Showing papers in "Plant Pathology in 2006"
TL;DR: In tomato, drought and salt stress stimulate different, but possibly overlapping, pathogen-defence pathways which may not necessarily involve ABA, but basal endogenous ABA levels suppress the resistance of tomato to O. cinerea, but an ABA increase above the basal level, resulting from exogenous application, does not increase susceptibility to these pathogens.
Abstract: Abiotic stress may affect plant response to pathogen attack through induced alterations in growth regulator and gene expression. Abscisic acid (ABA) mediates several plant responses to abiotic stress. The effects of drought, salt stress and ABA on the interaction of tomato (Lycopersicon esculentum) with the biotrophic fungus Oidium neolycopersici and the necrotrophic fungus Botrytis cinerea were investigated. Drought stress resulted in a twofold increase in endogenous ABA as well as a 50% reduction in B. cinerea infection and a significant suppression of O. neolycopersici on tomato cv. Moneymaker. Salt stress did not affect B. cinerea infection, but significantly reduced infection by O. neolycopersici, with no obvious increase in endogenous ABA. Compared with the wild type, the ABA-deficient sitiens mutant was more resistant to O. neolycopersici and B. cinerea. Exogenous ABA resulted in increased susceptibility of sitiens to both pathogens, but did not increase the basal susceptibility of wild-type plants. It is concluded that, in tomato, drought and salt stress stimulate different, but possibly overlapping, pathogen-defence pathways which may not necessarily involve ABA. Meanwhile, basal endogenous ABA levels suppress the resistance of tomato to O. neolycopersici and B. cinerea, but an ABA increase above the basal level, resulting from exogenous application, does not increase susceptibility to these pathogens.
204 citations
TL;DR: There is increasing interest in the use of biological control agents (BCAs) and plant resistance stimulants to suppress botrytis bunch rot in grapes, caused by Botrytis cinerea.
Abstract: There is increasing interest in the use of biological control agents (BCAs) and plant resistance stimulants to suppress botrytis bunch rot in grapes, caused by Botrytis cinerea. Numerous different filamentous fungi, bacteria and yeasts have been selected as potential BCAs for control of grey mould based upon demonstrated antagonism towards B. cinerea. Biological suppression of the pathogen arises via competition for nutrients and space, the production of inhibitory metabolites and/or parasitism. Preformed and inducible grapevine defence mechanisms also contribute to disease suppression by preventing or delaying pathogenic infection. Furthermore, various biotic and abiotic agents can stimulate grapevine defence mechanisms and so elevate resistance to B. cinerea infection. Biosuppression of B. cinerea in vineyards, using BCAs and resistance stimulants, has been inconsistent when compared with that observed in controlled glasshouse or laboratory conditions. This may be attributable, in part, to the innate variability of the field environment. Research to improve field efficacy has focused on formulation improvement, the use of BCA mixtures and combinational approaches involving BCAs and plant resistance stimulants with complementary modes of action.
203 citations
TL;DR: Co-dominant microsatellite molecular markers for Phytophthora infestans were developed and their potential for monitoring the genetic variation in populations was demonstrated in the UK, across Europe and worldwide.
Abstract: Co-dominant microsatellite molecular markers for Phytophthora infestans were developed and their potential for monitoring the genetic variation in populations was demonstrated in the UK, across Europe and worldwide. Markers were developed according to two strategies. First, several thousand P. infestans expressed sequence tag (EST) and bacterial artificial chromosome (BAC) sequences were screened for the presence of simple sequence repeat (SSR) motifs, and, of these, 100 candidate loci were selected for further investigation. Primer pairs developed to these loci were tested against a panel of 10 P. infestans isolates and approximately 10% were shown to be polymorphic and therefore appropriate for further testing. Secondly, the construction and screening of a partial genomic library resulted in the development of one additional polymorphic marker. The resulting 12 SSR markers were converted to higher-throughput fluorescence-based assays and used in combination with two previously published markers to characterize a wider collection of 90 P. infestans isolates from the UK and six other countries. Several isolates from the closely related species P. mirabilis, P. ipomoea and P. phaseoli collected from around the world were also genotyped using these markers. Amongst the 90 isolates of P. infestans examined, considerable SSR diversity was observed, with 68 different genotypes and an average of 3·9 (range 2–9) alleles per locus. When other Phytophthora species were genotyped, all loci were successfully amplified and the majority were polymorphic, indicating their transferability for the potential study of other closely related taxa.
177 citations
TL;DR: Nine plant volatiles were tested for their activity in vitro and in vivo against Penicillium expansum, the cause of blue mould of pear, and the four most active compounds in in vitro studies were tested in vivo as fumigants against blue mould on pear cv.
Abstract: Nine plant volatiles were tested for their activity in vitro and in vivo against Penicillium expansum, the cause of blue mould of pear. In vitro spore germination and mycelial growth assay showed a consistent fungicidal activity by trans-2-hexenal, carvacrol, trans-cinnamaldehyde and citral, while hexanal (-)- carvone, p-anisaldehyde, eugenol and 2-nonanone exhibited a progressively lower inhibition. trans-2-Hexenal was the best inhibitor of conidial germination [MIC (minimum inhibitory concentration) = 24·6 µL L−1; ED50 = 10·2 µL L−1], while carvacrol was the best inhibitor of mycelial growth (MIC = 24·6 µL L−1; ED50 = 9 µL L−1). The four most active compounds in in vitro studies were tested in vivo as fumigants against blue mould on pear cv. Conference. Best control was achieved by trans-2-hexenal vapour treatments (12·5 µL L−1) when applied over a 24-h period, beginning 24 h after inoculation. In contrast, carvacrol (12·5–200 µL L−1), and trans-cinnamaldehyde (50–400 µL L−1) were ineffective and citral (200 µL L−1) showed only slight effect.
161 citations
TL;DR: The synergistic interactions increased titres of SPMMV and SPFMV RNA by ∼ 1000fold as quantified by real-time PCR, whereas SPCSV titres were reduced twofold, indicating an antagonistic interaction.
Abstract: Novel and severe symptoms of chlorosis, rugosity, leaf strapping and dark green islands, designated as sweetpotato severe mosaic disease (SPSMD), were caused by dual infection of Sweet potato mild mottle virus (SPMMV; Ipomovirus ) and Sweet potato chlorotic stunt virus (SPCSV; Crinivirus ) in three East African sweetpotato cultivars (Tanzania, Dimbuka and New Kawogo). The storage root yield was reduced by ∼ 80%, as compared with healthy plants under screenhouse conditions in Uganda. Plants infected with SPMMV or SPCSV alone showed nonsignificant or 50% yield reduction, respectively. SPCSV reduced resistance to SPMMV in sweetpotato, similar to the situation with resistance to Sweet potato feathery mottle virus (SPFMV; Potyvirus ) that breaks down following infection with SPCSV, followed by development of sweet potato virus disease (SPVD). In single virus infections with SPMMV and SPFMV or their coinfection, cvs Tanzania and Dimbuka were initially systemically infected, displayed symptoms and contained readily detectable virus titres, but new leaves were symptomless with very low virus titres, indicating recovery from disease. In contrast, cv. New Kawogo remained symptomless and contained low SPMMV and SPFMV titres following graft inoculation. These moderate and high levels of resistance to SPMMV and SPFMV, respectively, were lost and cultivars succumbed to a severe disease following coinfection with SPCSV. The synergistic interactions increased titres of SPMMV and SPFMV RNA by ∼ 1000fold as quantified by real-time PCR, whereas SPCSV titres were reduced twofold, indicating an antagonistic interaction. Coinfection with SPMMV and SPFMV caused no detectable changes in virus titres or symptom severity.
143 citations
TL;DR: Results of the glasshouse evaluations revealed that two of the nonpathogenic F. oxysporum isolates reduced fusarium wilt incidence by 87·4 and 75·0%, respectively, and these isolates should be further evaluated for potential application in the field, independently and in combination.
Abstract: The aim of this study was to evaluate the ability of nonpathogenic F oxysporum and Trichoderma isolates from suppressive soils in South Africa to suppress fusarium wilt of banana in the glasshouse Several biological control agents and commercial biological control products were included in the study The isolates were first screened in vitro on potato dextrose agar In glasshouse evaluations, the fungal and bacterial isolates were established on banana roots before they were replanted in pathogen-infested soil, while the commercial biocontrol agents were applied as directed by the supplier Banana plantlets were evaluated for disease development after 7 weeks In vitro tests showed none of the nonpathogenic isolates suppressed Fusarium oxysporum fsp cubense ( Foc ), while slight suppression was observed with the two Trichoderma isolates Results of the glasshouse evaluations revealed that two of the nonpathogenic F oxysporum isolates, CAV 255 and CAV 241, reduced fusarium wilt incidence by 87·4 and 75·0%, respectively The known biological control agent Fo47 did not suppress Foc significantly Pseudomonas fluorescens strain WCS 417, known for its ability to suppress other fusarium wilt diseases (WCS 417), reduced disease incidence by 87·4% These isolates should be further evaluated for potential application in the field, independently and in combination
130 citations
TL;DR: Endophytic fungi were isolated from healthy stems and pods of cacao trees in natural forest ecosystems and agroecosystems in Latin America and West Africa and the role of these fungi within the host and their potential as biological control agents are discussed.
Abstract: Endophytic fungi were isolated from healthy stems and pods of cacao ( Theobroma cacao ) trees in natural forest ecosystems and agroecosystems in Latin America and West Africa. These fungi were collected for screening as a potential source of biocontrol agents for the basidiomycetous pathogens of cacao in South and Central America, Moniliophthora roreri (frosty pod rot) and Moniliophthora perniciosa (witches’ broom). Many of these isolates were morphologically unidentifiable as they failed to form fruiting structures in culture, or only produced arthrosporic stages. Affinities with basidiomycetes were suspected for many of these based on colony morphology. Fifty-nine of these morphologically unidentifiable isolates were selected for molecular identification by DNA extraction and sequence analysis of nuclear ribosomal DNA (rDNA). The large subunit (LSU) was chosen for initial sequencing because this region has been used most often for molecular systematics of basidiomycete fungi, and comprehensive LSU datasets were already available for sequence analyses. Results confirmed that the majority of the isolates tested belonged to the Basidiomycota, particularly to corticoid and polyporoid taxa. With LSU data alone, identification of the isolates was resolved at varying taxonomic levels (all to order, most to family, and many to genus). Some of the isolates came from rarely isolated genera, such as Byssomerulius , whilst the most commonly isolated basidiomycetous endophyte was a member of the cosmopolitan genus Coprinellus (Agaricales). The role of these fungi within the host and their potential as biological control agents are discussed.
123 citations
TL;DR: The specificity and sensitivity of the three protocols for detecting ‘ Candidatus Liberibacter asiaticus’ in total DNA extracts of midribs collected from infected citrus leaves with symptoms in Guangxi municipality, Jiangxi Province and Zhejiang Province were tested.
Abstract: Conventional PCR and two real-time PCR (RTi-PCR) methods were developed and compared using the primer pairs CQULA03F/CQULA03R and CQULA04F/CQULA04R, and TaqMan probe CQULAP1 designed from a speciesspecific sequence of the rplJ/rplL ribosomal protein gene, for diagnosis of citrus huanglongbing (HLB) disease in southern China. The specificity and sensitivity of the three protocols for detecting ‘ Candidatus Liberibacter asiaticus’ in total DNA extracts of midribs collected from infected citrus leaves with symptoms in Guangxi municipality, Jiangxi Province and Zhejiang Province, were tested. Sensitivities using extracted total DNA (measured as copy number, CN per µ L of recombinant plasmid solution) were 439·0 (1·30 × 10 − 1
114 citations
TL;DR: Two bacterial isolates, Bacillus megaterium and Burkholderia cepacia, demonstrated to be antagonistic against Fusarium oxysporum f.sp.
Abstract: Two bacterial isolates, Bacillus megaterium (c96) and Burkholderia cepacia (c91), demonstrated to be antagonistic against Fusarium oxysporum f.sp. radicis-lycopersici, the causal organism of fusarium crown and root rot of tomato, were evaluated as biocontrol agents alone and when integrated with the fungicide carbendazim. In an initial screening, these isolates reduced disease incidence by 75 and 88%, respectively. In vitro, both biocontrol agents were highly tolerant to the fungicide carbendazim, commonly used to control fusarium diseases. Carbendazim reduced disease symptoms by over 50% when used at > 50 µg mL−1, but had little effect at lower concentrations. Combination of the bacterial isolates and carbendazim gave significant (P ≤ 0·05) control of the disease when plants were artificially inoculated with the pathogen. Application of carbendazim at a low concentration (1 µg mL−1) in combination with B. cepacia c91 reduced disease symptoms by 46%, compared with a reduction of 20% obtained with the bacterium alone and no control with the chemical treatment alone. A combination of B. megaterium c96 with an increased application rate of 10 µg mL−1 carbendazim significantly reduced disease symptoms by 84% compared with inoculated controls and by 77% compared with carbendazim treatment alone. In this experiment, the integrated treatment also slightly outperformed application of 100 µg mL−1 carbendazim, and bacteria applied without fungicide also provided good disease control.
103 citations
TL;DR: The wheat cultivar Kariega expresses complete adult plant resistance against stripe rust, whereas cv.
Abstract: The wheat cultivar Kariega expresses complete adult plant resistance against stripe rust, whereas cv. Avocet S is susceptible. Using confocal laser scanning microscopy, initial fungal penetration into flag leaves was identical in both cultivars, with directional germ-tube growth towards stomata that were penetrated without the formation of an appressorium, followed by differentiation of a substomatal vesicle, infection hyphae, haustorial mother cells and haustoria. During the following 4 days, further fungal development occurred more quickly in the resistant than in the susceptible cultivar. However, by 7 days postinoculation (dpi) the situation changed, with exponential growth of the pathogen occurring only in the susceptible line. Induced cellular lignification, a typical defence reaction of cereals, was observed at 4 dpi in the resistant cultivar, and 2 days later lignified tissue completely surrounded the fungal colonies. In the susceptible cultivar, isolated lignified host cells occurred at 6 dpi, and long, unbranched fungal hyphae outgrowing the resistance reaction were observed.
101 citations
TL;DR: Information about the resistance of wheat lines to M. graminicola isolates will assist breeders to choose parents of crosses from which progeny with superior resistance to STB may be selected.
Abstract: From a total of 238 European cultivars and breeding lines screened for isolate-specific resistance to septoria tritici blotch (STB) with eight Mycosphaerella graminicola isolates from five different countries, 142 lines were resistant to Ethiopian isolate IPO88004, and 43 lines were specifically resistant to IPO323, with little or no leaf area bearing pycnidia of M. graminicola. These lines probably all have the resistance gene Stb6. Specific resistances to isolates CA30JI, IPO001, IPO89011, IPO92006 and ISR398 were less common. Seventy-three per cent of the lines were specifically resistant to at least one isolate and 36 lines were resistant to more than one isolate. The line with the greatest number of specific resistances was the spring cultivar Raffles, with five. The most resistant line in which no specific resistance was identified was the Italian landrace Rieti, an ancestor of many modern European wheat cultivars. There was also a wide range of partial resistance among the lines tested, expressed in detached seedling leaves. Information about the resistance of wheat lines to M. graminicola isolates will assist breeders to choose parents of crosses from which progeny with superior resistance to STB may be selected.
TL;DR: By detecting small but consistent differences in crown rot severity, the bioassay proved effective in large-scale screening for partial resistance, and makes this an invaluable tool in the understanding of pathogen aggressiveness, host specialization and parasitic fitness.
Abstract: A high-throughput and reliable seedling bioassay to screen wheat germplasm for crown rot resistance was developed. Single wheat seedlings were grown in square seedling punnets in a glasshouse and inoculated with a monoconidial Fusarium pseudograminearum isolate 10 days after emergence. The punnets were laid horizontally on their side and a 10-μL inoculum droplet placed on the stem base. Seedlings were incubated at near-saturated relative humidity, and crown rot severity was assessed 35 days after inoculation. Studies on the duration of incubation period, inoculum concentration and temperature were carried out to optimize these parameters. Seedling growth at 25/15(±5)°C in a glasshouse and 48-h incubation at near-saturated RH in darkness gave the best results. When crown rot resistance rankings of 16 Australian cultivars from the bioassay were compared with their field performance, Spearman's rank correlation coefficient was highly significant. This indicated that the seedling bioassay mimicked field resistance to crown rot in adult plants. A bootstrap resampling analysis showed little or no improvement in the coefficient of variation with an increasing number of replications, indicating a high level of precision and reproducibility. By detecting small but consistent differences in crown rot severity, the bioassay proved effective in large-scale screening for partial resistance: already over 1400 wheat genotypes have been screened. The high degree of precision makes this an invaluable tool in the understanding of pathogen aggressiveness, host specialization and parasitic fitness.
TL;DR: It is concluded that prior inoculation of olive plants with AMF may contribute to improving the health status and vigour of cvs Arbequina and Picual planting stocks during nursery propagation.
Abstract: The effects were investigated, under controlled conditions, of single and joint inoculation of olive planting stocks cvs Arbequina and Picual with the arbuscular mycorrhizal fungi (AMF) Glomus intraradices , Glomus mosseae or Glomus viscosum , and the root-knot nematodes Meloidogyne incognita and Meloidogyne javanica , on plant performance and nematode infection. Establishment of the fungal symbiosis significantly increased growth of olive plants by 88·9% within a range of 11·9‐214·0%, irrespective of olive cultivar, plant age and infection by M. incognita or M. javanica . In plants free from AMF, infection by Meloidogyne spp. significantly reduced the plant main stem diameter by 22·8‐38·6%, irrespective of cultivar and plant age. Establishment of AMF in olive plants significantly reduced severity of root galling by 6·3‐36·8% as well as reproduction of both Meloidogyne spp. by 11·8‐35·7%, indicating a protective effect against parasitism by root-knot nematodes. Infection by the nematodes influenced root colonization by AMF, but the net effect depended on the AMF isolate‐olive cultivar combination. It is concluded that prior inoculation of olive plants with AMF may contribute to improving the health status and vigour of cvs Arbequina and Picual planting stocks during nursery propagation.
TL;DR: A high degree of polymorphism in restriction patterns of the ITS region, including part of 25S rDNA, has been reported for the first time in the charcoal rot fungus.
Abstract: Phenotypic and genetic diversity of 59 Macrophomina phaseolina isolates collected from various host species growing in or near cluster bean (Cyamopsis tetragonoloba) fields in four states of north and north-west India were characterized using RAPD and PCR–RFLPs of the ITS region. These isolates, and 11 from various hosts from culture collections, were classified into three mycelial phenotypes: dense, feathery and restricted, based on variable growth patterns on nutrient agar containing 120 mm chlorate. Pathogenicity of isolates was evaluated by measuring the length of stem lesions 21 days post-inoculation on the susceptible cluster bean genotype FS 277. Isolates showed considerable variation in aggressiveness, with the isolates from cluster bean with dense chlorate phenotype producing relatively higher lesion lengths on cluster bean plants. The results of the RAPD assay clearly distinguished the isolates on the basis of chlorate phenotype and host origin. Isolates from a single host were generally similar to each other, but differed distinctly from those from other hosts. Chlorate-sensitive isolates were distinct from chlorate-resistant isolates within a given host. A high degree of polymorphism in restriction patterns of the ITS region, including part of 25S rDNA, has been reported for the first time in the charcoal rot fungus.
CABI1
TL;DR: It may be feasible to eradicate this outbreak by destroying affected plants and cleaning up affected fields, combined with removing male flower buds in surrounding healthy plants to prevent insect transmission.
Abstract: In May 2004, following reports from local farmers of a devastating new banana disease, the first three authors visited Masisi District, 72 km northwest from Goma in North Kivu Province, Democratic Republic of Congo (DRC) (altitude 1700 m above sea level) and diagnosed banana bacterial wilt caused by Xanthomonas campestris pv. musacearum (Xcm). Symptoms were similar to those seen in Uganda (Tushemereirwe et al., 2003) and included: progressive yellowing, wilting and blackening of leaves; yellow or brown vascular streaks throughout the plant; pockets of pale yellow bacterial ooze in airspaces within leaf bases; premature ripening and internal discoloration of fruits; and shrivelling of male inflorescence buds. Inflorescence symptoms probably result from transmission of bacteria by insects, and were uncommon. This may explain the limited spread in DRC since 2001, only c. 10 km from the original focus at Bashali Mokoto village, compared with 400 km in Uganda over the same period (see www.banana.go.ug). As in Uganda, ABB banana genotypes (especially 'Pisang Awak') appear to be the first to be infected, and matooke clones (Musa AAA-EA group) the last. Affected stools do not always die; new suckers emerge and these initially appear healthy but usually become infected from the mother plant, rarely surviving to flowering stage. The epicentre of the outbreak in Masisi was devastated, with total loss of yield and an alarming impact on food security. Using methods described by Tushemereirwe et al. (2004), yellow pigmented bacteria were isolated as almost pure cultures from samples of diseased inflorescence stalks sent to the Global Plant Clinic at CABI Bioscience, UK. Biochemical and molecular characteristics of two isolates were indistinguishable from Xcm from Uganda. Both caused rapid wilting within 7-10 days of inoculation into young banana plants from which the same organism was reisolated. It may be feasible to eradicate this outbreak by destroying affected plants and cleaning up affected fields, combined with removing male flower buds in surrounding healthy plants to prevent insect transmission. However, the first author has recently observed a new disease focus c. 20 km from the first, so continued vigilance and control action will be needed. The origin of these outbreaks is unknown. Until 2001, Xcm was known only from Ethiopia, where it causes disease in enset (Ensete ventricosum) and cultivated banana (Yirgou and Bradbury, 1968, 1974). It is possible that the disease has spread from wild or semicultivated enset plants, which can be found throughout the Masisi region. (Author's abstract).
TL;DR: It is suggested that pretreatment with biological or chemical defence activators can induce local and systemic resistance to L. maculans, with both short-term effects on the development of phoma leaf spotting and long-term results on theDevelopment of stem canker 8 months later.
Abstract: Effects of pretreatment of Brassica napus leaves with ascospores of Leptosphaeria biglobosa or chemical defence activators [acibenzolar-S-methyl (ASM) or menadione sodium bisulphite (MSB)] on infection by ascospores of Leptosphaeria maculans (phoma stem canker) and development of disease were studied in controlled-environment (phoma leaf spot) and field (phoma leaf spot and stem canker) experiments. In controlled-environment experiments, pretreatment of oilseed rape leaves (cv. Madrigal) with L. biglobosa, ASM or MSB delayed the appearance of L. maculans phoma leaf spot lesions. These pretreatments also decreased the phoma leaf spot lesion area in both pretreated leaves (local effect) and untreated leaves (systemic effect). In winter oilseed rape field experiments in the 2002/03 and 2003/04 growing seasons, pretreatment with L. biglobosa or ASM in October/November decreased not only the number of phoma leaf spot lesions per leaf caused by L. maculans in autumn/winter, but also the severity of phoma stem canker in the subsequent spring/summer. Effects were greater in 2002/03 (when natural L. maculans ascospore release began in September 2002) than in 2003/04 (when ascospore release began in December following a period of dry weather in August/September 2003). These results suggest that pretreatment with biological or chemical defence activators can induce local and systemic resistance to L. maculans, with both short-term effects on the development of phoma leaf spotting and long-term effects on the development of stem canker 8 months later.
TL;DR: The results demonstrate that different fungi inhabit decayed roots of conifer seedlings in different environments, and that pure-culture isolation and direct sequencing are complementary methods that are both necessary for a complete description of the fungal communities colonizing diseased conifer roots.
Abstract: Fungi colonizing decayed roots of Pinus sylvestris and Picea abies seedlings were assessed by pure-culture isolation and direct sequencing of DNA extracted from roots collected from three environments: bare-root forest nurseries; afforested clear-cuts; and abandoned farmland. Pure-culture isolation from 1500 roots collected from 480 seedlings (240 of each tree species) yielded 1110 isolates which, based on mycelial morphology and ITS rDNA sequencing, were found to represent 87 distinct taxa. Direct ITS rDNA sequencing from decayed sections of 140 roots (70 of each tree species) yielded 160 sequences representing 58 taxa. Direct sequencing revealed a significantly higher fungal diversity per root segment than pure-culture isolation. Overall, a total of 131 taxa were found, 92 of which (70·2%) were identified at least to genus level. Only 14 taxa (10·7%) were detected by both methods, while 73 (55·7%) were detected exclusively by isolation and 44 (33·6%), exclusively by sequencing. The pathogens Fusarium oxysporum (25·6%) and Nectria radicicola (14·9%) were the most common isolates. In contrast, direct sequencing most frequently detected endophyte Phialocephala fortinii (33·1%) and Chalara sp. NS234A2 (10·0%). There were no significant differences in species richness between roots from the different environments, but there was a marked effect on fungal community structure. The results demonstrate that different fungi inhabit decayed roots of conifer seedlings in different environments, and that pure-culture isolation and direct sequencing are complementary methods that are both necessary for a complete description of the fungal communities colonizing diseased conifer roots.
TL;DR: Cluster analyses clearly demonstrated the polyphyletic structures that exist in both pathogen populations, and genotypic diversity in F. pseudograminearum was significantly associated with its aggressiveness for the two diseases.
Abstract: Genotypic diversity in Fusarium pseudograminearum and F. graminearum from Australia and the relationship between diversity and pathogen aggressiveness for head blight and/or crown rot of wheat were examined. Amplified fragment length polymorphism (AFLP) analysis revealed a high level of genotypic diversity within each species. Sixty-three of the 149 AFLP loci were significantly different between the two species and 70 of 72 F. pseudograminearum and 56 of 59 F. graminearum isolates had distinct haplotypes. When head blight and crown rot severity data from a recently published work on isolates representing the entire range of aggressiveness were used, only the genotypic diversity of F. pseudograminearum was significantly associated with its aggressiveness for the two diseases. Cluster analyses clearly demonstrated the polyphyletic structures that exist in both pathogen populations. The spatial diversity within F. graminearum was high within a single field, while frequent gene flow (N-m similar to 14) and a low fixation index (G(st) = 0.03) were recorded among F. pseudograminearum isolates from the adjacent states of New South Wales and Queensland. The differences in population structure between the heterothallic F. pseudograminearum (teleomorph G. coronicola) and the homothallic F. graminearum (teleomorph G. zeae) were not as pronounced as expected given their contrasting mating systems. Neither species was panmictic or strictly clonal. This points to sexual recombination in F. pseudograminearum, suggesting that ascospores of G. coronicola may also play a role in its biology and epidemiology.
TL;DR: Transgenic lettuce lines containing the decarboxylase gene (oxdc) isolated from a Flammulina sp.
Abstract: Sclerotinia sclerotiorum causes rot in a broad range of crops including lettuce, soybean, dry bean and tomato. Pathogenesis of Sclerotinia has been associated with the copious production of oxalic acid. Enzymes capable of degrading oxalic acid have been utilized to produce transgenic resistant plants. Transgenic lettuce lines containing the decarboxylase gene (oxdc) isolated from a Flammulina sp. were produced by Agrobacterium-mediated transformation. Out of 80 regenerated plants, PCR analysis revealed the presence of the oxdc gene in 34 lines. Except for eight lines, the primary transformants transferred the foreign gene to the first generation in a Mendelian fashion. In a detached-leaf assay inoculated with agar plugs of a 2-day-old S. sclerotiorum culture, two lines (P100 and P43) were symptomless, while line P57 showed a delay in symptom development when compared with a nontransgenic control line. RT-PCR analysis carried out with the resistant lines showed the expression of oxdc gene transcripts.
TL;DR: PepMV was not found to reduce bulk yields in these trials, but the quality of tomato fruits harvested was reduced significantly, mainly a result of blotchy ripening, gold marbling, gold spot, and symptoms directly attributed to PepMV infection.
Abstract: Two fully replicated trials were conducted with glasshouse-grown tomatoes, under conditions similar to commercial production, to define the impact of Pepino mosaic virus (PepMV). PepMV was not found to reduce bulk yields in these trials, but the quality of tomato fruits harvested was reduced significantly. Compared with uninoculated, PepMV-free control plants, 6·5% of fruits of PepMV-affected cv. Espero were downgraded from class 1 in trial 1. In trial 2, an average 38% of class 1 fruits from PepMV-affected cvs Espero and Encore were lost as a result of downgrading. Loss of quality was mainly a result of blotchy ripening, gold marbling, gold spot, and symptoms directly attributed to PepMV infection. PepMV infection also affected fruit size. The results are discussed in relation to the demands of multiple retailers in the UK for class 1 tomatoes only.
TL;DR: Re-isolation studies, together with PCR analysis of samples taken from early growth stages through to fully ripe plants, showed that the onset of flowering was a critical phase for V. longisporum to colonize plants and the transition from the vegetative to generative phase is of importance for the spread of the fungus in oilseed rape plants.
Abstract: Verticillium wilt of oilseed rape (Brassica napus) is caused primarily by Verticillium longisporum and has become a serious problem in northern Europe. In order to evaluate whether V. longisporum and V. dahliae differ in their interaction with oilseed rape, phenotypical and molecular assessments were made. Oilseed rape plants for fungal assessments were inoculated with V. longisporum and V. dahliae via root-dipping and samples were taken from roots, stems, leaves, flowers, pods and seeds during plant development. The infection by V. longisporum was found to start mainly in lateral roots and root-hairs, followed by colonization of the xylem vessels and extensive spread in stems and leaves, whereas V. dahliae infected the main roots and remained in the region below the cotyledon node of the plants. Re-isolation studies, together with PCR analysis of samples taken from early growth stages through to fully ripe plants, showed that the onset of flowering was a critical phase for V. longisporum to colonize plants. No seeds infected with V. longisporum were found. Mycelial growth from V. dahliae but not V. longisporum was significantly reduced on media containing tissue from a low glucosinolate B. napus genotype compared with growth on media containing tissue from a high glucosinolate cultivar. The results of this study suggest that V. longisporum favours B. napus as host and that the transition from the vegetative to the generative phase is of importance for the spread of the fungus in oilseed rape plants.
TL;DR: Microsatellite and AFLP markers linked to these QTL should be useful for marker-assisted selection of QTL for low PSS and low DNA content from Wangshuibai.
Abstract: Wangshuibai is a Chinese landrace wheat with a high level of resistance to fusarium head blight (FHB) and deoxynivalenol (DON) accumulation. Using an F7 population of recombinant inbred lines (RILs) derived from the cross between Wangshuibai and Annong 8455 for molecular mapping of quantitative trait loci (QTL) for FHB resistance, the proportion of scabbed spikelets (PSS) and DON content were assessed under field conditions. Composite interval mapping revealed that two and three QTL were significantly associated with low PSS and low DON content, respectively, over 2 years. QTL on chromosomes 3B and 2A explained 17 and 11·5%, respectively, of the phenotypic variance for low PSS, whereas QTL on chromosomes 5A, 2A and 3B explained 12·4, 8·5 and 6·2%, respectively, of the phenotypic variance for low DON content. The 3B QTL appeared to be associated mainly with low PSS, and the 5A QTL primarily with low DON content in Wangshuibai. The 2A QTL had minor effects on both low PSS and DON content. Microsatellite and AFLP markers linked to these QTL should be useful for marker-assisted selection of QTL for low PSS and low DNA content from Wangshuibai.
TL;DR: PO has the potential to control bacterial wilt disease and that CWP may play an important role in the induction of resistance to R. solanacearum accompanying the activation of the ethylene-dependent signalling pathway.
Abstract: Pythium oligandrum (PO) is a mycoparasite on a wide range of fungi and suppresses diseases caused by fungal pathogens when colonizing the rhizosphere. PO and its cell wall proteins (CWPs) have elicitor activity that induces defence responses in plants. The potential of a mycelial homogenate of PO to suppress bacterial diseases was studied in roots of tomato (Lycopersicon esculentum cv. Micro-Tom) inoculated with Ralstonia solanacearum. PO-treated plants showed enhanced resistance to R. solanacearum and reduction in severity of wilt symptoms. As ethylene often acts as one of the signal molecules for induced resistance, its production following treatment of tomato roots with the mycelial homogenate or CWP of PO was measured. The level of ethylene in PO- and CWP-treated plants was transiently elevated six- to 11-fold at 4–8 h after treatment, followed by high expression of three basic ethylene-inducible defence-related genes (PR-2b, PR-3b and PR-5b). Analysis of PR-5b gene expression in the leaves of PO- and CWP-treated plants suggested that PR gene expression was induced systemically. The expression of LeERF2 and LeETR4, which confer an ethylene-dependent signalling pathway, was also significantly accelerated by such treatments. These results indicate that PO has the potential to control bacterial wilt disease and that CWP may play an important role in the induction of resistance to R. solanacearum accompanying the activation of the ethylene-dependent signalling pathway.
TL;DR: The objectives of this study were to isolate and identify nonpathogenic F. oxysporum strains from soils suppressive to banana wilt, and to determine the diversity of these isolates.
Abstract: One of the most serious diseases of banana is fusarium wilt, caused by Fusarium oxysporum f.sp. cubense (Foc). The objectives of this study were to isolate and identify nonpathogenic F. oxysporum strains from soils suppressive to banana wilt, and to determine the diversity of these isolates. More than 100 Fusarium strains were isolated from the rhizosphere of banana plants and identified to species level. Pathogenicity testing was carried out to confirm that these isolates were nonpathogens of banana. A PCR-based RFLP analysis of the intergenic spacer region of the ribosomal RNA operon was used to characterize the nonpathogens. The isolates were also compared with isolates of Foc from South Africa and the known biological control isolate of F. oxysporum, Fo47. The species-specific primers FOF1 and FOR1, in addition to morphological features, were used to confirm the identity of F. oxysporum isolates included in the PCR-RFLP analysis. Twelve different genotypes could be distinguished, identified by a six-letter code allocated to each isolate following digestion with the restriction enzymes HaeIII, HhaI, HinfI, MspI, RsaI and ScrfI. Eleven of these included nonpathogenic F. oxysporum isolates, and these groups could all be distinguished from the genotype that included Foc. Fo47 was included in one of the genotype groups consisting of nonpathogenic F. oxysporum isolates from South Africa.
TL;DR: An accurate image-analysis method was developed to assess quantitatively the spot-like lesions on fruits resulting from pathogen attack, applied to evaluation of the development and severity of anthracnose of mango fruit, caused by the fungus Colletotrichum gloeosporioides.
Abstract: An accurate image-analysis method was developed to assess quantitatively the spot-like lesions on fruits resulting from pathogen attack. The technique was applied to evaluation of the development and severity of anthracnose of mango fruit, caused by the fungus Colletotrichum gloeosporioides. In this method, a stepper motor rotates the mango fruit along its longitudinal axis while acquiring a sequence of 360 images of its total surface (one image for each degree). This set of images is used to create a pseudocylindrical ‘equal-area’ projection of the fruit in a two-dimensional map containing complete morphometrical and photometrical information of its surface. The lesion area can easily be evaluated from this map with image-analysis procedures. Quantitative data (percentage of area affected) can be used to establish an assessment scale for the disease based on lesion spots measured, as well as for detailed laboratory studies of mango anthracnose development. The average error of the method is −0·1%, standard deviation 0·44 (r2 = 0·99), and it may be adapted for use with most commercial image analysers and for other diseases with spot-like symptoms.
TL;DR: Results of a survey of olive knot disease in central Italy in 2002 and 2003 showed that Pantoea agglomerans was found associated with the pathogen Pseudomonas savastanoi pv.
Abstract: Results of a survey of olive knot disease in central Italy in 2002 and 2003 showed that Pantoea agglomerans was found associated with the pathogen Pseudomonas savastanoi pv. savastanoi (Ps. savastanoi) in 70% of the olive knots examined. Pathogenicity tests in which these two bacteria were co-inoculated on the stems of 1-year-old olive plants at ratios of 1:1, 1:100 and 100:1 showed that the growth of P. agglomerans was apparently aided by the presence of an actively growing population of Ps. savastanoi. At the same time, however, a dominant population of P. agglomerans at the inoculation site tended to depress the growth of Ps. savastanoi, probably because of competition for space and nutrients between these bacteria and by means of antibiotic production by P. agglomerans. In some cases the association of P. agglomerans, which in culture was found to produce indole-3-acetic acid but not cytokinins, with Ps. savastanoi resulted in an increase in the size of knots. This boosting effect of P. agglomerans on proliferation was probably due to the release of IAA by this bacterium at the inoculation sites.
TL;DR: The effects of a range of concentrations of four nutrients – nitrogen, phosphorus, potassium and calcium – in fertilizer solutions on the severity of anthracnose on strawberry cv.
Abstract: The effects of a range of concentrations of four nutrients – nitrogen, phosphorus, potassium and calcium – in fertilizer solutions on the severity of anthracnose on strawberry cv. Nyoho cultivated under a noncirculation hydroponics system were determined after inoculation with Colletotrichum gloeosporioides. Crop growth and tissue nitrogen, phosphorus, potassium and calcium contents of the entire above-ground parts of the plant were also investigated. Elevated nitrogen and potassium concentrations in the fertilizer solution increased disease severity in contrast to phosphorus and calcium. Treatment with either NH4 or NO3 nitrogen was not significantly different. The dry weight of the strawberry plants increased significantly with elevated concentrations of nitrogen (R2 = 0·9078) and phosphorus (R2 = 0·8842), but was not influenced by the elevated amounts of potassium (R2 = 0·8587) and calcium (R2 = 0·6526) concentrations.
TL;DR: A viral complex causing golden mosaic and leaf distortion in tomato plants was obtained from viruliferous whiteflies, and named TGV-Ub1, and clones corresponding to full-length viral genomes indicated that they constitute novel western hemisphere begomoviruses.
Abstract: A viral complex causing golden mosaic and leaf distortion (rugosity) in tomato plants was obtained from viruliferous whiteflies, and named TGV-Ub1. This complex was sap-transmitted from tomato to Nicotiana benthamiana. PCR amplification using universal begomovirus primers yielded two distinct fragments for DNA-A, suggesting that the TGV-Ub1 complex comprised at least two distinct viruses. Clones corresponding to full-length viral genomes were obtained from tomato plants infected with TGV-Ub1. Comparisons of the complete sequences of clones pUb1-49 (DNA-A), pUb1-62 and pUb1-81 (both DNA-B) indicated that they constitute novel western hemisphere begomoviruses. Clones pUb1-49 and pUB1-81 have identical common regions, thus representing the cognate DNA-A and -B of a novel begomovirus, named Tomato rugose mosaic virus (ToRMV). Clone pUb1-62 has a distinct common region from ToRMV and all other geminiviruses. A cognate DNA-A for pUb1-62 was not found. Clones containing 1·8 copies of the genomic components were constructed. Infectivity assays of these clones in tomato and N. benthamiana demonstrated that the clones corresponding to ToRMV systemically infected both hosts. Symptoms were analogous to those observed when using the pure isolates obtained in this study. The combination of pUb1-49 and -62 did not result in systemic infection, indicating that these components do not form a viable virus. ToRMV was sap-transmitted from N. benthamiana to N. benthamiana, and by grafting to Solanum tuberosum and Datura stramonium. ToRMV-A and ToRMV-B were detected in plants of Nicandra physaloides and Phaseolus vulgaris, respectively, growing in nearby tomato fields, in association with distinct DNA components.