoctonia Disease of Potato and Soil Microbial Communities

Rotation crops and biocontrol amendments were investigated for suppression of Rhizoctonia solani on potato (Solanum tuberosum) and their interactive effects on soil microbial communities. Greenhouse trials were conducted to evaluate selected rotation crops, including barley, common and “Lemtal” ryegrass, clover, potato, and combinations of barley with ryegrass or clover, for their effects on populations of R. solani and Rhizoctonia disease. Potato and clover preceding potato resulted in higher disease severity than most other rotations, whereas ryegrass reduced stem canker severity. In addition, all ryegrass treatments resulted in substantially higher populations of R. zeae. Field trials evaluating selected biocontrol treatments in combination with different rotations were conducted at two locations in Maine. Potatoes were treated with the biocontrol organisms Laetisaria arvalis, Trichoderma virens, or Bacillus subtilis and planted following rotation crops of barley and ryegrass, barley and clover, or potato. The barley/ryegrass rotation significantly reduced incidence and severity of stem canker and increased tuber yield at one location. Efficacy of the biocontrol treatments varied by rotation and location, with L. arvalis and T. virens reducing black scurf in some rotations and increasing some aspects of tuber yield at one location. Soil microbial community characteristics differed among rotation crops and biocontrol treatments. Significant crop by biocontrol interactions were observed demonstrating the complex interactions among rotation crops, biocontrol treatments, and soil microbial communities, as well as indicating that biocontrol can be enhanced within beneficial rotations.

Mycovirus-Induced Hypervirulence of Leptosphaeria biglobosa Enhances Systemic Acquired Resistance to Leptosphaeria maculans in Brassica napus.

Phoma stem canker (blackleg) is one of the most important diseases of winter oilseed rape (Brassica napus) worldwide and is caused by a complex that comprises at least two species: Leptosphaeria maculans and L. biglobosa. Screening a panel of field Leptosphaeria isolates from B. napus for the presence of mycoviruses revealed the presence of a novel double-stranded RNA quadrivirus in L. biglobosa and no viruses in L. maculans. Following elimination of the mycovirus, virus-infected and virus-free isogenic lines of L. biglobosa were created. A direct comparison of the growth and virulence of these isogenic lines illustrated that virus infection caused hypervirulence and resulted in induced systemic resistance toward L. maculans in B. napus following lower leaf preinoculation with the virus-infected isolate. Analysis of the plant transcriptome suggests that the presence of the virus leads to subtle alterations in metabolism and plant defenses. For instance, transcripts involved in carbohydrate and amino acid metabolism are enriched in plants treated with the virus-infected isolate, while pathogenesis-related proteins, chitinases and WRKY transcription factors are differentially expressed. These results illustrate the potential for deliberate inoculation of plants with hypervirulent L. biglobosa to decrease the severity of Phoma stem canker later in the growing season.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Automated image processing framework for analysis of the density of fruiting bodies of Leptosphaeria maculans on oilseed rape stems

Understanding the transmission of plant pathogen inoculum during the periods when the host plants are not present is crucial for predicting the initiation of epidemics and optimizing mitigation strategies. However, inoculum production at the end of the cropping season, survival during the intercrop period, and the emergence or release of inoculum can be highly variable, difficult to assess, and generally inferred indirectly from symptom data. As a result, a lack of large datasets hampers the study of these epidemiological processes. Here, inoculum production was studied in Leptosphaeria maculans, the cause of phoma stem canker of oilseed rape. The fungus survives on stubble left in the field, from which ascospores are released at the beginning of the next cropping season. An image processing framework was developed to estimate the density of fruiting bodies produced on stem pieces following incubation in field conditions, and a quality assessment of the processing chain was performed. A total of 2540 standardized RGB digital images of stems were then analysed, collected from 27 oilseed rape fields in Brittany over four cropping seasons. Manual post‐processing removed 16% of the pictures, e.g. when moisture‐induced darkening of the oilseed rape stems caused overestimation of the area covered with fruiting bodies. The potential level of inoculum increased with increasing phoma stem canker severity at harvest, and depended on the source field and the cropping season. This work shows how image‐based phenotyping generates high‐throughput disease data, opening up the prospect of substantially increased precision in epidemiological studies.

Oilseed rape (Brassica napus) resistance to growth of Leptosphaeria maculans in leaves of young plants contributes to quantitative resistance in stems of adult plants.

Key message: One QTL for resistance against Leptosphaeria maculans growth in leaves of young plants in controlled environments overlapped with one QTL detected in adult plants in field experiments. The fungal pathogen Leptosphaeria maculans initially infects leaves of oilseed rape (Brassica napus) in autumn in Europe and then grows systemically from leaf lesions along the leaf petiole to the stem, where it causes damaging phoma stem canker (blackleg) in summer before harvest. Due to the difficulties of investigating resistance to L. maculans growth in leaves and petioles under field conditions, identification of quantitative resistance typically relies on end of season stem canker assessment on adult plants. To investigate whether quantitative resistance can be detected in young plants, we first selected nine representative DH (doubled haploid) lines from an oilseed rape DY ('Darmor-bzh' × 'Yudal') mapping population segregating for quantitative resistance against L. maculans for controlled environment experiment (CE). We observed a significant correlation between distance grown by L. maculans along the leaf petiole towards the stem (r = 0.91) in CE experiments and the severity of phoma stem canker in field experiments. To further investigate quantitative trait loci (QTL) related to resistance against growth of L. maculans in leaves of young plants in CE experiments, we selected 190 DH lines and compared the QTL detected in CE experiments with QTL related to stem canker severity in stems of adult plants in field experiments. Five QTL for resistance to L. maculans growth along the leaf petiole were detected; collectively they explained 35% of the variance. Two of these were also detected in leaf lesion area assessments and each explained 10-12% of the variance. One QTL on A02 co-localized with a QTL detected in stems of adult plants in field experiments. This suggests that resistance to the growth of L. maculans from leaves along the petioles towards the stems contributes to the quantitative resistance assessed in stems of adult plants in field experiments at the end of the growing season.

Analysis of quantitative adult plant resistance to blackleg in Brassica napus

Leptosphaeria maculans L. causing blackleg is a highly evolved fungal pathogen that damages the rapeseed industry in Canada, Australia, and Europe. Advanced backcross populations segregating for intermediate resistance (BLMR2) and susceptible (blmr2) alleles were planted and inoculated using cotyledons. Near iso-genic lines (NILs) containing the intermediate blackleg resistance locus along with the parent lines and other controls were planted in a blackleg nursery at the University of Manitoba. ANOVA indicated a correlated significant difference between the genotypes with BLMR2 and blmr2 alleles at the seedling and adult plant stages. Heterozygous plants with both BLMR2 and blmr2 alleles showed relatively lower disease severity index (DSI) while other plants with only the blmr2 allele induced a higher DSI at seedling and adult plant stages, respectively, on a scale of 0–9. In the field, the NILs with the BLMR2 alleles consistently showed lower stem canker severity index similar to the resistant parent Surpass 400 in three consecutive years. In comparison, lines with known R genes (BLMR1 and Rlm2) showed relatively higher DSI in the field test. This demonstrated that the intermediate resistance locus performed well under severe blackleg disease pressure in the field while the NIL lines with single dominant R genes were ineffective. Because a group of isolates carrying various Avr/avr alleles was used in the field evaluation, our results suggest that the intermediate resistance locus confers horizontal resistance and has excellent potential in blackleg management in western Canadian canola production regions.

Correction to: Using multilocus sequence analysis to distinguish pathogenic from saprotrophic strains of Pseudomonas from stone fruit and kiwifruit

A variety of pseudomonads are associated with diseases of Actinidia and Prunus plants. A recently emerged virulent haplotype of Pseudomonas syringae pv. actinidiae (biovar 3) causes severe stem cankers on kiwifruit associated with dieback of canes. Pseudomonas syringae also causes one of the most important bacterial diseases affecting cherry orchards worldwide. Bacterial canker of cherry limits production in orchards in New Zealand. Less virulent and non-pathogenic pseudomonads also exist on both hosts, providing an opportunity to investigate the diversity of pseudomonads on these hosts, to find identifiers for new emerging highly pathogenic strains that could be used at the border to prevent incursions. In this study, genetic typing was used to explore the diversity of Pseudomonas on kiwifruit and a variety of Prunus plants. Multilocus sequence analysis (MLSA) of four house-keeping genes separated pseudomonads from kiwifruit and stone fruit plants into two major groups: one corresponding to P. syringae sensu lato and one corresponding to other Pseudomonas species. Within P. syringae sensu lato, strains were assigned to six of the nine previously described genomospecies or five of the seven previously described phylogroups. In the other major group, strains from both hosts clustered with a variety of well characterised non-pathogenic pseudomonads. The classification of strains into the two major groups is of practical diagnostic value since the most common pathogens of fruit trees belong to the P. syringae species complex. Furthermore, our analysis suggests that molecular diagnostics might be possible for the classification of strains into these two groups as a first tool to screen for exotic pathogenic pseudomonads on germplasm imports of both commodities.

ANALYSIS OF CAUSATIVE FACTORS FOR CANKER DISEASE EPIDEMIC ON RUBBEr PLANTS IN SOUTH SUMATRA

Analysis of Causative Factors for Canker Disease Epidemic on Rubber Plants in South Sumatra. Lasiodiplodia theobromae is the cause of stem canker in the rubber plant (Hevea brasiliensis), this is a new disease and has never been reported in Indonesia. Currently, data on environmental factors and cultivation techniques related to the development of stem canker are not available so research needs to be done on this subject. The objective of the study was to analyze environmental factors and cultivation techniques related to stem canker on rubber trees in southern Sumatra. The study was conducted by collecting data on environmental factors and cultivation techniques and then analyzed the association with stem canker. Field observation was carried out at 21 locations in PT. Perkebunan Nusantara VII Business Unit Padang Plawi Bengkulu, Tulung Buyut Lampung, Musilandas Palembang and Experimental Garden of Sembawa Research Center, Indonesian Rubber Research Institute at Palembang to measure the severity and incidence of this disease. Besides that, the data of cultivation technique and the condition of the garden were taken from the officer at those locations. Soil sample from each location was taken in a composite way, then its physical and chemical components were analyzed. The analysis was conducted to find out the correlation between cultivation and disease severity using chi-square test. The relationship between chemical and physical factors of the soil with the disease severity was analyzed using a multivariate test of principal component analysis/PCA. Regression analysis was conducted to show the possible relation to the incidence and severity of this disease. The location, clones, weed control and the number of plants per hectare showed a correlation with the disease severity. There was a positive correlation between incidence and severity of the disease. The highest disease severity occurred at the Sembawa location on clone BPM 24, with the number of plant population per ha was ≥ 555 trees and weed control using herbicide. Content of soil nitrogen, dust, clay and water as well as cation exchange capacity (CEC)were suggested to significantly contribute to the severity of stem canker.

Temporal Interactions between Root-Lesion Nematodes and the Fungus Rhizoctonia Solani Lead to Reduced Potato Yield

Soil microorganisms and soil fauna may have a large impact on the tuber yield of potato crops. The interaction between root-lesion nematodes and the pathogenic fungus Rhizoctonia solani Kühn was studied on potato plants grown in pots under controlled conditions. In two similar experiments, different combinations of nematodes and fungal mycelium were added to the pots at three occasions; at planting, after 14 days, and after 28 days. The nematodes reduced root biomass and the combination of nematodes and R. solani resulted in reduced tuber yield in both experiments, but the interaction was not synergistic. In contrast, the number of stem canker lesions decreased in the presence of nematodes compared to treatments with R. solani only. The time of inoculation influenced the severity of both fungal and nematode damage. The nematode damage on tubers was less severe if the nematodes were added at 28 days, while the number of severe stem canker lesions increased if the fungus was added at 28 days. However, the time of nematode inoculation did not affect the incidence of fungal damage, hence the nematodes did not assist R. solani to infect the plant. Our results highlight the underestimated importance of root-lesion nematodes, not resulting in obvious above ground symptoms or misshaped tubers yet affecting the performance of other pathogens.

Using multilocus sequence analysis to distinguish pathogenic from saprotrophic strains of Pseudomonas from stone fruit and kiwifruit

A variety of pseudomonads are associated with diseases of Actinidia and Prunus plants. A recently emerged virulent haplotype of Pseudomonas syringae pv. actinidiae (biovar 3) causes severe stem cankers on kiwifruit associated with dieback of canes. Pseudomonas syringae also causes one of the most important bacterial diseases affecting cherry orchards worldwide. Bacterial canker of cherry limits production in orchards in New Zealand. Less virulent and non-pathogenic pseudomonads also exist on both hosts, providing an opportunity to investigate the diversity of pseudomonads on these hosts, to find identifiers for new emerging highly pathogenic strains that could be used at the border to prevent incursions. In this study, genetic typing was used to explore the diversity of Pseudomonas on kiwifruit and a variety of Prunus plants. Multilocus sequence analysis (MLSA) of four house-keeping genes separated pseudomonads from kiwifruit and stone fruit plants into two major groups: one corresponding to P. syringae sensu lato and one corresponding to other Pseudomonas species. Within P. syringae sensu lato, strains were assigned to six of the nine previously described genomospecies or five of the seven previously described phylogroups. In the other major group, strains from both hosts clustered with a variety of well characterised non-pathogenic pseudomonads. The classification of strains into the two major groups is of practical diagnostic value since the most common pathogens of fruit trees belong to the P. syringae species complex. Furthermore, our analysis suggests that molecular diagnostics might be possible for the classification of strains into these two groups as a first tool to screen for exotic pathogenic pseudomonads on germplasm imports of both commodities.

Mycovirus induced hypervirulence of Leptosphaeria biglobosa enhances systemic acquired resistance to Leptosphaeria maculans in Brassica napus

Phoma stem canker is one of the most important diseases of winter oil seed rape (Brassica napus) world-wide and is caused by a complex that comprises at least two species: Leptosphaeria maculans and Leptosphaeria biglobosa. Screening a panel of field Leptosphaeria isolates from B. napus for the presence of mycoviruses revealed the presence of a novel double stranded (ds) RNA virus in L. biglobosa and no viruses in L. maculans. The virus forms isometric particles ca. 40–45 nm in diameter and has four genomic segments, each possessing a single open reading frame flanked by untranslated regions. Phylogenetic analysis revealed modest similarities to known and suspected members of the family Quadriviridaeand therefore the virus was nominated Leptosphaeria biglobosa quadrivirus-1. Following eradication of the mycovirus, virus-infected and virus-free isogenic lines of L. biglobosa were created. A direct comparison of the growth and virulence of these isogenic lines illustrated that virus infection caused hypervirulence and resulted in induced systemic resistance towards L. maculans in B. napus following lower leaf pre-inoculation with the virus-infected isolate. Analysis of the plant transcriptome suggests that the presence of the virus leads to subtle alterations in metabolism and plant defences. For instance, transcripts involved in carbohydrate and amino acid metabolism are enriched in plants treated with the virus-infected isolate, while pathogenesis-related proteins, chitinases and WRKY transcription factors are differentially expressed. These results illustrate the potential for deliberate inoculation of plants with hypervirulent L. biglobosa to decrease the severity of phoma stem canker later in the growing season.

Combining R gene and quantitative resistance increases effectiveness of cultivar resistance against Leptosphaeria maculans in Brassica napus in different environments.

Using cultivar resistance against pathogens is one of the most economical and environmentally friendly methods for control of crop diseases. However, cultivar resistance can be easily rendered ineffective due to changes in pathogen populations or environments. To test the hypothesis that combining R gene-mediated resistance and quantitative resistance (QR) in one cultivar can provide more effective resistance than use of either type of resistance on its own, effectiveness of resistance in eight oilseed rape (Brassica napus) cultivars with different R genes and/or QR against Leptosphaeria maculans (phoma stem canker) was investigated in 13 different environments/sites over three growing seasons (2010/2011, 2011/2012 and 2012/2013). Cultivar Drakkar with no R genes and no QR was used as susceptible control and for sampling L. maculans populations. Isolates of L. maculans were obtained from the 13 sites in 2010/2011 to assess frequencies of avirulent alleles of different effector genes (AvrLm1, AvrLm4 or AvrLm7) corresponding to the resistance genes (Rlm1, Rlm4 or Rlm7) used in the field experiments. Results of field experiments showed that cultivars DK Cabernet (Rlm1 + QR) and Adriana (Rlm4 + QR) had significantly less severe phoma stem canker than cultivars Capitol (Rlm1) and Bilbao (Rlm4), respectively. Results of controlled environment experiments confirmed the presence of Rlm genes and/or QR in these four cultivars. Analysis of L. maculans populations from different sites showed that the mean frequencies of AvrLm1 (10%) and AvrLm4 (41%) were less than that of AvrLm7 (100%), suggesting that Rlm1 and Rlm4 gene-mediated resistances were partially rendered ineffective while Rlm7 resistance was still effective. Cultivar Excel (Rlm7 + QR) had less severe canker than cultivar Roxet (Rlm7), but the difference between them was not significant due to influence of the effective resistance gene Rlm7. For the two cultivars with only QR, Es-Astrid (QR) had less severe stem canker than NK Grandia (QR). Analysis of the relationship between severity of stem canker and weather data among the 13 sites in the three growing seasons showed that increased severity of stem canker was associated with increased rainfall during the phoma leaf spot development stage and increased temperature during the stem canker development stage. Further analysis of cultivar response to environmental factors showed that cultivars with both an Rlm gene and QR (e.g. DK Cabernet, Adriana and Excel) were less sensitive to a change in environment than cultivars with only Rlm genes (e.g. Capitol, Bilbao) or only QR (e.g. DK Grandia). These results suggest that combining R gene and QR can provide effective, stable control of phoma stem canker in different environments.