Management of rice blast (Pyricularia oryzae): implications of alternative hosts

Rice blast (Pyricularia oryzae) has become a serious disease on commercial rice (Oryza sativa) crops in northern Australia and is present there on wild rice (O. australiensis). Characterisation of the host range of P. oryzae is fundamental to both reducing disease spread and to preventing development of epidemics via better management of Poaceae inoculum reservoirs in Australia. Studies on response of three different wild O. australiensis sources toward four isolates of P. oryzae showed all genotypes very susceptible to three isolates [WAC13466 (race IA-1), BRIP53376 (race IB-3), NT2014a (race unknown)], but resistant to isolate BRIP39772 (race IA-3). Studies to investigate levels of blast disease development following inoculation on a range of Poaceae hosts showed both P. oryzae isolates (WAC13466, BRIP53376) were highly virulent on barley (disease index, DI?=?100%), and on Phalaris and O. australiensis (DI?=?70%). However, isolate BRIP53376 showed a significantly higher level of aggressiveness (DI ~80%) on ryegrass, wild oat and rice. Of the two wheat cultivars tested, only one cultivar showed disease and only from WAC13466 (DI ~30%). Sweet corn and goosegrass were also susceptible to both blast isolates, but DI was <50%. That P. oryzae was virulent across these diverse Poaceae hosts, highlights, for Australia, the possibility for these species in, first, harbouring P. oryzae isolates highly virulent to commercial rice, second, fostering spread of rice-attacking P. oryzae strains into regions currently free of rice blast, and third, potential for these alternative host species to encourage development of more virulent host-specific strains of P. oryzae. The current study is an important step towards facilitating improved crop protection in the medium and long term from reducing P. oryzae disease epidemics via a better understanding and management of inoculum reservoirs in Australia.

     

Factors Influencing Development of Root Rot on Ginseng Caused by Cylindrocarpon destructans

ABSTRACT The fungus Cylindrocarpon destructans (Zins) Scholten is the cause of root rot (disappearing root rot) in many ginseng production areas in Canada. A total of 80 isolates of C. destructans were recovered from diseased roots in a survey of ginseng gardens in British Columbia from 2002-2004. Among these isolates, 49% were classified as highly virulent (causing lesions on unwounded mature roots) and 51% were weakly virulent (causing lesions only on previously wounded roots). Pectinase and polyphenoloxidase enzymes were produced in vitro by C. destructans isolates when they were grown on pectin and phenol as a substrate, respectively. However, highly virulent isolates produced significantly (P < 0.001) higher enzyme levels compared with weakly virulent isolates. Histopathological studies of ginseng roots inoculated with a highly virulent isolate revealed direct hyphal penetration through the epidermis, followed by intracellular hyphal growth in the cortex. Subsequent cell disintegration and accumulation of phenolic compounds was observed. Radial growth of highly and weakly virulent isolates on potato dextrose agar was highest at 18 and 21 degrees C, respectively and there was no growth at 35 degrees C. Mycelial mass production was significantly (P 相似文献   

Suppression of Dollar Spot by Hypovirulent Isolates of Sclerotinia homoeocarpa

ABSTRACT Selected hypovirulent isolates of Sclerotinia homoeocarpa were evaluated for efficacy in suppressing dollar spot of turfgrass under growth room and field conditions. Under growth room conditions, hypovirulent isolates Sh12B, Sh09B, or Sh08D of S. homoeocarpa caused 3.4 to 30.4% diseased turf in comparison to virulent isolates Sh48B and Sh14D, which caused 80.2 to 90.2% disease. In treatments that received both virulent and hypovirulent isolates, only hypovirulent isolate Sh12B significantly reduced disease as compared with the control with virulent isolates alone. In a field experiment in 1993 on swards of creeping bentgrass artificially inoculated with a virulent isolate of the pathogen, all treatments containing hypovirulent isolate Sh12B applied as a mycelial suspension, granular mix, or alginate pellets developed significantly less disease (6.3 to 20.8% diseased turf) compared with their respective formulation controls (23.8 to 31.2%). Suppression of dollar spot by treatment with mycelial suspensions of isolate Sh12B was evident up to 45 days postinoculation, and disease suppression was still significant 1 year after application when compared with the water control. Applications of hypovirulent isolate Sh09B did not reduce dollar spot in any treatments. Significant suppression of dollar spot by isolate Sh12B was also observed in the experiment conducted in 1994. In addition, suppression of dollar spot by hypovirulent isolate Sh12B was evaluated on swards with naturally occurring inoculum during 1994. Treatments with a mycelial suspension and alginate pellets of hypovirulent isolate Sh12B significantly reduced dollar spot compared with their respective formulation controls. With few exceptions, there was no statistical difference between treatments with hypovirulent isolate Sh12B and the fungicide chlorothalonil (Daconil 2787). Multiple applications of the hypovirulent isolate did not result in greater suppression of dollar spot as compared with a single application. The results indicate that hypovirulence has potential as an effective strategy for the management of dollar spot.     

稻梨孢菌与其他寄主梨孢菌在水稻植株上的交互作用

作者研究了1个稻梨孢菌株与4个稻以外寄主梨孢菌株在混合接种和间隔接种条件下,在水稻植株上的交互作用。结果表明:弱致病菌与强致病菌间交互作用较强,非致病菌与强致病菌间较弱。弱致病菌可明显降低强致病菌的致病性,非致病菌对强致病菌的作用,多数组合表现为减轻病害,但有少数组合表现为促进发病。弱致病菌先接种;2天后接种强致病菌,比两者混合接种具有更强烈的互作效应,病斑数减少达35.7％—38.1％。先接种菌的孢子浓度对后接种菌的致病性也有影响。     

Incidence and pathogenicity of races and isolates of Verticillium dahliae in Crete, southern Greece

Classification of 32 Verticillium dahliae isolates originating from 19 plant species in eight different botanical families to races and determination of host range pathogenicity were carried out. The physiological races of isolates were identified using the two differential tomato cultivars ??Belladonna?? (susceptible to both races 1 and 2 of V. dahliae) and ??Ace 55VF?? (resistant to race 1, susceptible to race 2 of V. dahliae). Among these isolates, 14 were race 2 (43.8%), 12 race 1 (37.5%) and six nonpathogenic (18.7%) on tomato. The host range pathogenicity of isolates was determined using four differential hosts (eggplant, turnip, tomato (Ve ^? ) and sweet pepper). Among isolates, five were pathogenic to both eggplant and turnip (15.6%), 21 to eggplant, turnip and tomato (65.6%), five to eggplant, turnip, tomato and sweet pepper (15.6%) and one was pathogenic to eggplant, turnip and sweet pepper (3.2%). The pathogenicity of isolates on the aforementioned five hosts was investigated on the basis of external symptoms and by calculating the relative areas under disease progress curves (relative AUDPC). Results showed that eggplant was the most susceptible, followed by turnip and tomato cv. Belladonna, while sweet pepper and tomato cv. Ace 55VF were less susceptible to all the isolates used. The pathogenicity of isolates varied from highly to mildly virulent on eggplant and turnip while on Belladonna, Ace 55VF and sweet pepper it varied from highly virulent to nonpathogenic. Belladonna exhibited a similar level of susceptibility to races 1 and 2 of V. dahliae, but was more susceptible than Ace 55VF to race 2. Interestingly, the isolates originating from eggplant were clearly more virulent than those originating from tomato and black nightshade on all solanaceous plants tested.     

Present status of verticillium wilt of olive in Andalucía (southern Spain)1

Verticillium wilt caused by Verticillium dahliae is a disease highly prevalent in newly established olive orchards in Andalucía, southern Spain. Two syndromes of the disease occur in Andalucia, namely apoplexy and slow decline. Apoplexy is characterized by quick dieback of twigs and branches while slow decline consists of rapid drying out of inflorescences together with leaf chlorosis and necrosis. Systematic disease observations carried out in two experimental orchards planted with susceptible cv. Picual indicated that natural recovery of diseased trees occurred over time. Infection and vascular colonization of olive plants by V. dahliae were studied in susceptible (Picual) and resistant (Oblonga) cultivars inoculated with a mildly virulent or a highly virulent cotton-defoliating isolate of V. dahliae. Disease symptoms developed 24–32 days after inoculation in cv. Picual, but at that time plants of cv. Oblonga remained free from symptoms. However anatomical observations and isolations indicated that systemic infections by the two isolates had occurred to a large extent in both cultivars.     

稻梨孢菌与其他寄主梨孢菌在水稻植株上的交互作用

作者研究了1个稻梨孢菌株与4个稻以外寄主梨孢菌株在混合接种和间隔接种条件下,在水稻植株上的交互作用。结果表明:弱致病菌与强致病菌间交互作用较强,非致病菌与强致病菌间较弱。弱致病菌可明显降低强致病菌的致病性,非致病菌对强致病菌的作用,多数组合表现为减轻病害,但有少数组合表现为促进发病。弱致病菌先接种;2天后接种强致病菌,比两者混合接种具有更强烈的互作效应,病斑数减少达35.7%-38.1%。先接种菌的孢子浓度对后接种菌的致病性也有影响。     

Genetic and Virulence Diversity in Verticillium dahliae Populations Infecting Artichoke in Eastern-Central Spain

ABSTRACT Severe Verticillium dahliae attacks have occurred in artichoke crops in the Comunidad Valenciana region of eastern-central Spain since the late 1990s. Knowledge of genetic and virulence diversity in the pathogen population is a key factor for the management of the disease through disease risk assessment as well as development and use of resistant cultivars. V. dahliae isolates from artichoke (109 isolates) and cotton (three isolates) in that region were characterized by vegetative compatibility grouping (VCG), and specific polymerase chain reaction assays using three sets of primer pairs that differentiate the cotton-defoliating (D) and -nondefoliating (ND) V. dahliae pathotypes. In all, 35 and 39 V. dahliae isolates representative of the identified VCGs and geographic origins were tested for virulence to artichoke cvs. Nun 6374 and Nun 9444, and cotton cv. Acala SJ-2, respectively. Four VCGs were identified among 107 artichoke isolates, and 2 isolates were heterokaryon self-incompatible: VCG1A (one isolate), VCG2A (31 isolates), VCG2B (72 isolates), and VCG4B (three isolates). The three cotton isolates were VCG1A. Isolates in VCG2B were distributed across the region and were the most prevalent isolates in the northern part. Conversely, 83.9% of isolates in VCG2A were recovered from the southern part of the region. Two subgroups of isolates were identified in VCG2B based on heterokaryon compatibility with either international or local tester isolates, which further showed diversity in the amplification of 334- and 824-bp DNA fragments which are markers of the D and ND pathotypes, respectively. Virulence of isolates to artichoke and cotton correlated with VCG but the pattern of correlation varied with the host. VCG1A isolates from artichoke and cotton induced defoliation in cotton but not in artichoke. Collectively, isolates of VCG2B and VCG4B were the most virulent and isolates of VCG1A or HSI were the least virulent to artichoke; but isolates of VCG1A were more virulent to cotton than those of any other VCG. Also, molecular subgrouping in VCG2B determined by amplification of the 334- and 824-bp markers correlated with virulence of isolates to the two hosts tested.     

Evaluation of Olive Cultivars for Resistance to Verticillium dahliae

Resistance of 23 important olive cultivars to Verticillium dahliae has been evaluated in four experiments under controlled conditions. Nine-month-old nursery olive plants were inoculated with a cotton non-defoliating (ND) (V4) or a cotton defoliating (D) (V117) isolate of V. dahliae. Resistance was evaluated by assessing symptom severity using a 0–4 rating scale and estimating the area under disease progress curves. The percentage of plants killed and of those which recovered from the disease were used as additional parameters for including a particular cultivar into a defined category. Most of the evaluated cultivars were susceptible, although at different levels, to both isolates of V. dahliae. All cultivars were more susceptible to the D pathotype than to the ND one. A group of 11 cultivars, including several important Spanish cultivars, were susceptible or extremely susceptible to both pathotypes of V. dahliae. A second group showed differences of resistance depending on the pathotype used. They were susceptible or extremely susceptible to the D pathotype but resistant or moderately susceptible to the ND one. Finally, 'Frantoio', 'Oblonga' and 'Empeltre' were moderately susceptible to the D isolate of V. dahliae and resistant to the ND one. The resistance of 'Empeltre' was evident by the plant ability to recover from infection with either isolates. 'Empeltre' is considered to be a valuable cultivar for inclusion in breeding programmes for resistance to Verticillium wilt.     

Systemic growth of Leptosphaeria maculans from cotyledons to hypocotyls in oilseed rape: influence of number of infection sites, competitive growth and host polygenic resistance

The influence of competitive effects between two isolates, of the number of infection sites on cotyledons and of host polygenic resistance on the systemic growth of Leptosphaeria maculans , the cause of phoma stem canker in oilseed rape ( Brassica napus ), were investigated. Controlled-condition experiments were conducted with two oilseed rape doubled haploid lines, one susceptible and the other with a high level of polygenic resistance, inoculated via wounded cotyledons with conidial suspensions obtained from two isolates. Expression of cankers in plants was enhanced by exposing inoculated plants to low temperature (6°C) followed by warm temperature (20°C). The fungus was detected by PCR amplifications of three minisatellite markers in all stems with visible canker symptoms and also in the stems of 14 of the 59 plants without visible cankers on the hypocotyls. Disease severity increased with the number of infection sites on cotyledons: in one of the three replicate experiments, the mean external necrosis length on the hypocotyl ranged from 6·47 to 35·3 mm for one and eight infections sites on cotyledons, respectively. The probability of an isolate reaching the hypocotyl from inoculated cotyledons decreased with increasing competing inoculum load on cotyledons: for instance, for isolate A290v it decreased from 1 when inoculated alone to 0·28 when coinoculated with six drops of competing isolate P27d. Polygenic resistance significantly reduced disease incidence and severity. For instance, in one of the three replicate experiments, disease incidence ranged from more than 74% in susceptible plants to 16% in resistant ones, while mean external necrosis length was up to 35·3 and 6·5 mm on susceptible and on resistant plants, respectively. This study offers new possibilities for assessing levels of polygenic resistance to stem canker in B. napus and studying the aggressiveness of L. maculans isolates.     

Variation of pathotypes and races and their correlations with clonal lineages in Verticillium dahliae

Understanding pathogenic variation in plant pathogen populations is key for the development and use of host resistance for managing verticillium wilt diseases. A highly virulent defoliating (D) pathotype in Verticillium dahliae has previously been shown to occur only in one clonal lineage (lineage 1A). By contrast, no clear association has yet been shown for race 1 with clonal lineages. Race 1 carries the effector gene Ave1 and is avirulent on hosts that carry resistance gene Ve1 or its homologues. The hypothesis tested was that race 1 arose once in a single clonal lineage, which might be expected if V. dahliae acquired Ave1 by horizontal gene transfer from plants, as hypothesized previously. In a diverse sample of 195 V. dahliae isolates from nine clonal lineages, all race 1 isolates were present only in lineage 2A. Conversely, all lineage 2A isolates displayed the race 1 phenotype. Moreover, 900‐bp nucleotide sequences from Ave1 were identical among 27 lineage 2A isolates and identical to sequences from other V. dahliae race 1 isolates in GenBank. The finding of race 1 in a single clonal lineage, with identical Ave1 sequences, is consistent with the hypothesis that race 1 arose once in V. dahliae. Molecular markers and virulence assays also confirmed the well‐established finding that the D pathotype is found only in lineage 1A. Pathogenicity assays indicated that cotton and olive isolates of the D pathotype (lineage 1A) were highly virulent on cotton and olive, but had low virulence on tomato.     

Effect of wheat cultivar mixtures on populations of Puccinia striiformis races*

This study quantifies the frequency of simple and complex races (races that can infect two or more components) of Puccinia striiformis in mixtures of wheat cultivars possessing different race-specific resistance genes. Treatments were designed so that the complex race changed depending on the host mixture, thus enabling us to observe the influence of pathogen complexity in different genetic backgrounds. Six cultivar mixtures and one pure stand of winter wheat were inoculated with three races of P. striiformis at two locations for two seasons. Potted plants of three winter wheat cultivars, each susceptible to one of the three races of the pathogen, were used to sample the pathogen during the field epidemics. Disease incidence on the differential cultivars was used to calculate the proportion of the three races in each treatment. The specific cultivars included in the mixtures influenced the frequencies of the three races. Increasing the number of virulent races in a mixture reduced the frequency of the complex race relative to the other two races. The results suggest that genetic background of the pathogen race, host composition, and interaction among pathogen races may be as important as cost of virulence in determining race frequencies in mixtures.     

Influence of Temperature and Inoculum Density of Fusarium oxysporum f. sp. ciceris on Suppression of Fusarium Wilt of Chickpea by Rhizosphere Bacteria

The effects of temperature and inoculum density of Fusarium oxysporum f. sp. ciceris race 5 on suppression of Fusarium wilt in chickpea (Cicer arietinum) cv. PV 61 by seed and soil treatments with rhizobacteria isolated from the chickpea rhizosphere were studied in a model system. Disease development over a range of temperatures (20, 25, and 30 degrees C) and inoculum densities (25 to 1,000 chlamydospores per gram of soil) was described by the Gompertz model. The Gompertz relative rate of disease progress and final amount of disease increased exponentially and monomolecularly, respectively, with increasing inoculum densities. Disease development was greater at 25 degrees C compared with 20 and 30 degrees C. At 20 and 30 degrees C, disease development was greater at 250 to 1,000 chlamydospores per gram of soil compared with 25 to 100 chlamydospores per gram of soil. At 25 degrees C, increasing inoculum densities of the pathogen did not influence disease. Nineteen Bacillus, Paenibacillus, Pseudomonas, and Stenotrophomonas spp. out of 23 bacterial isolates tested inhibited F. oxysporum f. sp. ciceris in vitro. Pseudomonas fluorescens RGAF 19 and RG 26, which did not inhibit the pathogen, showed the greatest Fusarium wilt suppression. Disease was suppressed only at 20 or 30 degrees C and at inoculum densities below 250 chlamydospores per gram of soil. Bacterial treatments increased the time to initial symptoms, reduced the Gompertz relative rate of disease progress, and reduced the overall amount of disease developed.     

Quantifying Aphanomyces euteiches in Alfalfa with a Fluorescent Polymerase Chain Reaction Assay

ABSTRACT A polymerase chain reaction (PCR) assay using a set of specific primers and a dual-labeled probe (TaqMan) was developed to quantify the amount of Aphanomyces euteiches DNA in alfalfa plants exhibiting varying levels of disease severity. The study included isolates of race 1 and race 2 of A. euteiches. The assay also discriminated between alfalfa populations for resistance based on analysis of DNA extracted from bulked plant samples. Analysis of individual plants and bulked plant samples of standard check populations with both pathogen isolates resulted in Spearman rank correlations between pathogen DNA content and disease severity index ratings that were greater than 0.75 and highly significant (P < 0.0005). In experiments with a race 1 isolate, the amount of pathogen DNA present in the resistant check WAPH-1 was significantly less than in the susceptible check Saranac. In experiments with a race 2 isolate, the amount of pathogen DNA in the resistant check WAPH-5 was significantly less than in either of the susceptible checks, Saranac and WAPH-1. Discrimination between commercial cultivars based on quantitative PCR analysis of bulked plant samples was similar to classification based on visual assessment of disease severity.     

Pathogenic variability in Ethiopian isolates of Fusarium oxysporum f. sp. ciceris and reaction of chickpea improved varieties to the isolates

Twenty-four isolates of Fusarium oxysporum f. sp. ciceris were isolated from wilted chickpea plants obtained from different districts and ‘wilt sickplots’ of central Ethiopia to assess variability in pathogenecity of the populations. Each isolate was tested on 10 different chickpea lines and eight improved chickpea varieties. Isolates showed highly significant variation in wilt severity on the differential lines and improved varieties. Based on the reaction types induced on differential lines, isolates were grouped into four corresponding races. Of the 24 isolates, F13, F20 and F22 were the most virulent. Isolates of race 3 were found in all of the districts and ‘wilt sickplots’ studied. Improved chickpea varieties also showed differential reactions to the isolates. All varieties were resistant to isolates of race 3, while varieties Arerti and DZ-10-4 were resistant to all isolates tested, showing the lowest mean wilt severity. Varieties DZ-10-11 and Maryie were susceptible to isolates F13, F20 and F22 and showed the highest mean wilt severity. Identification of races can be useful in breeding chickpea varieties resistant to wilt. The differential reactions of the improved varieties against different races might be important in managing chickpea wilt through gene deployment.     

Isolation of Nonpathogenic Mutants of Fusarium oxysporum f. sp. melonis for Biological Control of Fusarium Wilt in Cucurbits

ABSTRACT Two nonpathogenic mutant strains 4/4 and 15/15 of Fusarium oxysporum f. sp. melonis (race 1,2) were isolated by a continuous dipinoculation technique following UV mutagenesis of the virulent wild-type isolate FOM1.2. No disease symptoms or detrimental effects were observed following inoculation of muskmelon seedlings by strain 4/4. In contrast, strain 15/15 caused mortality of susceptible cultivars although to a lesser extent than the wild-type isolate. Strain 4/4 colonized a variety of muskmelon and watermelon cultivars. In muskmelon cv. Ein Dor, seedlings were dipped in a conidial suspension of strain 4/4 and planted in medium amended with the mutant to achieve 100% colonization of roots and between 30 to 70% of the lower stem tissues 7 days after planting. Similar percent colonization of watermelon seedlings by strain 4/4 was recorded. In cross-protection experiments with muskmelon cultivars, significant reduction in seedling mortality was observed between 4/4-colonized FOM1.2. challenged plants compared with that of wild-type challenged plants alone. Similarly, strain 4/4 was able to significantly reduce mortality of watermelon seedlings caused by F. oxysporum f. sp. niveum race 2. This novel approach of generating nonpathogenic mutants for biological control in Fusarium spp. and other fungal pathogens from virulent wild-type isolates may be beneficial for control, because the mutant strains, lacking only in pathogenicity, may compete more efficiently than other biocontrol organisms against the pathogen of origin.     

小麦对白粉菌的抗性和感病性的诱导

 用小麦白粉菌(Erysiphe graminis f.sp.tritici)的无毒和有毒菌株,以3种接种顺序:1.先接无毒菌株,间隔一定时间后,再接有毒菌株;2.无毒和有毒菌株同时接;3.和"1"相反,接种具有不同抗性基因的小麦品种的幼苗或叶段,以研究小麦对白粉菌的诱导抗性和诱导感病性。结果表明:先接无毒菌株诱导的抗性最强;后接无毒菌株或无毒、有毒菌株同时接,当诱导菌(inducer)的密度很高时,也可以诱导出抗性,但较微弱;诱导的抗性随诱导菌的密度增高而增强;一经诱导,抗性可在被诱导部位维持到第6天;所使用的3种接种顺序都可以诱导小麦对白粉菌产生感病性,但较之诱导的抗性弱得多;在抗病性的诱导中,存在着小麦品种与诱导菌,或诱导菌与挑战菌(challenger)之间的相互作用;一个无毒菌株可以通过诱导而获得在抗病品种上生长的毒性,但当缺少有毒菌株存在时,获得毒性即消失;没有发现所诱导的抗性具有系统传导性。     

Pathogenic Variation in Colletotrichum gloeosporioides Infecting Stylosanthes spp. in a Center of Diversity in Brazil

ABSTRACT Pathogenic variation in Colletotrichum gloeosporioides infecting species of the tropical pasture legume Stylosanthes at its center of diversity was determined from 296 isolates collected from wild host population and selected germ plasm of S. capitata, S. guianensis, S. scabra, and S. macrocephala in Brazil. A putative host differential set comprising 11 accessions was selected from a bioassay of 18 isolates on 19 host accessions using principal component analysis. A similar analysis of anthracnose severity data for a subset of 195 isolates on the 11 differentials indicated that an adequate summary of pathogenic variation could be obtained using only five of these differentials. Of the five differentials, S. seabrana 'Primar' was resistant and S. scabra 'Fitzroy' was susceptible to most isolates. A cluster analysis was used to determine eight natural race clusters using the 195 isolates. Linear discriminant functions were developed for eight race clusters using the 195 isolates as the training data set, and these were applied to classify a test data set of the remaining 101 isolates. All except 11 isolates of the test data set were classified into one of the eight race clusters. Over 10% of the 296 isolates were weakly pathogenic to all five differentials and another 40% were virulent on just one differential. The unclassified isolates represent six new races with unique virulence combinations, of which one isolate is virulent on all five differentials. The majority of isolates came from six field sites, and Shannon's index of diversity indicated considerable variation between sites. Pathogenic diversity was extensive at three sites where selected germ plasm were under evaluation, and complex race clusters and unclassified isolates representing new races were more prevalent at these sites compared with sites containing wild Stylosanthes populations.