Asexual Genetic Exchange in the Barley Pathogen Rhynchosporium secalis

ABSTRACT The causal agent of barley scald, Rhynchosporium secalis, is a haploid anamorphic ascomycete with no known sexual stage. Nevertheless, a high degree of genetic variation has been observed in fungal populations on commercial barley cultivars and parasexuality has been suggested to contribute to this variation. In order to test whether asexual genetic exchange can occur, isolates of R. secalis were transformed to hygromycin B resistance or phleomycin resistance. Mixtures of transformants were co-inoculated either on agar or in planta and screened for the occurrence of dual-antibiotic-resistant colonies. No dual-antibiotic-resistant colonies resulted from mixing transformants of different fungal isolates. In contrast, with transformants originating from the same fungal isolate, asexual exchange of markers was demonstrated on agar plates and in planta. This is the first definitive evidence of asexual genetic exchange in R. secalis.     

Analyzing and Modeling Temporal Disease Progress of Barley yellow dwarf virus Serotypes in Barley Fields

ABSTRACT Population dynamics of Padi avenae (PAV), Macrosiphum avenae (MAV), and Rhopalosiphum padi (RPV) virus serotypes of Barley yellow dwarf virus (BYDV) and of their main aphid vectors were studied in winter barley (Hordeum vulgare) fields for three successive years in western France. An epidemiological model of the spread of viruses in the field was developed based on vector populations as forcing variables and the population dynamics of each virus serotype. This model accurately simulated the kinetics of the epidemic for PAV serotypes, which are the most common ones. For RPV and to some extent for MAV, the results were less satisfactory. The occurrence and spread of PAV and MAV serotypes in the field was clearly and easily related to that of their main vector species. Conversely, the spread of RPV serotypes showed no consistent relationships with the dynamics of their vectors. Incidence of PAV in 1989 to 1990 and 1990 to 1991 showed a bimodal distribution, with maximums in fall (December) and spring (May) that were linked to fall infestations by R. padi and spring infestations by three (R. padi, Sitobion avenae, and Metopolophium dirhodum) or two (S. avenae and M. dirhodum) aphid species. In 1991 to 1992, the PAV infection curve was monomodal and mainly due to a primary spread of the virus by very large populations of alate R. padi. MAV incidence was low in fall and winter and reached a maximum in spring 1990 and 1991 related to the occurrence of S. avenae and M. dirhodum. RPV incidence was low every year, despite the abundance of its vector, R. padi. Mixed infections were more frequent than expected by chance and were assumed to be partly related to heterologous encapsidation. The occurrence of each serotype is discussed in relation to the time of crop infection and possible damage.     

大麦黄矮病毒-GAV在燕麦植株体内运动规律的初步研究

 利用RT-PCR方法研究了大麦黄矮病毒-GAV在燕麦植株内的移动规律。先将介体麦二叉蚜(Schizaphis graminum)在BYDV-GAV新鲜病叶上饲毒,再将获毒蚜虫放置到二叶期的健康燕麦植株接种48h,随后分期提取接种植株的第1~6片叶和根组织的总RNA,利用特异引物扩增BYDV-GAV的外壳蛋白(CP)基因以检测病毒在燕麦植株内的复制和移动。结果表明,在接种5d后,接种叶片(第2片叶)呈现阳性,接种7d后,植株新生的第4片叶被侵染,接种9d后,部分的第3片叶呈现阳性,至接种16d,几乎所有的叶片均呈现阳性。仅在接种的第5、7和9d收集的根组织呈现阳性,而所有的第1片叶均为阴性,可能是由于这些组织内病毒含量太低所致。本研究初步揭示了BYDV-GAV长距离运动的规律并且发现该病毒在燕麦根部从接种到系统发病都没有进行大量增殖,为今后进一步研究病毒运动机制选取适当的植物材料提供了基本信息。     

Weed Control in Wheat and Barley in the Middle East

Abstract

Wheat and barley constitute the bulk of agricultural production in the Middle East. Unfortunately, these crops are grown on marginal soils which are infested with annual and perennial weeds. The principal weed species include Avena sterilis L., Convolvulus arvensis L., and Sinapis arvensis L. and it has been estimated that yield losses due to weeds are between 20 and 30%. Control is mostly by hand pulling, hoeing or mechanical tillage, the use of herbicides being limited to about 2–5%, of the cereal area. The chemical used is usually 2,4-D but there are a few applications of barban. With the increase in wages, and unavailability of agricultural labour it is suggested that herbicide application should be widely used in the Middle East. Mixtures with benzonitriles and dicamba should be used in addition to the economical phenoxy herbicides to widen the spectrum of control of broadleaved weeds. Glyphosate may also be used after grain harvest for the control of perennial weeds and tri-allate, chlortoluron and other chemicals for the control of wild oats and other grasses. Plant breeders should study the genetic response of newly developed varieties to the most widely used herbicides.     

Barley seed vigour and mechanical weed control

Two field experiments investigated the influences of crop seed vigour on the effect of weed harrowing and crop:weed interactions in spring barley. Artificially reduced seed vigour, which was similar to the variation within commercial seed lots, caused a reduction in germination rate, delayed time of emergence and, consequently, caused reduced competitive ability against weeds. During both years, the reduced seed vigour increased the average weed biomass by 169% and 210%, and reduced the average crop yield by 16% and 21%. Without the influence of weeds, the yield reduction was estimated to be 8% and 10%. A three‐times harrowing strategy reduced the weed biomass by 75% and 72% on average. However, it also caused damage to the crop and reduced yield. There was no clear interaction between barley seed vigour and weed harrowing in the experiments but, in one year, reduced seed vigour tended to decrease the effect of weed harrowing and also increased crop damage. Results in both years, however, indicate potential possibilities for successful integrated weed control by adding the use of high seed quality to a weed harrowing strategy.