Assessing resistance of rubber tree clones to <Emphasis Type="Italic">Microcyclus ulei</Emphasis> in large-scale clone trials in Ecuador: a less time-consuming field method

Resistance of rubber tree clones to South American Leaf Blight (SALB) caused by the fungus Microcyclus ulei is normally assessed in specific large-scale clone trials, which in general entail a considerable amount of work. Four variables are observed monthly on each tree over many years: disease severity and conidial sporulation intensity on young leaves, and severity and stroma density on mature leaves. In order to simplify this field assessment method, we tested the resistance of eight rubber tree clones to M. ulei in Ecuador in a Fisher block design with four replicates per treatment. Three months after planting, monthly observations were made for a period of 12 months on the foliage focusing on the four variables, in order to quantify disease development. Given the correlations between the four variables, assessment of conidial sporulation intensity on young leaves and stroma density on mature leaves should be sufficient. The most suitable period to start the assessment was 6 months after planting, for a duration of six to nine months. As repeated observations on the same trees were autocorrelated, it was possible to reduce the assessment frequency to once every 2 months. To conclude, assessing the resistance of rubber tree clones to SALB in large-scale clone trials can be optimized to reduce the number of observation times by 50%.     

Influence of host resistance and phenology on South American leaf blight of the rubber tree with special consideration of temporal dynamics

South American leaf blight (SALB), the most dangerous disease of the rubber tree, is responsible for the lack of significant natural rubber production in South America and is a major threat to rubber tree plantations in Asia and Africa. Although the selection of resistant clones is the preferred disease control method, greater knowledge is required of the relationship between host and pathogen, in order to construct more durable resistance. Based on small-scale trials, this study set out to compare the dynamics of SALB on two highly susceptible and one moderately susceptible clone and to analyse the effect of host phenology on disease severity, at leaflet and flush scales. Clonal resistance was found to have a noticeable effect on disease severity, asexual sporulation and stromatal density at both leaflet and flush levels, and on disease dynamics at a leaflet level; time for symptom and sporulation appearance were longer on the moderately susceptible clone than on the susceptible clones. On the moderately susceptible clone, the stromatal density was largely dependent on disease severity. The phenology did not differ among the three clones and could not be considered as a factor in genetic resistance to SALB. However, for the three clones, the position of the leaflet in the flush affected the duration of the immature stages and the disease: the shorter the duration of leaflet development, the lower the disease severity, the sporulation intensity and the stromatal density.     

橡胶树重要品系对胶孢炭疽菌抗性评价

橡胶树抗性种质的鉴定与利用是控制橡胶树炭疽病的一项重要措施。采用室内离体橡胶叶接种法、田间种质圃人工接种和自然发病条件下定期田间病害调查方法, 2011-2012年在海南省儋州开展46份重要橡胶树种质对胶孢炭疽菌病抗性鉴定和评价。把抗炭疽病程度划分为高度抗病(HR)、抗病(R)、中度感病(MS)、感病(S)和高度感病(HS)共5个级别,根据抗性评价结果分析得出：高度抗病(HR)类型的0份;抗病(R)类型8份, 占17.39%; 中度感病(MS)类型19份,占41.30%;感病(S)类型16份,占34.78%;高度感病(HS)类型3 份,占6.52%。筛选出的抗病品种,是今后可利用的抗性种质资源。本研究结果为炭疽病抗性种质资源的利用、培育及病害管理提供参考数据。     

Validation and use of a qPCR protocol to quantify the spread of Ralstonia solanacearum in susceptible and resistant eucalypt plants

Bacterial wilt caused by Ralstonia solanacearum is a serious disease of eucalypt in humid and high temperature areas worldwide. Spreading of the bacterium in the field or to other nurseries occurs mainly by symptomless infected plant material. The use of pathogen-free propagating material as well as planting of resistant genotypes are currently the only strategies used for disease control. Therefore, a reliable and sensitive method for detection of low titres of R. solanacearum in infected plant tissue is essential for the success of management programmes. In this work, we adapted an efficient intercalating dye-based real-time PCR protocol to detect the bacterium in symptomless eucalypt plants as well as to investigate its movement in eucalypt clones CLR172 and CLR371, which exhibit resistant and susceptible phenotypes, respectively. We found that the bacterium translocates acropetally and basipetally in inoculated but symptomless cuttings of the resistant clone, as in cuttings of the susceptible clone displaying symptoms. Nevertheless, a smaller concentration of bacterial DNA was detected in tissues of the resistant clone. Mature biofilms occluding the xylem vessels were present in the susceptible clone whereas only single cells or small aggregates were observed in the resistant clone. This work contributes to improve our knowledge of the colonization process of R. solanacearum in eucalypt clones with different levels of susceptibility and to understand how the defence mechanisms against bacterial wilt in Eucalyptus work. Our findings could aid in the selection of the most resistant eucalypt clones to be used in wilt disease management programmes.     

Induction of peroxidase, scopoletin, phenolic compounds and resistance in Hevea brasiliensis by elicitin and a novel protein elicitor purified from Phytophthora palmivora

Elicitin and a new protein 75 kDa elicitor were purified from the culture filtrate of Phytophthora palmivora, a pathogen of Hevea brasiliensis (rubber plant). Elicitin was obtained by using a one step of DEAE cellulose chromatography and the new elicitor was obtained by two steps of chromatography: a DEAE cellulose column followed by a hydrophobic column. Both elicitors were stable to heat and a wide range of pH values, but were sensitive to ProteaseK. Both elicitors induced scopoletin, peroxidase isozymes (with substrate o-dianisidine and scopoletin) and total phenolic compounds in cell suspension of H. brasiliensis with similar kinetics. In addition, both elicitors induced peroxidase enzyme (o-dianisidine), total phenolic compounds and enhanced local resistance against P. palmivora on young rubber tree seedlings. However, the increase of peroxidase enzyme and total phenolic compounds in rubber tree seedlings was different from those in cell suspension. Furthermore, during the expression of local resistance the zoospore of P. palmivora induced the peroxidase enzyme (o-dianisidine) more rapidly and with higher level than the control plants. H. brasiliensis is more responsive to the new elicitor than elicitin in triggering defense responses. That is the new elicitor was active at a concentration lower than those required for elicitin, about a 30-fold decrease for activation defense responses in cell suspension. For induction of peroxidase enzyme (o-dianisidine), phenolic compounds and local resistance of rubber plants against P. palmivora, the 75 kDa protein was active at about a 2-fold lower concentration when compared to elicitin.     

Single-chain variable fragments antibody specific to <Emphasis Type="Italic">Corynespora cassiicola</Emphasis> toxin,cassiicolin, reduces necrotic lesion formation in <Emphasis Type="Italic">Hevea brasiliensis</Emphasis>

Corynespora leaf disease poses a serious threat to rubber cultivation because infected leaves develop necrotic lesions and abscise, leaving the tree unproductive. The destructiveness of Corynespora cassiicola has been largely attributed to cassiicolin, a protein toxin secreted by the fungus. Recombinant antibody technology offers hope to curtail the disease whereby single-chain variable fragments (scFv) specific to cassiicolin could bind and deactivate the toxin in genetically modified rubber trees that harbour the antibody gene. A scFv phage library was constructed from heavy and light variable chains of IgG from cassiicolin immunized Balb/C mice spleen. Biopanning of the phage library yielded a scFv clone with high specificity to cassiicolin. The nucleotide sequence and deduced amino acid sequence information of the scFv were obtained. Hemagglutinin (HA)-tagged scFv expressed in Escherichia coli is discerned as a band at ca. 30 kDa on SDS-PAGE, and the corresponding band was detected by anti-HA IgG on a Western immunoblot. Deactivation of cassiicolin by the affinity-purified scFv was demonstrated in a detached-leaf bio-assay on selected susceptible Hevea clones (PB 260, RRIM 2020, RRIM 901 and RRIM 929). The assay was also performed on clones that are relatively more resistant to the fungus (RRIM 600 and GT-1), and the results were as expected. Thus, we have successfully demonstrated that the cassicolin-specific scFv can effectively reduce cassicolin toxicity.     

Pathogenicity of Phragmidium violaceum isolates on European blackberry clones and on leaves of different ages

Phragmidium violaceum (Pv) isolate F15 was released for biological control of invasive European blackberry in Australia in 1991–1992. This study reports variation in the pathogenicity of isolate F15 and Pv isolates SA1, V1 and V2, collected in southeastern Australia in 1997 or 1998, on a broad sample of genetically variable European blackberry clones collected in 1996–2000. The use of whole shoots or detached leaflet discs led to the same conclusions about the relative susceptibility of a blackberry clone to a particular Pv isolate. Moderate to relatively high numbers of uredinia developed on all 25 blackberry clones inoculated with isolate V1, whereas no or few uredinia of isolates V2 and SA1 developed on Rubus erythrops clone EB20, three clones of Rubus leucostachys (EB9, EB19 and 960804) or Rubus sp. clone SR43. Pv isolate F15 differed from the three isolates collected in Australia in that no or few uredinia of F15 developed on Rubus polyanthemus clone 961107 or three clones of Rubus laciniatus (EB22, KE1, SR14). Isolate F15 also differed from isolates V1 and V2 for the density of uredinia as a function of leaf age in a clone of Rubus anglocandicans found widely in Australia. Isolates V1 and V2 ceased production of uredinia on leaves older than 16 days, whereas Pv isolate F15 continued production of uredinia beyond this leaf age. This study informed selection of additional Pv isolates from Europe for release in Australia in 2004.     

Role of ascospores and conidia in the initiation and spread of South American leaf blight in a rubber tree plantation

Microcyclus ulei, the fungus causing South American leaf blight (SALB) on rubber tree leaves, produces two main types of spores: ascospores and conidia. To assess their respective epidemiological role, a field experiment was conducted in French Guiana over 3 years. Tree phenology, disease severity and climate variables were recorded while airborne spores were trapped and quantified. Ascospores were shown to play an essential role in the perpetuation of the disease outside the host's growth periods, in the resumption of epidemics, and in the spread of the disease to disease‐free zones. Conidia were trapped in visibly infected plots only, during periods of host growth. Disseminated over short distances and present only temporarily on leaves, the conidia enabled the disease to spread stepwise when the climate was conducive. Segmentation analysis revealed that the duration of high relative humidity was the climatic variable most related to ascospore trapping. Ascospore release did not require low temperatures. Considering the essential role of the ascospores in the initiation and spread of disease, artificial defoliation as a means of reducing the inoculum pressure during tree refoliation is proposed to control SALB. To check the validity of this method, a survey over several years of natural defoliation–refoliation in relation to climate and other leaf diseases is needed.     

A biochemical mechanism of resistance to pirimicarb in two glasshouse clones of Aphis gossypii

The susceptibility of three clones of Aphis gossypii Glover to 15 insecticides was established by bioassay. A high level of resistance towards pirimicarb was confirmed for a clone from Holland (Dutch R) and a clone from Japan (Jap R), while the susceptible clone (S) was killed by very low doses of the insecticide. However, only limited cross-resistance was shown towards other carbamates and organophosphates, and no marked resistance to the pyrethroids tested. The acetylcholinesterase (AChE) of both resistant clones hydrolysed acetyl-choline faster than that of susceptible aphids, with greatest enzyme activity shown by the Dutch R clone. Inter-clone differences in these rates were consistent with differences in catalytic centre activities. Inhibition (I₅₀) of AChE by pirimicarb was approximately 900-fold higher for the resistant clones than for the S clone. First-order kinetics revealed that resistance to pirimicarb in Dutch R and Jap R involved a modified AChE which had a reduced (approximately 350-fold) affinity (K_d) for pirimicarb. The marked change in AChE affinity for pirimicarb was not repeated with the other carbamates tested, ethiofencarb and aldicarb. It was considered that the resistant aphids would not require mechanisms in addition to insensitive AChE in order to show the high level of resistance to pirimicarb shown in the bioassay.     

Segmentation applied to weather-disease relationships in South American leaf blight of the rubber tree

South American leaf blight (SALB) is a severe threat to world rubber production. One way of controlling it is to set up plantations in zones not conducive to the disease. Such zones are known once a plantation has been set up, but few data are available on how climate affects the disease, especially in the Amazon region. With better knowledge of conditions that are favourable to SALB epidemics it would be possible to more accurately identify risk zones in Asia and Africa, continents that are still SALB-free. Based on a trial design involving detailed and frequent observations, and with a method rarely used in plant epidemiology, the segmentation method, the results presented in this article make it possible to list, in order of importance, climatic factors that influence disease severity under conditions where the climate varies little over the year.     

Assessment of resistance to <Emphasis Type="Italic">Ceratocystis cacaofunesta</Emphasis> in cacao genotypes

Ceratocystis wilt of cacao (caused by Ceratocystis cacaofunesta) is a dangerous disease and results in the death of the plant. This fungus was recently identified in the major cacao-producing regions of Brazil, and was observed to be more aggressive than isolates from other geographical locations. The objective of this study was to develop and test a consistent method to assess cacao genotype response to C. cacaofunesta, based on young plants (seedlings or cuttings). The fungus was inoculated by the deposition of propagule suspensions on cut stems. The parameters to assess disease progress were (a) disease incidence, (b) differences in mortality between the most contrasting cacao genotypes for resistance and susceptibility, (c) disease index, (d) consistency of response over time and (e) relative lesion heights. When seedlings were used for the analyses, the ICS-1 and TSH-1188 genotypes proved to be useful as genetic standards for susceptibility and resistance to C. cacaofunesta, respectively. Inoculum concentrations between 10⁴ and 10⁵ propagules ml⁻¹ and the moment at which the disease incidence stabilized provided appropriate conditions for genotypic comparison. When ten cacao genotypes propagated by cuttings (clones) were assessed, the results confirmed TSH-1188 as the reference genotype for resistance to C. cacaofunesta, while the remaining clones could be grouped as resistant (CEPEC-2008), moderately resistant (CEPEC-2002, CEPEC-2007) and susceptible (CEPEC-2009, CCN-10, CCN-51, HW-25, PH-16, SJ-02). The analytical concepts and results were discussed in terms of their application in breeding programmes aimed at developing genetic resistance to Ceratocystis wilt of cacao.     

Identification of the anthracnose fungus of walnut (Juglans spp.) and resistance evaluation through physiological responses of resistant vs. susceptible hosts

Walnut anthracnose is one of the most serious diseases of walnut globally. In this study, the pathogen was identified as Colletotrichum gloeosporioides by morphological and molecular approaches. Then, the resistance of 18 walnut genotypes was evaluated by field investigation as well as field and detached leaflet inoculation. A field inoculation assay was used as a benchmark for comparing field evaluation and the detached leaflet inoculation assay. Seven genotypes, Panhe1 and clones 71 and 200 of Juglans sigillata, Yanyuanzao and Qingxiang of J. regia, and Shujiang1 and Chuanzao1 of J. regia × J. sigillata, displayed high resistance in all assays, suggesting that they may be good candidates for further evaluation. Subsequently, two genotypes (Panhe1 with high resistance, and clone 199 with high susceptibility) were selected to analyse the different physiological responses between highly resistant and susceptible genotypes after inoculation. C. gloeosporioides infection induced the activities of superoxide dismutase (SOD), peroxidase (POD), polyphenol oxidase (PPO), and chitinase; they were all significantly increased in both Panhe1 and clone 199. However, the POD activity and its change rate in Panhe1 were significantly higher than those in clone 199. The change rates of PPO activity, and total phenol and proline content in Panhe1 were significantly higher than those in clone 199. Moreover, the change rates of SOD activity, malondialdehyde, and soluble sugar content in clone 199 were significantly higher than those in Panhe1. These findings not only enhance our understanding of the resistance of walnut to anthracnose, but also lay the foundation for breeding anthracnose-resistant walnuts in the future.     

The significance of guttation in the secondary spread of Clavibacter michiganensis subsp. michiganensis in tomato greenhouses

Bacterial canker, caused by Clavibacter michiganensis subsp. michiganensis (Cmm), can spread in commercial tomato greenhouses causing epidemics. Results of greenhouse experiments with Cmm‐contaminated tools demonstrated disease spread for only a limited distance (<4 plants) from infected plants. However, touching symptomless infected plants bearing guttation droplets prior to touching nearby plants spread the pathogen over longer distances within rows (>22 plants). The pathogen was exuded in large numbers in the guttation fluid of infected plants; its presence in the guttation fluid was not influenced by the inoculation procedures, leaf age or the volume of the guttation droplets. Population size of Cmm and the incidence of leaflets with epiphytic bacteria were significantly higher in plants placed in a guttation‐induction chamber than in those kept in a growth chamber with high humidity, suggesting exudation through guttation contributed to the formation of epiphytic populations on leaflets. This new knowledge may provide a simple and environmentally friendly means for decreasing the spread of the disease by avoiding contact with plants during periods when they bear guttation droplets.     

Diseases of Rubber and their Control

Abstract

Most of the major diseases of Hevea brasiliensis are of worldwide distribution (with the notable exception of South American leaf blight, against which strict quarantine regulations are enforced by rubber-growing countries outside the Americas to prevent the unauthorised import of Hevea) but their local severity and importance vary from one region to another. Root diseases are a serious problem nearly everywhere and regular rounds of inspection and treatment are essential while the trees are still young in order to prevent serious losses. The recently introduced prophylactic collar protectant dressings specific to each of the three main diseases are valuable aids to control.

Diseases of the tapping panel may prevent tapping or hinder bark regeneration; black stripe in particular (Phytophthora palmivora) can be very persistent and is often eradicated only by repeated therapeutic fungicidal treatment of the bark. Stem diseases, chiefly pink disease (Corticium salmonicolor), are important in certain areas in wet weather, and can lead to severe damage and even dieback in the absence of effective treatment.

Four major leaf diseases can have a debilitating effect on the tree. Abnormal leaf fall (Phytophthora spp.) is severe in India, causing the abscission of mature leaves during the monsoon rains; treatment necessitates the application of a pre-monsoon prophylactic copper spray. Secondary leaf fall, caused both by Oidium heveae and Colletotrichum gloeosporioides which infect the flushes of new leaves produced after the annual leaf change (wintering), varies greatly in severity according to local weather and cultivar. It can be controlled by repeated rounds of an appropriate prophylactic fungicide or, under certain conditions, by artificially hastening the onset of wintering (and thus of refoliation) by removing the old leaves with a contact herbicide. By far the most important leaf disease is South American leaf blight (Microcyclus ulei); it is largely responsible for the lack of a vigorous rubber industry in South America, the home of the rubber tree. It can only be effectively countered by the use of resistant cultivars.

The economic importance of the above diseases is discussed and current control measures described.     

P450-mediated resistance in Myzus persicae (Sulzer) (Hemiptera: Aphididae) reduces the efficacy of neonicotinoid seed treatments in Brassica napus

BACKGROUND

The prophylactic use of seeds treated with neonicotinoid insecticides remains an important means of controlling aphid pests in canola (Brassica napus) crops in many countries. Yet, one of the most economically important aphid species worldwide, the peach potato aphid (Myzus persicae), has evolved mechanisms which confer resistance to neonicotinoids, including amplification of the cytochrome P450 gene, CYP6CY3. While CYP6CY3 amplification has been associated with low-level resistance to several neonicotinoids in laboratory acute toxicity bioassays, its impact on insecticide efficacy in the field remains unresolved. In this study, we investigated the impact of CYP6CY3 amplification on the ability of M. persicae to survive neonicotinoid exposure under laboratory and semi-field conditions.

RESULTS

Three M. persicae clones, possessing different copy numbers of CYP6CY3, were shown to respond differently when exposed to the neonicotinoids, imidacloprid and thiamethoxam, in laboratory bioassays. Two clones, EastNaernup209 and Osborne171, displayed low levels of resistance (3–20-fold), which is consistent with previous studies. However, in a large-scale semi-field trial, both clones showed a surprising ability to survive and reproduce on B. napus seedlings grown from commercial rates of neonicotinoid-treated seed. In contrast, an insecticide-susceptible clone, of wild-type CYP6CY3 copy number, was unable to survive on seedlings treated in the same manner.

CONCLUSION

Our findings suggest that amplification of CYP6CY3 in M. persicae clones substantially impairs the efficacy of neonicotinoid seed treatments when applied to B. napus. These findings highlight the potentially important real-world implications of resistances typically considered to be ‘low level’ as defined through laboratory bioassays. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

Intraspecific variation in susceptibility to dothistroma needle blight within native Scottish Pinus sylvestris

Dothistroma needle blight (DNB), caused by Dothistroma septosporum, is the most important disease currently affecting pine plantations in Britain. Intraspecific variation in susceptibility to DNB has been observed in several pine species, but it is not clear if similar variation occurs in Pinus sylvestris (Scots pine), Britain's only native pine. In three separate experiments 2‐ and 3‐year‐old Scots pine saplings from six native Scottish populations were artificially inoculated with D. septosporum conidial suspensions and incubated under conditions optimal for disease development. Conidial suspensions were produced using a single isolate from northeast Scotland. In one experiment, plants were also treated with various spore suspension concentrations to assess the impact of inoculum load on disease severity. There were no significant interactions between host population, plant height, and experiment/inoculum load (anova , P > 0·05), but population, height and inoculum load all significantly affected disease severity (anova , P < 0·05). Among the 2‐year‐old trees, those from Amat were less susceptible than those from Glen Loyne and Glen Cannich (anova , P < 0·05). Among the 3‐year‐old trees, those from Beinn Eighe were less susceptible than those from Abernethy. Plant height and DNB susceptibility had a slightly negative relationship. The use of a spore suspension with a concentration of 1·6 × 10⁶ spores mL^?1 was optimum for disease development. In an in vitro experiment, production of conidia was greater when cultures were incubated in darkness. This paper is the first to report intraspecific variation in DNB susceptibility within Scots pine.     

Further studies on the genetics of the carboxylesterase regulatory system involved in resistance to organophosphorus insecticides in Myzus persicae (Sulzer)

Levels of carboxylesterase activity in F₁ clones of Myzus persicae, obtained by crossing sexuales from a resistant, high esterase clone with those from a susceptible, low esterase clone, fell into two distinct groups intermediate between the levels of carboxylesterase in the parent clones. When sexuales of F₁ clones of the lower of these two intermediate levels of carboxylesterase activity were crossed, segregation ratios in the F₂ generation indicated that this lower intermediate activity level (about 0.4 μmol mg^?1 h^?1). which is about twice the level in susceptible clones, is due to mutation at a single regulatory locus. The results obtained with backcrosses, between sexuales of an F₁ clone having the higher intermediate level of carboxylesterase activity and a parent susceptible, low esterase clone, suggest that a second locus may be involved in the expression of higher levels of esterase activity.     

Fusariosis in rubber tree: pathogenic,morphological, and molecular characterization of the causal agent

Fusariosis, one of the phytosanitary problems found in rubber producing areas in the northwest of the state of São Paulo, is a disease that affects the bark of the adult plants, affecting the exploitation of latex. The typical symptoms appear as cracks in the bark that expand from the rootstock towards the tapping panel, causing a drying of the latex flux in the injured region, impeding latex tapping. Due to the recent incidence of this disease in rubber plantations, the goal of this study was to characterize the Fusarium associated with symptomatic rubber tissue in three different locations in the state of São Paulo. In order to identify Fusarium species, pathogenicity, morphological, cultural and molecular studies were carried out. A total of 51 isolates were obtained and separated into three groups based on macroconidium morphology, presence or absence of sporodochia, types of chlamydospores, formation of phialides and conidiogenesis of microconidia and mesoconidia, mycelial growth rate and coloring of the colonies. These groups were corroborated using DNA sequence information for five different genetic loci, and were subsequently recognized as Fusarium oxysporum, F. incarnatum and F. decemcellulare. Our results further showed that all 51 of the Fusarium isolates recovered were pathogenic to rubber tree seedlings of RRIM 600 standard clone. This study also reports for the presence of F. oxysporum and F. incarnatum in rubber plantations in the state of São Paulo and in Brazil.

     

利用MaxEnt预测橡胶南美叶疫病菌在全球的潜在地理分布

橡胶南美叶疫病菌在美洲地区引起橡胶树严重的落叶病害,是亚太地区国家重要的检疫性有害生物。本文在MaxEnt预测全球范围内气候条件适宜该菌存活区域的基础上,结合寄主分布推测该菌的潜在地理分布。预测结果显示,在54个天然橡胶种植国家或地区中,除了该菌目前分布国家或地区、几内亚比绍和马里,其余33个国家或地区气候适宜值大于10的区域为该菌的潜在地理分布区,加强这些区域的调查监测对保护当地橡胶种植有重要意义。     

Intra-specific variation and inheritance of BYDV-PAV transmission in the aphid <Emphasis Type="Italic">Sitobion avenae</Emphasis>

Vector efficiency of 44 clonal lines (clones) of Sitobion avenae belonging to 31 different genotypes (distinct patterns for five microsatellite loci) originating from Western France was evaluated by transmitting the isolate PAV4 of BYDV-PAV to barley seedlings. Variation in transmission rates from 3.7% to 92.5% was observed, with significant effects of the aphid clone, of the plant species on which clones were collected, and of the reproductive mode of the clones. When genotypes are considered instead of clones, a continuum in transmission rates was observed. A subset of S. avenae clones was tested for transmission of one (10 clones) and 13 (4 clones) other BYDV-PAV isolates, and a clear clone effect modulated by an isolate effect was observed. Crosses were made between clones with different vectoring phenotypes and their F1 progeny were tested for PAV4 transmission. The narrow sense heritability of the PAV transmission character was rather high in the F1 families (h²=0.5) and the segregation analyses suggested an oligo/polygenic determinism of this character. The possibility of generating new transmission variants by sexual reproduction and its consequences on transmission mechanism studies and on BYD epidemics are discussed.