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1.
Aflatoxin contamination of crops is a world-wide problem. Lethal aflatoxicosis of humans has been associated with maize produced in Kenya for over three decades. The S strain morphotype of Aspergillus flavus was identified as the primary cause of aflatoxin contamination events occurring between 2004 and 2006 in Kenya. Because the S strain was first described in the U.S., it was suggested that the agent causing lethal levels of aflatoxins was introduced to Kenya with maize. DNA sequence comparisons among 68 S strain isolates from Kenya, the Americas, Asia, and Australia suggest the Kenyan isolates are distinct from those causing aflatoxin contaminations in the U.S. Analyses of 4.06 kb representing three loci from distinct chromosomes indicate that most S strain isolates from the U.S. resolved into a clade distinct from one containing the 30 Kenyan isolates. S strain isolates from Kenya were more closely related to the recently described species A. minisclerotigenes than to A. flavus. Furthermore, failure of the Kenyan isolates to produce G aflatoxins was attributed to a previously undescribed deletion in the cypA gene, suggesting that different deletion events led to loss of G aflatoxin production in S strain isolates from the U.S. and Kenya. Thus, although the Kenyan isolates have S strain morphology and produce large quantities of only B aflatoxins like A. flavus S strain isolates, these isolates are phylogenetically divergent from those described from other regions. The molecular characteristics of the Kenyan S strain isolates described herein are valuable tools to identify and track these highly aflatoxigenic fungi.  相似文献   

2.
Compositions of Aspergillus flavus populations determine the extent to which crops become contaminated with aflatoxins. In the current study, influences of diverse crop hosts on competition among A. flavus isolates were quantified with pyrosequencing. Maize, cotton, soyabean and sorghum supported different levels of sporulation, but intraspecific differences in sporulation were not detected on any host. However, hosts differentially influenced competition during infection, allowing greater sporulation by some isolates and increased host tissue invasion by others. Furthermore, competitive interactions during host invasion did not predict isolate success during sporulation. Isolates were similarly competitive on maize and sorghum, the two most closely related hosts. Host‐specific influences on intraspecific competition may dictate compositions of A. flavus populations and, as a result, the severity of aflatoxin contamination. Host factors should be considered when designing and implementing aflatoxin management strategies including biocontrol with atoxigenic strains.  相似文献   

3.
The aim of this study was to identify agronomic, ecological and sociocultural factors that could be modified to reduce the risk of aflatoxin contamination of peanuts from western Kenya. Presence of fungi within section Flavi of the genus Aspergillus and levels of total aflatoxin were determined for 436 peanut samples from the Busia and Homa bay districts. A total of 1458 cultures of Aspergillus flavus or A. parasiticus isolated from the samples were assayed for production of aflatoxin B1, B2, G1 and G2. Associations among the incidences of fungal species, incidences of samples with ≥10 μg kg?1 aflatoxin, production of specific aflatoxin types and various agronomic, ecological and sociocultural factors were modelled with chi‐squared and logistic regression methods. The predominant species were A. flavus L‐strain (78% incidence), A. flavus S‐strain (68%) and A. niger (65%). Occurrence of A. caelatus, A. alliaceus and A. tamarii in Kenya was also documented. Samples from the Busia district were three times (odds ratio = 3·01) as likely to contain ≥10 μg kg?1 of total aflatoxin as were samples from the Homa bay district, while samples containing A. flavus S‐strain were 96% more likely to exceed this threshold compared with samples from which this fungus was not isolated. Grading, planting improved cultivars and membership of a producer marketing group were negatively associated with the incidence of A. flavus, while crop rotation was negatively correlated with the incidence of B aflatoxins. These sociocultural factors can be modified to reduce the risk of peanut contamination with aflatoxin.  相似文献   

4.
A test performance study (TPS) was organized in 2018 with ten official testing laboratories to evaluate the performance of different real-time PCR tests for the detection of Clavibacter sepedonicus and/or Ralstonia solanacearum in potato tubers. Participants were sent spiked potato extracts with low (0.8–1.2 × 104 cfu mL-1), medium (1.6–2.4 × 105 cfu mL-1) and high (1.6–2.4 × 107 cfu mL-1) bacterial loads, DNA extracts thereof and heel-end cores from symptomatic potato tubers. The four real-time PCR tests in this TPS for detection of C. sepedonicus were considered fit for purpose as principal screening methods. Two real-time PCRs in this TPS were considered fit for purpose as principal screening methods for detection of R. solanacearum. A third real-time PCR missed 23% of the DNA samples from low-level R. solanacearum spikes and is considered not fit for purpose as a principal screening method. Correct identification of spiked samples was lower when DNA extraction from the spiked samples was performed by the participating laboratories, highlighting the importance of appropriate DNA extraction protocols.  相似文献   

5.
Two isolates of Streptomyces spp. DAUFPE 11470 and DAUFPE 14632 were evaluated to determine the antagonist–pathogen inoculum concentration relationship under greenhouse conditions. Pathogen and antagonist concentration, significantly (P < 0.05) affected development of Fusarium disease in maize with a significant interaction between pathogen and antagonist concentration. Dose–response relationship also differed significantly (P < 0.05) between the two isolates, but both isolates demonstrated effective control of Fusarium disease, regardless of pathogen concentration. The isolate DAUFPE 11470 provided the most effective control. The lowest value for disease incidence occurred at low pathogen (103 chlamydospore g−1 soil) and high antagonist concentration (106 cfu ml−1) for both isolates. The disease incidence for control plants ranged from 19% to 76%. However, in relation to control the lowest disease reduction occurred at low pathogen (103 chlamydospore g−1 soil) and high antagonist concentrations (106 cfu ml−1). These reductions were 10.6% and 13% for DAUFPE 14632 and DAUFPE 11470, respectively. The highest disease reductions, in relation to control plants, occurred at high pathogen (106 chlamydospore g−1 soil) and antagonist (106 cfu ml−1) concentrations for both isolates. These values were 55% and 62.2% for DAUFPE 14632 and DAUFPE 11470, respectively. The chlamydospore germination of Fusarium moniliforme was affected by glucose addition, antagonist isolates and type of inoculum. The lowest chlamydospore germination was observed with bacterial suspensions of the isolates, for all glucose additions. The results suggested that both Streptomyces spp. isolates were effective at different doses as biocontrol agents against F. moniliforme. Also, there was evidence for at least two mechanisms of biocontrol and that apparently, both isolates showed the same mechanisms of biocontrol action related to production of bioactive compounds and competition for carbon. Further studies will be developed to improve the level and effectiveness of control by these isolates.  相似文献   

6.
Push–pull involves intercropping of cereals with Desmodium as a “push” crop and planting Napier grass/Brachiaria as the “pull” crop at the border. The technology has been reported to effectively control stemborers, striga weed, and fall armyworm (FAW), and to improve soil nutrition, resulting in increased grain yield. This study evaluated the impact of stemborer and FAW management using this technology on incidence of maize ear rots and preharvest contamination of grains with aflatoxin and fumonisin in western Kenya. The study was conducted during three cropping seasons on maize grown under the push–pull system and as a monocrop. Incidence of stemborer and FAW damage was determined as percentage of damaged plants, while incidence of ear rots was determined as percentage of ears with symptoms. At harvest, fungi were isolated from kernels and aflatoxin and fumonisin were quantified using enzyme-linked immunosorbent assay. Stemborer and FAW damage was significantly (p = .001) reduced by over 50% under the push–pull system. There was also a significant (p < .001) reduction in the incidence of Fusarium verticillioides (60%) and Aspergillus flavus (86%), which was reflected in a reduced incidence of ear rots (50%) with the push–pull system (p = .001). Fumonisin in maize from push-pull farms was significantly (p = .048) reduced (39%) but the technology had no significant (p > .05) effect on aflatoxin. The study showed that push–pull is an effective strategy for managing maize ear rots and fumonisins, and therefore could play a role in improving food safety among smallholder maize farmers in the region.  相似文献   

7.
A detached leaf protocol for rapid screening of germplasm for resistance to citrus canker (Xanthomonas citri subsp. citri, Xcc) and citrus bacterial spot (Xanthomonas alfalfae subsp. citrumelonis, Xac) was developed to evaluate limited quantities of leaf material. Bacterial inocula of Xcc or Xac at 104, 105, or 108 cfu ml−1 were injection-infiltrated into the abaxial surface of disinfested, immature leaves of susceptible and resistant genotypes. Inoculated detached leaves were placed on the surface of 0.5% water agar plates and incubated at 28°C under a 12 h photoperiod. Likewise, inocula were infiltrated into attached leaves of greenhouse plants. At high inoculum concentrations of Xcc or Xac (108 cfu ml−1), resistant cultivars of kumquat developed a hypersensitive-like reaction within 3 days post inoculation (dpi). At 105 cfu ml−1, populations 14 dpi were <104 per inoculation site. In canker-susceptible Citrus spp. (‘Duncan’ grapefruit and ‘Rough’ lemon), water-soaked areas occurred by 3 dpi and typical canker lesions developed by 7 to14 dpi. Concentration of Xcc recovered from inoculation sites was approximately 105 cfu ml−1 by 14 dpi. In citrus bacterial spot-susceptible citrus (‘Swingle’ citrumelo and grapefruit), symptoms developed within 7 dpi. Populations of Xac after inoculation at 105 cfu ml−1 were comparable to Xcc in susceptible hosts 14 dpi (>105). The detached leaf assay is useful for the characterization and differentiation of lesion phenotype for each Xanthomonas pathogen permitting rapid screening of germplasm resistance based on the quantification of number of lesions and bacterial concentration.  相似文献   

8.
The vertical diversity and distribution of arbuscular mycorrhizal (AM) fungi were investigated in the Mu Us Desert, northwest China. Soils were sampled to 50?cm in depth in the rhizospheres of Hedysarum laeve, Artemisia ordosica, and Psammochloa villosa and 44 AM fungal species belonging to 10 genera were isolated. Several of these species have peculiar morphological features, which are distinct from other habitats. AM fungal diversity and distribution differed significantly among the three host plants and the five soil layers. Spore density, species richness, and the Shannon-Wiener index of AM fungi were 0.55–4.3 spores g?1 soil, 7–36 and 1.78–2.89, respectively. Spore density and species richness had a significant positive correlation with soil total phosphorus content (0.0377–0.1129?mg?g?1), and a negative correlation with soil pH (7.19–7.64). Nonmetric multidimensional scaling, PerMANOVA, and structural equation model analysis demonstrated that host plant species and soil depth significantly and directly influenced the structure of AM fungal communities. We concluded that diversity and distribution of AM fungi might be influenced by plant species, soil depth patterns, and soil nutrient availability in desert ecosystems. This research into AM fungal communities may lead to the development of AM fungi treatment for the mitigation of soil erosion and desertification using mycorrhizal plants, such as H. laeve, A. ordosica, and P. villosa.  相似文献   

9.
Verticillium wilt can cause high losses in tree nurseries. To be able to predict disease and unravel disease dynamics over time and space, the relationship between verticillium wilt and soil inoculum densities of Verticillium dahliae and the nematode Pratylenchus fallax was studied in two 4-year field experiments with Acer platanoides and Catalpa bignonioides in the Netherlands. Best-fit regression equations showed that pre-planting inoculum densities of V. dahliae can be used to predict verticillium wilt over a period of at least 4 years. Pratylenchus fallax contributed significantly to disease severity in A. platanoides in some years. Disease can already occur at the detection limit of the pathogens. The 5% infection thresholds for V. dahliae were at 1 (A. platanoides) vs. 3 (C. bignonioides) colony-forming units (CFU) g−1 soil. Analysis of spatial relationships indicated that diseased plants had a higher influence on neighbouring plants at low V. dahliae inoculum densities (<5 CFU g−1 soil) than at high densities (≥5 CFU g−1 soil). Seventy-four percent of the diseased plants recovered during the following year. After that year, recovered plants had a significantly higher probability of becoming diseased again than plants that were healthy during the two previous years, at high inoculum densities of V. dahliae, indicating that inoculum density in the soil, rather than incomplete recovery, was the most important factor for disease development.  相似文献   

10.
A conventional PCR and a SYBR Green real-time PCR assays for the detection and quantification of Phytophthora cryptogea, an economically important pathogen, have been developed and tested. A conventional primer set (Cryp1 and Cryp2) was designed from the Ypt1 gene of P. cryptogea. A 369 bp product was amplified on DNA from 17 isolates of P. cryptogea. No product was amplified on DNA from 34 other Phytophthora spp., water moulds, true fungi and bacteria. In addition, Cryp1/Cryp2 primers were successfully adapted to real-time PCR. The conventional PCR and real-time PCR assays were compared. The PCR was able to detect the pathogen on naturally infected gerbera plants and on symptomatic artificially infected plants collected 21 days after pathogen inoculation. The detection limit was 5 × 103 P. cryptogea zoospores and 16 fg of DNA. Real-time PCR showed a detection limit 100 times lower (50 zoospores, 160 ag of DNA) and the possibility of detecting the pathogen in symptomless artificially infected plants and in the re-circulating nutrient solution of closed soilless cultivation systems.  相似文献   

11.
Sorption–desorption of the fungicide triadimefon in field‐moist silt loam and sandy loam soils were determined using low‐density supercritical fluid extraction (SFE). The selectivity of SFE enables extraction of triadimefon from the soil water phase only, thus allowing calculation of sorption coefficients (Kd) at field‐moist or unsaturated conditions. Triadimefon sorption was influenced by factors such as soil moisture content and temperature; sorption increased with increased moisture content up to saturation, and decreased with increased temperature. For instance, Kd values for triadimefon on the silt loam and the sandy loam soils at 40 °C and 10% water content were 1.9 and 2.5 ml g−1, respectively, and at 18% water content, 3.3 and 6.4 ml g−1, respectively. Isosteric heats of sorption (ΔHi) were −42 and −7 kJ mol−1 for the silt loam and sandy loam soils, respectively. Sorption–desorption was also determined using an automated accelerated solvent extraction system (ASE), in which triadimefon was extracted from silt loam soil by 0.01 M CaCl2. Using the ASE system, which is basically a fast alternative to the batch equilibration system, gave a similar ΔHi value (−29 kJ mol−1) for the silt loam soil (Kf = 27 µg1 − 1/n ml1/n g−1). In order to predict transport of pesticides through the soil profile more accurately on the basis of these data, information is needed on sorption as a function of soil water content. © 2000 Society of Chemical Industry  相似文献   

12.
Fusarium graminearum and F. verticillioides are among the most important pathogens causing ear rot of maize in Central Europe. Our objectives were to (1) compare eight isolates of each species on two susceptible inbred lines for their variation in ear rot rating and mycotoxin production across 3 years, and (2) analyse two susceptible and three resistant inbred lines for potential isolate x line interactions across 2 years by silk-channel inoculation. Ear rot rating, zearalenone (ZEA) and deoxynivalenol (DON) concentrations were evaluated for all F. graminearum isolates. In addition, nivalenol (NIV) concentrations were analysed for two NIV producers. Fumonisin (FUM) concentrations were measured for all F. verticillioides isolates. Mean ear rot severity was highest for DON producers of F. graminearum (62.9% of the ear covered by mycelium), followed by NIV producers of the same species (24.2%) and lowest for F. verticillioides isolates (9.8%). For the latter species, ear rot severities differed highly among years (2006: 24%, 2007: 3%, 2008: 7%). Mycotoxin concentrations among isolates showed a broad range (DON: 100–284 mg kg−1, NIV: 15–38 mg kg−1, ZEA: 1.1–49.5 mg kg−1, FUM: 14.5–57.5 mg kg−1). Genotypic variances were significant for isolates and inbred lines in all traits and for both species. Isolate x line interactions were significant only for ear rot rating (P < 0.01) and DON concentration (P < 0.05) of the F. graminearum isolates, but no rank reversals occurred. Most isolates were capable of differentiating the susceptible from the resistant lines for ear rot severity. For resistance screening, a sufficiently aggressive isolate should be used to warrant maximal differentiation among inbred lines. With respect to F. verticillioides infections, high FUM concentrations were found in grains from ears with minimal disease symptoms.  相似文献   

13.
This study aimed to assess the extent and distribution of Fusarium graminearum species complex (FGSC) diversity in rice seeds produced in southern Brazil. Four species and two trichothecene genotypes were detected among 89 FGSC isolates, based on a multilocus genotyping assay: F. asiaticum (69·6%) with the nivalenol (NIV) genotype, F. graminearum (14·6%) with the 15‐acetyldeoxynivalenol (ADON) genotype, and F. cortaderiae (14·6%) and F. meridionale (1·1%), both with the NIV genotype. Seven selected F. asiaticum isolates from rice produced NIV in rice‐based substrate in vitro, at levels ranging from 4·7 to 84·1 μg g?1. Similarly, two F. graminearum isolates from rice produced mainly 15‐ADON (c. 15–41 μg g?1) and a smaller amount of 3‐ADON (c. 6–12 μg g?1). One F. meridionale and two F. cortaderiae isolates did not produce detectable levels of trichothecenes. Two F. asiaticum isolates from rice and two from wheat (from a previous study), and one F. graminearum isolate from wheat, were pathogenic to both crops at various levels of aggressiveness based on measures of disease severity in wheat spikes and rice kernel infection in a greenhouse assay. Fusarium asiaticum and the reference F. graminearum isolate from wheat produced NIV, and deoxynivalenol and acetylates, respectively, in the kernels of inoculated wheat heads. No trichothecene was produced in kernels from inoculated rice panicles by any of the isolates. These findings constitute the first report of FGSC composition in rice outside Asia, and confirm the dominance of F. asiaticum in rice agroecosystems.  相似文献   

14.
Orchard and garden papaya crops grown in 47 Cuban municipalities were surveyed from 2008 to 2013, revealing the widespread distribution of papaya ringspot virus (PRSV) in Cuba. Phylodynamic analyses performed with the partial coat protein gene of all Cuban PRSV-P isolates (34 sequences) and 107 sequences of isolates from the American continent and the Caribbean islands showed a most recent common ancestor in 1942 (95% highest posterior density, HPD 95% = 1911–1967). The substitution rate was estimated to be 7.7 × 10−4 substitutions per site per year (HPD 95% = 4.6 × 10−4 to 1.1 × 10−3), which is equivalent to those detected in other RNA viruses. Demographic reconstruction of PRSV showed that viral diversity increased in the 1985–1990 period, which coincides with the implementation of extensive production practices. Moreover in Cuba, viral dispersion occurred from Mexico and other unknown ancestral locations. The spatiotemporal diffusion analysis proposed Mexico as an ancestral area for the origin of diversification in the American continent and suggests new dispersion events between American and Caribbean isolates. The observed widespread distribution, clear geographic grouping of Cuban isolates, virus growth and genetic diversity provide strong evidence of the PRSV dispersion patterns, which has implications for the control strategies of PRSV.  相似文献   

15.
The endoparasitic nematophagous fungusHirsutella rhossiliensis was tested for its ability to suppress root penetration and cyst formation by the potato cyst nematode speciesGlobodera pallida. Isolates ofH. rhossiliensis were obtained from infected potato cyst nematode juveniles from different starch potato fields in The Netherlands. The isolates showed no difference in spore adhesion to juveniles on agar plates (adhesion rate: ±90%). The most rapid growing isolate, CBS 108.94, was used for experiments. Vegetative mycelial colonies ofH. rhossiliensis CBS 108.94, grown in potato dextrose broth, were used as soil inoculum. During submerged cultivation the mycelial colonies produced phialides (spore-bearing cells) but no spores. Exposed to the air, however, spores were rapidly formed. The effect of different soil inoculum densities of mycelial colonies on root penetration byGlobodera pallida was examined in an experiment in 250-ml pots. Up to a mycelial colony concentration representing a potential spore density of 104 g–1 soil no suppression occurred. At approximated densities of 2.5×104 and 105 spores g–1 soil the numbers of juveniles which penetrated roots were reduced by 30% and 34%, respectively. The distribution of the inoculum could be improved by fragmentation of the mycelial colonies before soil inoculation. Using mycelial fragments, again no suppression of root penetration was observed up to a potential spore density of 104 g–1 soil, but at densities of 105 and 106 g–1 a suppression of 54% and 88%, respectively, was measured. In a greenhouse experiment, soil inoculation with mycelial colonies with a potential spore production of 2.5×105 g–1 soil resulted in a suppression of root penetration of 37% and 51% after 5 and 6 weeks, respectively, but the number of newly formed cysts after 18 weeks in soil was not different for control and inoculated pots. It is concluded thatH. rhossiliensis may be useful for the reduction of root damage caused by juveniles of potato cyst nematodes, but the usefulness for population control is doubtful.  相似文献   

16.
Sclerotinia stem rot (Sclerotinia sclerotiorum) is a serious disease in oilseed Brassica crops worldwide. In this study, temperature adaptation in isolates of S. sclerotiorum collected from differing climatic zones is reported for the first time on any crop. Sclerotinia sclerotiorum isolates from oilseed rape (Brassica napus) crops in warmer northern agricultural regions of Western Australia (WW3, UWA 7S3) differed in their reaction to temperature from those from cooler southern regions (MBRS‐1, UWA 10S2) in virulence on Brassica carinata, growth on agar, and oxalic acid production. Increasing temperature from 22/18°C (day/night) to 28/24°C increased lesion diameter on cotyledons of B. carinataBC054113 more than tenfold for warmer region isolates, but did not affect lesion size for cooler region isolates. Mean lesion length averaged across two B. carinata genotypes (resistant and susceptible) fell from 4·6 to 2·4 mm for MBRS‐1 when temperature increased from 25/21°C to 28/24°C but rose for WW3 (2·35 and 3·21 mm, respectively). WW3, usually designated as low in virulence, caused as much disease on stems at 28/24°C as MBRS‐1, historically designated as highly virulent. Isolates collected from cooler areas grew better at low temperatures on agar. While all grew on potato dextrose agar between 5 and 30°C, with maximum growth at 20–25°C, growth was severely restricted above 32°C, and only UWA 7S3 grew at 35°C. Oxalate production increased as temperature increased from 10 to 25°C for isolates MBRS‐1, WW3 and UWA 7S3, but declined from a maximum level of 101 mg g?1 mycelium at 20°C to 24 mg g?1 mycelium at 25°C for UWA 10S2.  相似文献   

17.
A large collection (n = 539) of Fusarium graminearum species complex (FGSC) isolates was obtained from Brazilian maize, and collections formed according to geography and maize part: (i) kernel (n = 110) from south and south‐central Brazil; (ii) stalk (n = 134) from Paraná state (south); and (iii) stubble (n = 295) from Rio Grande do Sul state (south). Species composition, identified using a multilocus genotype approach, was assessed separately in each collection due to differences in geographic sampling. Overall, three species were found: F. meridionale (Fmer; 67% prevalence) with the nivalenol (NIV) genotype, F. graminearum (Fgra; 19%) with the 15‐acetyl (A) deoxynivalenol (DON) genotype, and F. cortaderiae (Fcor; 14%) with the NIV (49/74) or the 3‐ADON (25/74) genotype. In kernels, Fmer was spread across all locations and Fgra and Fcor were found mostly at high elevation (>800 m a.s.l.). The majority (97·8%) of stalk isolates was assigned to Fmer; three were assigned to Fgra. In the stubble, Fmer was less dominant (53%), with a shift towards Fcor as the most frequent species at high elevation sites (>600 m a.s.l.). No differences in the mycelial growth rate were observed among isolates from each species grown at 15°C. Fgra grew faster at 25°C and Fmer showed the widest range of variation across the isolates at both temperatures. The survey data suggest that Fmer may outcompete other species on ears and stalks in comparison to stubble. Additional sampling that controls for other factors, as well as direct testing of aggressiveness on ears and stalk tissue, will be needed to fully evaluate this hypothesis.  相似文献   

18.
A protocol using real‐time polymerase chain reaction (PCR) for the direct detection and quantification of Plasmodiophora brassicae in soil samples was developed and used on naturally and artificially infested soil samples containing different concentrations of P. brassicae. Species‐specific primers and a TaqMan fluorogenic probe were designed to amplify a small region of P. brassicae ribosomal DNA. Total genomic DNA was extracted and purified from soil samples using commercial kits. The amount of pathogen DNA was quantified using a standard curve generated by including reactions containing different amounts of a plasmid carrying the P. brassicae target sequence. The PCR assay was optimized to give high amplification efficiency and three to four copies of the target DNA sequence were detected. Regression analysis showed that the standard curve was linear over at least six orders of magnitude (R2 > 0·99) and that the amplification efficiency was >92%. The detection limit in soil samples corresponded to 500 resting spores g?1 soil. The intersample reproducibility was similar to, or higher than, that of assays for other pathogens quantified in soil samples. Bait plants were used to validate the real‐time PCR assay. The protocol developed was used to investigate the spatial distribution of P. brassicae DNA in different fields and a significant difference was found between in‐field sampling points. The reproducibility of soil sampling was evaluated and showed no significant differences for samples with low levels of inoculum, whereas at higher levels differences occurred. Indicator kriging was used for mapping the probability of detecting P. brassicae within a 2‐ha area of a field. A threshold level of 5 fg plasmid DNA g?1 soil, corresponding to approximately 3 × 103P. brassicae resting spores g?1 soil, is suggested for growing resistant cultivars. The results provide a robust and reliable technique for predicting the risk of disease development and for assessing the distribution of disease within fields.  相似文献   

19.
Exserohilum turcicum is the causal agent of northern leaf blight, a devastating foliar disease of maize and sorghum. Specificity of E. turcicum to either maize or sorghum has been observed previously, but molecular evidence supporting host specialization is lacking. The aim of this study was to compare the genetic structure of E. turcicum isolates collected from adjacent maize and sorghum fields in Delmas and Greytown in South Africa. In addition, the mode of reproduction of this pathogen was investigated. Isolates from maize (N = 62) and sorghum (N = 64) were screened with 12 microsatellite markers as well as a multiplex mating type PCR assay. No shared haplotypes were observed between isolates from different hosts, although shared haplotypes were detected between isolates from maize from Delmas and Greytown. Population structure and principal coordinate analyses revealed genetic differentiation between E. turcicum isolates from maize and sorghum. Analysis of molecular variance indicated higher among‐population variation when comparing populations from different hosts, than comparing populations from different locations. Lack of shared haplotypes, high proportion of private alleles, greater among‐population variance between hosts than locations and significant pairwise population differentiation indicates genetic separation between isolates from maize and sorghum. The high haplotypic diversity in combination with unequal mating type ratios and significant linkage equilibrium indicates that both sexual and asexual reproduction contributes to the population genetic structure of E. turcicum in South Africa.  相似文献   

20.
Controlling Verticillium dahliae through irrigation systems should be an important measure within integrated management of verticillium wilt of olive in Spain. Pathogen content of water infested by V. dahliae conidia and sclerotia can be diminished following in vitro treatments with the disinfectants OX-VIRIN and OX-AGUA AL25. Three concentrations per disinfectant were assessed for their effectiveness under operational conditions of modern irrigated olive orchards. Sterilized potted soil was drip-irrigated with conidia- or sclerotia-containing water that was pumped from a storage tank and disinfected (or not, control) within the pipelines via metering pumps. The trial was carried out in autumn and spring for each type of propagule infesting the water. The inoculum dispensed through drippers and the total inoculum density accumulated in soil were estimated. Furthermore, the treated residual inoculum in soil was assessed for pathogenicity on olive plants. Conidial incorporation in soil was prevented by most disinfectant treatments in spring; while for sclerotia, prevention was observed only at the highest OX-VIRIN (51.2 mL L−1; in both seasons) and OX-AGUA AL25 (11.27 mL L−1; in autumn) concentration. The remaining disinfectant treatments reduced conidia and sclerotia accumulation in soil by over 99% and 95%, respectively. Season particularly impacted the efficacy of lower concentrations. Expression of symptoms was not observed in olive plants grown in previously treated soils. The infectivity of the residual inoculum present in some treated soils was prevented, markedly reduced or limited to the roots. These results provide a novel, interesting and feasible approach in the management of the disease.  相似文献   

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