Genetic differentiation in populations of Exserohilum turcicum from maize and sorghum in South Africa

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.     

Viral inclusions in monocotyledons infected by maize streak and related geminiviruses

Isolates of maize streak virus (MSV) were examined by thin-section electron microscopy in plants, assessed for characteristic features of infection and compared with other related geminiviruses infecting monocotyledons from Africa, islands in the Indian Ocean, and the Pacific Island of Vanuatu. Arrays of virus particles, often crystalline, were most often seen in the nucleus. The morphology of the nuclear crystalline arrays was characteristic of certain isolates or groups of isolates (strains). Infected nuclei could be seen in cells from the phloem parenchyma, vascular bundle sheath and mesophyll tissue, and also in epidermal guard cells of plants infected with the maize strain of MSV. The particle arrays varied in morphology from regular rows of virions forming distinctive blocks, to randomly arranged aggregates in certain areas of the nucleus. We consistently failed to find viral crystalline arrays associated with infection of panicum streak virus (PSV) and sugar cane streak virus (SSV) isolates either in these hosts or in maize. Occasionally arrays of MSV particles were found outside the nuclear envelope in physiologically active cells. Accumulations or sheets of MSV particles were seen lining the walls of some phloem companion cells. Crystalline aggregates of particles were frequently observed in the cell vacuole, after lysis of the nuclear membrane of dead cells which made up the chlorotic lesions, the typical symptom of virus infection. Virus preparations from all hosts contained typical geminate particles regardless of the morphology of the virion arrays. The effect on chloroplasts appeared to vary between isolates and this is discussed in relation to lesion colour. The arrangement of virions in the nucleus as a taxonomic character is diagnostic for MSV. Inclusions with crystalline structure found in sieve elements of infected plants were not immunogold labelled when thin sections were probed using antiserum to the virus particles.     

Incidence and geographical distribution of downy mildew on maize caused by Peronosclerospora sorghi in Nigeria

Abstract

Incidence and geographical distribution of downy mildew (Peronosclerospora sorghi) on maize (Zea mays) were determined in a systematic survey of 181 maize fields in the major maize‐growing regions and ecological zones of Nigeria. Downy mildew was observed in the forest and Guinea savanna ecologies, but no disease was detected in other ecologies. Highest levels of disease incidence (63%) were observed in the forest zone, and progressively less disease incidence was found at the southern and northern Guinea savanna zones. Spatial pattern analysis indicated an aggregation and randomness of infected plants in the northern Guinea savanna and the forest zone, respectively. Disease incidence was positively correlated with the presence of downy mildew infected sorghum plants and relative amount of sporulation on infected sorghum (Sorghum bicolor) but negatively correlated with plant age and cropping pattern. Relative sporulation on maize plants was negatively correlated with age of maize plants. Higher levels of disease incidence were observed when the previous crop was either maize or sorghum than when previous crops were cassava, okra, tomatoes, or rice.     

Influence of plant host species on intraspecific competition during infection by Aspergillus flavus

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.     

北京玉米和高粱上的玉米矮花叶病毒

 北京郊区玉米和高粱上的三个汁传病毒分离物鉴定为玉米矮花叶病毒株系B (MDMV),它可被桃蚜(Myzus persicoe)由玉米传至玉米,这三个分离物不侵染约翰逊草(Sorghum halepense(L) pers)。稀释终点是10^-2-10^-3;致死温度是50-55℃;体外保毒期是1-2天。MDMV的颗拉长度是735±15毫微米,在染病高粱叶的细胞质中观察到凤轮状内含体。狗尾草(Setaria viridis(L) Beauv),马唐(Digitavia Sanguinalis(L) Scop),蟋蟀草(Eleusine indica(L) Gaertn),矛叶荩草(Arthraxan Lancelatus(Roxb) Hochst)是北京地区MDMV的天然寄主。该病毒也侵染大油芒(Spodiopogon Sibiricus Trin)。     

Reaction of maize,sorghum and Johnson grass to Peronosclerospora sorghi

Development of sorghum downy mildew, incited by Peronoscleospora sorghi (Weston and Uppal) C.G. Shaw, on maize, sorghum and Johnson grass was investigated at two locations in Uganda during three seasons (1994 and 1995). More sorghum downy mildew developed on the Johnson grass and sorghum than on the maize at all locations and in all seasons. No significant differences were observed in sporulation of P. soghi on the three hosts. Leaf shredding occurred on the three hosts but was the least on maize. Cross-inoculation with both conidia and oospores was achieved on the three hosts. Since the fungal population formed oospores and sporulated readily on the three hosts, which is typical of the sorghum strain, the disease in Uganda is attributed to the sorghum strain.     

侵染番红花的芜菁花叶病毒

 从表现花叶症状的德国进口番红花上获得一线状病毒分离物SA,线状病毒粒子长度为700~800nm。其在人工接种的11科39种植物中能侵染6科14种植物,在克里芙兰烟上产生系统坏死,在小白菜、花椰菜等十字花科植物上产生系统花叶或为隐症,在昆诺藜等藜科植物上为局部侵染。在番红花病叶、球茎及克里芙兰烟病叶组织中观察到卷轴状和片层状聚集的风轮状内含体。免疫吸附和免疫修饰电镜观察结果显示,SA与芜菁花叶病毒((TuMV)具有紧密的血清学关系。根据这些特征将SA鉴定为TuMV的一个分离物。这是TuMV侵染番红花的首次报道。     

Impact of tobacco recessive resistance gene va on biological properties of Brazilian Potato virus Y (PVY) isolates

To investigate the role of environmental conditions on the selection of virulent Potato virus Y (PVY) isolates subject to pressure from the recessive resistance gene va in tobacco, a field survey was performed in Brazil where va‐derived genotypes have been recently introduced and now represent less than one‐third of cultivated tobacco genotypes. A serological analysis of 397 leaves collected from different Brazilian tobacco‐growing areas and mainly from plants with symptoms indicated that 52·4% of samples were infected by at least one of the viral species tested. PVY was present in 72·1% of infected samples. The probability of a plant being infected with PVY was reduced in va hosts. However, the biological characterization of PVY isolates on indicator hosts showed that 20 of the 29 tested isolates were able to overcome the alleles of the va gene. Moreover, the observed biological diversity of isolates was higher in susceptible tobacco genotypes than in va‐resistant ones. Comparison of these data with the PVY diversity in French tobacco fields shows that the use of va‐derived genotypes in two environments with contrasting climatic conditions, local hosts and cultural contexts, leads to a similar outcome: the prevalence of virulent isolates. These results strongly suggest an important role of the va gene in the modification of PVY populations.     

Susceptibility of different plant species and tomato cultivars to two isolates of <Emphasis Type="Italic">Pepino mosaic virus</Emphasis>

As Pepino mosaic virus has become a pathogen of major importance in worldwide tomato production, information is needed on possible differences between the sensitivity of cultivars towards infection. Furthermore, it is important what hosts other than Solanaceae may be virus reservoirs and are, therefore, threats for tomato cultivation. Two PepMV isolates (PepMV-Sav, E397, a European tomato isolate and PV-0554, a Peruvian pepino isolate) differing in their origin and virulence were used for several experiments to investigate these issues. The response to mechanical inoculation with PepMV was studied using 25 tomato cultivars, seven indicator plant species, and nine other possible horticultural host plants. Symptom development after infection with PepMV was monitored and the virus was detected by DAS-ELISA and IC-RT-PCR. Garlic and broad bean were shown to be additional hosts of PepMV depending on the virus isolate. Nicotiana benthamiana seems to be the most sensitive indicator among all tested indicator plants developing symptoms. Both PepMV isolates infected all tested tomato cultivars. Development of disease symptoms depended on the cultivar and the virus isolate but symptoms were not visible in all cases. None of the cultivars showed tolerance against the two isolates but two responded with a lower susceptibility at an absorbance level of 0.2 (healthy control 0.09). It was observed that some cultivars grown hydroponically showed also lower losses in biomass and yield. Data indicated a correlation between absorbance level in DAS-ELISA and reduction in total tomato growth.     

Disease severity,incidence and races of <Emphasis Type="Italic">Setosphaeria turcica</Emphasis> on sorghum in Uganda

In order to understand the underlaying causes of new severe turcicum leaf blight outbreaks in East Africa, a survey was undertaken in Uganda to examine the sorghum—Setosphaeria turcica interaction in terms of disease severity and incidence, the overall fungal population structure, and new resistant resources. Highest disease severities were recorded on caudatum accessions, whereas kafir genotypes were most resistant. The disease was more severe in the most humid farmlands compared to moderately dry agro-ecologies. In districts with wide adoption of the Epuripur variety a very high incidence (100%) of turcicum leaf blight was found. The two S. turcica mating type genes MAT1-1 and MAT1-2 assessed on fungal isolates deriving from both sorghum and maize diseased leaves were found in 20 of 23 districts sampled and in equal proportions. Upon cross inoculation on maize differential lines, four S. turcica isolates were identified as race 1, two as race 2, and one isolate corresponded to race 0 and race 3, respectively. The remaining 10 S. turcica isolates did not cause any disease symptoms on the maize lines assessed. Highly resistant accessions originating from a regional collection were found among the five sorghum races (kafir, guinea, caudatum, bicolor and durra), and are now implemented in new sorghum disease resistance programs.     

Complete genome sequences and properties of Spartina mottle virus isolates from hybrid Bermuda grass (Cynodon dactylon × Cynodon transvaalensis)

Full genome sequences are presented for two isolates of Spartina mottle virus (SpMoV) from two accessions of the hybrid turf grass Cynodon dactylon × C. transvaalensis (Bermuda grass, green couch), originally from the USA but detected during post-entry quarantine in Australia. Both isolates had a genome of 9,346 nucleotides, encoding a single polyprotein of 3,029 amino acids and predicted to be cleaved into 10 mature proteins. Phylogenetic analysis strongly supported placement in a new genus within the family Potyviridae. The isolates possess 715–780 × 11–12 nm flexuous virions and produce cylindrical (pinwheel) inclusions in infected cells. They were mechanically transmitted to several species in the Poaceae including Zea mays (maize, sweet corn), Chloris gayana (Rhodes grass), and Echinochloa colona (awnless barnyard grass).     

Genetic differentiation between hosts and locations in populations of latent Botrytis cinerea in southern England

This study tested the hypothesis that aggressive, localized infections and asymptomatic systemic infections were caused by distinct specialized groups of Botrytis cinerea, using microsatellite genotypes at nine loci of 243 isolates of B. cinerea obtained from four hosts (strawberry (Fragaria×ananassa), blackberry (Rubus fruticosus agg.), dandelion, (Taraxacum officinale agg.) and primrose (Primula vulgaris)) in three regions in southern England (in the vicinities of Brighton, Reading and Bath). The populations were extremely variable, with up to 20 alleles per locus and high genic diversity. Each host in each region had a population of B. cinerea with distinctive genetic features, and there were also consistent host and regional distinctions. The B. cinerea population from strawberry was distinguished from that on other hosts, including blackberry, most notably by a common 154‐bp amplicon at locus 5 (present in 35 of 77 samples) that was rare in isolates from other hosts (9/166), and by the rarity (3/77) of a 112‐bp allele at locus 7 that was common (58/166) in isolates from other hosts. There was significant linkage disequilibrium overall within the B. cinerea populations on blackberry and strawberry, but with quite different patterns of association among isolates from the two hosts. No evidence was found for differentiation between populations of B. cinerea from systemically infected hosts and those from locally infected fruits.     

Biological characterization of French Potato virus Y (PVY) isolates collected from PVY‐susceptible or ‐resistant tobacco plants possessing the recessive resistance gene va

Improved tobacco cultivars introgressed with alleles of the recessive resistance va gene have been widely deployed in France to limit agronomical consequences associated with Potato virus Y (PVY) infections. Unfortunately, necrotic symptoms associated with PVY have been reported on these cultivars suggesting that PVY is able to overcome the resistance. A field survey was performed in France in 2007 to (i) estimate the prevalence of PVY in tobacco plants showing symptoms and (ii) characterize PVY isolates present in susceptible and va‐derived tobacco cultivars. A serological typing procedure, applied to 556 leaves collected from different French tobacco growing areas, was performed using polyclonal antisera raised against different viral species including PVY. Viral species were detected in 80·8% of leaves and PVY was present in 83·5% of infected samples. However, statistical analysis confirmed that the probability of a tobacco plant being infected with PVY is reduced in va hosts. Eighty‐six PVY isolates were mechanically inoculated on one susceptible and three va‐derived tobacco cultivars used as indicator hosts to define virulence of these isolates against alleles 0, 1 and 2 of the va gene. Both qualitative and quantitative analyses showed that 55 PVY isolates were able to overcome the three va alleles. Moreover, the monitored biological diversity of PVY isolates was higher in the susceptible tobacco hosts than in the va‐derived ones. This study helps to understand consequences of the deployment of the va gene in tobacco on diversity and virulence of PVY isolates.     

Ornamental plants and thrips populations associated with tomato spotted wilt virus in Greece

A survey was conducted in order to record the ornamental plants that are hosts of tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV) in Greece. Polyclonal antibodies prepared against the N protein of a Greek isolate of TSWV fromGerbera jamesonii (GR-34) were used. Leaf samples were taken from plants showing typical symptoms of tospovirus infection such as chlorotic and necrotic rings on the leaves and malformation and necrosis of the flowers. The samples were tested by double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) using polyclonal antibodies to the N proteins of TSWV and INSV (NL-07). ELIS A-positive samples were mechanically transmitted to plants ofPetunia hybrida, Nicotiana rustica andN. benthamiana to confirm infection. Although none of the samples was found infected with INSV, TSWV presence was recorded in 42 botanical species that belong to 40 genera in 27 families. Among them the speciesBeloperone guttata, Coleus barbatus, Impatiens petersiana andLilium auratum are reported for the first time as hosts of TSWV, whereasBegonia sp.,Catharanthus roseus Celosia cristata, Dianthus chinensis, Fuchsia hybrida andStephanotis floribunda are found as new hosts of the virus in Greece. Thrips collected from TSWV-infected plants were in most cases identified asFrankliniella occidentalis, except from plants ofDendranthema sp. andDianthus caryophyllus whereThrips tabaci individuals were also identified. Different percentages of transmitters were noticed when the thrips populations collected from TSWV-infected ornamental hosts were tested for transmission of TSWV.     

Characterization of fluorescent Pseudomonas spp. associated with roots and soil of two sorghum genotypes

Sorghum is used as bioenergy feedstock, animal feed, and food. Economical methods for disease prevention and control are valuable for producers. Fluorescent Pseudomonas spp. were isolated from sorghum roots and surrounding soil with the goal of finding isolates that significantly inhibited sorghum fungal pathogens. Fluorescent pseudomonads were collected from seedlings of sorghum cultivars RTx433 and Redlan and wheat cultivar Lewjain, grown in two soils. Lewjain is known to support growth of producers of the antibiotic, 2,4-diacetylphloroglucinol (2,4-DAPG). Isolates from all three plants were assessed for hydrogen cyanide (HCN) and extracellular protease production, and for a 2,4-DAPG gene, phlD. Both soil type and plant type affected HCN- and protease-production, but phlD was not affected. Subsets of phlD ⁺ isolates were chosen to determine phlD genotypes and to conduct in vitro inhibition assays against sorghum pathogens. Most isolates from sorghum and wheat were genotype D, previously associated with superior root colonization. phlD ⁺ sorghum isolates were co-cultured with five sorghum pathogens. One isolate from each sorghum line exhibited inhibition to all five pathogens but more Redlan isolates were inhibitory to the virulent pathogen, Fusarium thapsinum, than RTx433 isolates. Nearly all inhibitory isolates from either sorghum cultivar were from one soil type. This is consistent with what had been previously observed in field studies: that soil type played a significant role in determining characteristics of fluorescent Pseudomonas spp. isolated from roots or soil, but sorghum genotype also had a considerable effect.     

Five new hosts of Pseudomonas andropogonis occurring in eastern Australia: host range and characterization of isolates

Pseudomonas andropogonis was recorded for the first time as the cause of lesions on blueberry ( Vaccinium sp.). carob ( Ceratonia siliqua L.), Gypsophila paniculata L. G. elegans Bieb. and statice ( Limonium sinuatum (L.) Mill). Cross-inoculation studies with 13 isolates from 10 host plants showed that sorghum, sweet corn, vetch, carnation and Gypsophila elegans were hosts in common, whereas clovers ( Trifolium repens and T. pratense ) were infected only by isolates from clover, Gypsophila paniculata and vetch. Seventeen isolates subjected to bacteriological characterization tests formed a uniform phenotype. A single isolate from sorghum produced an antimetabolite inhibitor of Escherichia coli B which was reversed by L-glutamine. but not by other amino acids.     

苹果茎沟病毒柑橘碎叶株系的分布与序列分析

为明确苹果茎沟病毒(apple stem grooving virus,ASGV)柑橘碎叶株系(citrus tatter leaf virus,CTLV)在中国柑橘产区的发生分布及其分子特性,于2017—2021年对我国12个柑橘主产区开展系统调查,并采用多重比对以及系统发育分析法对CTLV的全序列进行分析。结果表明,从12个柑橘主产区采集的2 012份疑似样品中检测出413份感CTLV阳性样品。除陕西省外,其余11个柑橘主产区样品中均检测出CTLV,并且柚类样品中CTLV的检出率最高,达到32.31%。对分离获得的22株CTLV毒株和已知的7株CTLV以及5株ASGV毒株进行全序列分析,发现所有34株毒株的核苷酸序列相似性为78.6%～99.8%,且CTLV序列的保守性较高。此外,与其他4株ASGV毒株相比,本研究中22株CTLV毒株与ASGV-MK毒株(GenBank登录号为MZ127820.1)具有更近的亲缘关系。推测CTLV中国毒株间的亲缘关系可能与其采样地和寄主品种有关。     

Variation in Pathogenicity Associated with the Genetic Diversity of Fusarium graminearum

We screened 188 isolates of Fusarium graminearum, which originated from northwest Europe, the USA and Nepal, for genetic diversity using a sequence-characterised amplified region polymorphism (SCAR). On the basis of this analysis, 42 of the 118 isolates were selected for random amplified polymorphic DNA (RAPD) analysis. Three groups were identified, two of which, A and B, contained the isolates from Nepal, and a third, group C, contained the isolates from Europe and the USA. In pathogenicity tests on wheat and maize seedlings, group C isolates were more pathogenic than the group A and B isolates. The isolates were assigned chemotypes based on their ability to produce the trichothecene mycotoxins nivalenol (NIV) and deoxynivalenol (DON). Isolates from group A were equally likely to produce NIV or DON while group B isolates produced predominantly NIV, and group C isolates produced predominantly DON. Within group A, isolates of the two chemotypes were equally pathogenic to wheat but isolates with the NIV chemotype were significantly more pathogenic to maize. The results confirm that distinct genetic groups exist within F. graminearum and demonstrate that these groups have different biological properties, especially with respect to their pathogenicity to two of the most economically important hosts of this pathogen.     

Genetic Structure of the Population of Phytophthora infestans Attacking Solanum ochranthum in the Highlands of Ecuador

Thirty-nine isolates of Phytophthora infestans were collected from the wild host Solanum ochranthum in the highland tropics of Ecuador and characterized with a set of phenotypic and molecular markers (mating type, metalaxyl sensitivity, the allozyme loci Gpi, and Pep, mitochondrial DNA haplotype, RFLP, and SSR), as well as for pathogenicity on various hosts. Three groups of isolates (A, B, and C) were identified based on their multilocus genotypes and variable abilities to cause disease on different hosts. Group A had a combination of alleles for the Gpi (86/100), Pep (96/100) and mtDNA (Ia) loci, as well as an RFLP fingerprint, that have not been reported for P. infestans in Ecuador, or elsewhere. Group B shares many marker characteristics with the US-1 lineage described in Ecuador on tomato, pear melon (S. muricatum), and S. caripense, but has SSR alleles not present in typical US-1 isolates. Group C for all markers tested is identical to the EC-1 lineage described on cultivated and wild potatoes in Ecuador. All isolates from S. ochranthum were able to re-infect their host of origin in the detached leaf assay; however, we did not draw clear conclusions as to the relative aggressiveness of the three groups on this host. Isolates of group A were the most specialized and were generally non-pathogenic or weakly pathogenic on all hosts other than S. ochranthum. Groups B and C infected tuber-bearing hosts, including the cultivated potato but were generally non-pathogenic on other non-tuber bearing hosts. Solanum ochranthum was infected by isolates coming from tuber-bearing Solanum hosts (i.e., the EC-1 lineage of P. infestans) and some US-1 isolates from non-tuber bearing hosts. Thus, in nature this species might be a potential reservoir of inoculum of different pathogen populations able to infect the cultivated hosts potato, tomato and pear melon (S.␣muricatum). Unlike potato and tomato in Ecuador, each of which is primarily attacked by a highly specialized pathogen population, S. ochranthum appears to harbour at least three pathogen groups of␣different genetic make-up. The unresolved issue of potential host specificity in isolates found on S.␣ochranthum could complicate efforts to use this species in tomato improvement.     

Identification of bacterial leaf streak of cereals by their phenotypic characteristics and host range in Iran

Forty-four bacterial isolates were obtained from infected wheat, barley and various grasses from different regions of Iran. All isolates were bacteriologically similar toXanthomonas campestris and some of their physiological and biochemical features can be useful for a primary differentiation between them. Depending on their pathogenicity, the isolates were split into two groups; the wheat group isolated from wheat, barley and grasses could infect artificially wheat, barley, rye,Agropyron elongatum, Bromus inermis, andLolium multiflorum but not oat, whereas the barley group obtained from cultivated or wild barley was pathogenic to barley only. From their bacteriological characteristics and host range, the barley and the wheat group isolated were identified asX. campestris pvs.hordei andcerealis, respectively.Aegilops sp.,Sclerochloa dura, andHeteranthelium sp. were, for the first time, shown to be hosts ofX. c. pv.cerealis.