Impact of proquinazid on appressorial development of the barley powdery mildew fungus Blumeria graminis f.sp hordei

The effect of a novel mildew-specific quinazolinone fungicide, proquinazid, on the barley powdery mildew fungus, Blumeria graminis f.sp. hordei, has been studied using scanning electron microscopy and quantitative RT-PCR. Proquinazid has previously been shown to perturb conidial morphogenesis, similar to quinoxyfen, a currently widely used mildewicide. In this study, we confirm an effect of proquinazid on appressorial differentiation. By comparison to quinoxyfen, however, proquinazid affects this highly coordinated process differently, with more deformed appressorial germ tubes observed, often growing away from the leaf surface. Comparison of the expression of genes involved in the transduction of signals directing conidial development has also suggested differences in the affects of proquinazid and quinoxyfen. In particular, the expression of the Ras-type GTPase activating gene, previously implicated in quinoxyfen resistance, is distinctly affected by proquinazid treatment at time points critical to normal conidial morphogenesis. Together, these data indicate differences in the mechanisms by which proquinazid perturbs appressorial differentiation in comparison with quinoxyfen.     

Virulence phenotypes of Blumeria graminis f. sp. hordei in South Africa

Virulence analysis of 224 isolates of Blumeria graminis f. sp. hordei (barley powdery mildew) from South Africa was performed. The isolates were collected from eight fields and a greenhouse in 2004 and 2007. The isolates were tested for virulence on a set of 20 differential varieties. All isolates were virulent on the resistance genes Mla8 and Ml(Ch) and avirulent for the resistance genes Mla3, Mla6, Mla7, Mla9, Mla13, Mla23, Mlp1 and MlaN81. Virulence frequencies of field isolates for the resistance genes Mla12 + MlaEm2, Mlat, Mla22, Mlk1 and Mlh were 52.9–99.5 % and for Mla1 + MlaAl2, MlLa, Mlra, Mlg + MlCP and Ml(Ru2) were 0.5–23.5 %. In total, 46 pathotypes were detected in the field and seven other pathotypes in the greenhouse. Only nine pathotypes were found in both years, but they included 61.8 % of the isolates. The predominant pathotype represented 15.9 % of the isolates, and was the only one common to all three field populations. The average relative virulence complexity per field isolate increased from 0.405 in 2004 to 0.486 in 2007. Two powdery mildew metapopulations in geographically distant and separated areas (North West and Western Cape) were deduced. The South African population of Blumeria graminis f. sp. hordei had unique virulence frequencies and virulence associations when compared to populations from other parts of the world.     

Relative tolerances of wild and cultivated barleys to infection by Blumeria graminis f.sp. hordei (Syn. Erysiphe graminis f.sp. hordei). I. The effects of infection on growth and development

The two lines of wild barley, B19909 and I-17-40 and the cultivated barley, cv. Prisma used in this investigation were found to be the most susceptible to infection of 25 wild and four cultivated barley lines when exposed to the local population of Blumeria graminis f.sp. hordei. All three lines were susceptible during the early stages of growth but expressed some level of adult plant resistance although this level of resistance was significantly higher in line I-17-40 than in either of the other two.The relative tolerances of the lines to the mildew were determined by comparing the effects of infection on their growth and development in growth cabinet experiments. Mildew developed more slowly on line I-17-40 than on the other two lines and by the third week after inoculation, when mildew cover on B19909 and cv. Prisma had reached about 27%, only about 15% of the green leaf area of line I-17-40 was covered. Mildew continued to increase on line B19909 and cv. Prisma so that 6 weeks after inoculation it covered 40% of their leaf-blades. On line I-17-40 30% of the green leaf area was colonised by 4 weeks after inoculation but because of adult plant resistance coupled with the loss of the earlier infected leaves through senescence mildew cover then reduced falling to 15% by 6 weeks. Although total mildew biomass, measured as conidial production was higher on line B19909 than on cv. Prisma all its growth parameters were reduced less indicating that it was the more tolerant line. Conidial production on the lower susceptible leaves of line I-17-40 was slightly lower than on cv. Prisma yet the reaction of these leaves to infection was the same on both lines indicating that tissues of I-17-40 were slightly less tolerant than those of the cultivated barley. However, during the later stages of growth when its upper leaves expressed high levels of ‘adult plant resistance’ dry matter production in this line increased to levels higher even than in the controls. This capacity for compensatory photosynthesis ensured that by the end of growth few differences in any of the measured growth parameters between infected and uninfected plants of line I-17-40 were significant.The greater tolerance of line B19909 over the other two lines and of cv. Prisma over line I-17-40 during the early stages of growth appears to be due to a lower sensitivity to infection of those processes which regulate dry matter accumulation and its distribution around the plant.     

白粉病菌侵染诱导的大麦叶肉细胞变化的超微结构与细胞化学

 从超微结构和细胞化学角度对大麦白粉病菌侵染诱导的寄主叶肉细胞变化进行了研究。结果发现,病原菌接种后12 h,叶肉细胞光合作用的标志性酶RCA和Rubisco含量发生迅速的变化,感病大麦(Ingrid)中RCA和Rubisco含量急剧减少,而抗病大麦(mlo-3)中则趋势相反。随后,不论在感病或抗病大麦中这2种蛋白的含量一直下降,但感病品种中一直维持相对较高水平。超微结构观察表明,病原菌侵染能够诱导侵染点附近叶肉细胞中叶绿体的增殖,新叶绿体的不断产生,从而延缓了叶肉细胞的衰老和死亡,增加了光合产物的形成。以上结果说明,叶肉细胞可能从养分供给上影响到了表皮细胞中病原菌吸器的发育。     

Effect of early infection on pathotype frequencies in barley powdery mildew (Blumeria graminis f.sp. hordei) populations in field plots

A field experiment with barley powdery mildew ( Blumeria graminis f.sp. hordei ) was designed in order to study how the time of arrival of inoculum in the field influenced pathotype frequencies in the resulting populations. Three isolates belonging to pathotypes that were absent or rare in the local aerial inoculum were used to inoculate field plots of winter barley cv. Plaisant. Two successive inoculations with different combinations of the three isolates were performed with an approximately two-generation delay, and frequencies of inoculated pathotypes were assessed four and nine generations after the first inoculation. Pathotypes of the first inoculated isolates generally persisted throughout the period of sampling; this is described as an 'early arrival' effect. During the epidemics the inoculated isolates were not replaced by isolates from the natural airborne inoculum. Pathotype frequencies depended mainly on the time of arrival of inoculum in the plot, but frequencies also depended on the isolate that had been inoculated. The most frequent isolate, GL1, belonged to the clonal lineage dominant in powdery mildew populations on winter barley in the north of France. These results confirmed that the composition of a powdery mildew population in a field is largely determined by the composition of the initial inoculum.     

Development of genetic SSR markers in Blumeria graminis f. sp. hordei and application to isolates from Australia

The barley powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh), exists in numerous haplotypes and displays significant differences in fungicide sensitivity. It causes considerable yield losses throughout the world. Microsatellite SSRs are useful tools to study the population level and biogeographic aspects of intraspecific diversity, but so far none have been defined for Bgh. Here, eight polymorphic microsatellite loci were identified and characterized. Primer pairs amplifying the loci were then applied to 111 isolates of Bgh from Australia. The number of alleles per locus ranged from 4 to 13, and Nei's genetic diversity ranged from 0·25 to 0·76. The microsatellite primers detected several clones among the isolates and defined 97 unique haplotypes. There was little evidence for regional genotypic subdivision, suggesting that gene flow may not be restricted among geographic regions. All data was consistent with high levels of genetic diversity, potentially resulting from random mating and spread within each region.     

The influence of the primary germ tube on infection of barley by Erysiphe graminis f. sp. hordei

In each of three separate genotype combinations of Erysiphe graminis f. sp. hordei and barley the probability of infection success by an appressorium was reduced by close proximity to a primary germ tube interacting with the same host cell, whether this germ tube was produced by the same or another conidium. Infection success of appressoria was unaffected by events in an adjacent cell, whether associated with other conidia or with the primary germ tube of the same conidium. This interaction between germ tubes and underlying host cells should be considered when mechanisms of resistance to E. graminis f. sp. hordei are investigated.     

An apparatus for collecting total conidia of Blumeria graminis f.sp. hordei from leaf colonies using electrostatic attraction

Conidia from living conidiophores of barley powdery mildew ( Blumeria graminis f.sp. hordei ) on host leaves were collected consecutively using an electrostatic spore collector. The collector consisted of an electrical conductor plate linked to an electrostatic voltage generator and insulator plates placed abreast on a timed conveyer. The conductor plate was negatively charged by the potential supplied from the voltage generator. The negatively charged conductor plate caused dielectric polarization of the insulator plate, and the surface charge on the insulator plate attracted mature conidia abstricted from conidiophores on colonies growing on leaves placed 2 cm from the insulator plate. The surface charge on the insulator plate was proportional to the voltage applied to the conductor plate. Under optimized conditions, abstricted conidia were attracted to the electrostatically activated insulator plates without any detriment to their survival. During a colony's life span of c . 460 h, conidia were released throughout the day and c . 12 × 10⁴ conidia were collected during the lifetime of the colony. This is the first report on the direct quantification of progeny conidia produced by powdery mildew infecting host leaves.     

Effect of Film-forming Polymers on Infection of Barley with the Powdery Mildew Fungus, Blumeria graminis f. sp. hordei

The effects of three film-forming compounds, Ethokem, Bond and Vapor Gard, on infection of barley by the powdery mildew fungus Blumeria graminis f. sp. hordei were examined in glasshouse and field experiments. The three compounds provided significant control of powdery mildew infection when applied as pre- or post-inoculation treatments in the glasshouse. Such treatment had no effect on plant growth. Bond and Vapor Gard reduced the germination of conidia of B. graminis by 78% and 85% respectively, and reduced the subsequent formation of appressoria (73% and 85% respectively) and haustoria (75% and 79% respectively). The three compounds were less effective in field experiments, although they provided significant control of mildew infection and had no impact on plant growth and grain yield.     

Genetics of avirulences in Erysiphe graminis f.sp. hordei

The genetics of avirulences towards barley mildew resistances were analysed in crosses of the Ervsiphe graminis f.sp. hordei isolate DH14 with CC107 and with CC138. Nine avirulences, Av r_a9, Avr _a10, Avr _a11, Avr _a12, Avr _Ab, Avr _CP, Avr _h, Avr _k and Avr _La, segregated as single genes in one or other cross. However. F₁ segregation data were consistent with avirulence matching the Mla7 resistance gene being controlled by two genes, designated Avr _a7 1 and Avr _a7 2. Infection types of avirulent isolates differed on varieties in which Mla7 had been derived from each of the four known sources of that resistance. Linkage was detected between Avr _a7 1 and Avr _h in the cross CC107 × DH14, and between Avr _a10 and Avr _k, Avr _a11 and Avr _La, and Avr _h and the triadimenol response gene Tdl2 in CC138 × DH14.     

Sensitivity of Wheat Powdery Mildew (Blumeria graminis f.sp. tritici) towards Metrafenone

A monitoring study on the current sensitivity situation of wheat powdery mildew (Blumeria graminis f.sp. tritici) towards metrafenone was performed in 2009 with 2509 isolates. Airborne isolates were randomly collected in different regions of the most important European cereal growing regions. The sensitivities of the majority of isolates were comparable to the baseline sensitivity, which was determined in 2000 before market launch of the compound. Ninety-two isolates showed sensitivities outside the baseline. Eighty-four of them were classified as moderately adapted and could still be inhibited with registered rates of metrafenone in glasshouse tests under various preventive or curative conditions. Eight isolates were identified to be resistant, which were not fully inhibited at registered rates. Resistance management strategies and further extensive monitoring studies are indicated for a sustainable use of metrafenone against wheat powdery mildew.     

小麦白粉菌有性时期闭囊壳在侵染循环中作用的初步研究

通过对陕西省和河南省小麦白粉病越夏期间场院麦秸垛中闭囊壳发育、存活情况及场院附近自生麦苗发病情况的调查,结果发现闭囊壳保湿培养7~8 d后,子囊孢子形成达到高峰;自生麦苗发病率与闭囊壳存活率之间存在极显著的正相关性,且闭囊壳存活率与海拔高度之间存在显著的负相关性,采自海拔500~900 m的闭囊壳有较高的存活率,海拔超过1 000 m的存活率较低或为零。此结果说明小麦白粉菌有性时期闭囊壳可能是通过侵染自生麦苗在侵染循环中起作用。     

High diversity, low spatial structure and rapid pathotype evolution in Moroccan populations of Blumeria graminis f.sp. hordei

Limited information is available about the spatial distribution and evolution of Blumeria graminis f.sp. hordei populations in North African countries, such as Morocco. Frequencies of virulence alleles in B. graminis populations are mainly driven by selection exerted by host resistance genes in addition to neutral processes such as migration and genetic drift. In Morocco, in contrast to Europe, there has been no systematic deployment of resistant cultivars, although some R genes are present in the traditional varieties. This is expected to result in the evolution of pathotypes with virulence to different R genes, and higher diversity in Morocco compared to Europe. To test this, we used 24 differential cultivars to characterise 72 isolates from Morocco in 2009. We assessed diversity and spatial structure of pathotypes and compared them to past isolates from the same area (collected in 1992). There was a high diversity of pathotypes. Isolates from 2009 were distinct from isolates from 1992, due to loss of virulence to Mla12, increased virulence to Mla8, Mla3 and Mlk1, and decreased virulence to Mla6, Ml(Ru2), Mlg and MlLa. Many virulences were different from those observed in European and Asian populations of B. graminis. At the spatial scale investigated, airborne dispersal and a lack of strong selection in the host population likely prevented the formation of population structure and allowed the accumulation of high isolate diversity. The evolution of novel and distinct pathotypes since 1992 is likely attributable to gene flow from Europe and selection by the host population in Morocco.     

Infection-inhibition activity of avenacin saponins against the fungal pathogens Blumeria graminis f. sp. hordei, Bipolaris oryzae, and Magnaporthe oryzae

Triterpenoid saponins are sugar-modified triterpene derivatives. Cereals and other grasses are generally deficient in these secondary metabolites with the exception of oat. Oat accumulates antimicrobial triterpenoid saponins in its roots. These oat-root-derived compounds, called avenacins, confer broad-spectrum resistance to soil-borne pathogens. Here, we tested the effect of avenacins on the development of infection structures of fungal pathogens Blumeria graminis f. sp. hordei and Bipolaris oryzae and Magnaporthe oryzae. We show that avenacins are able to inhibit the infection process of these phytopathogens on plant hosts.     

小麦白粉病菌闭囊壳在初侵染中的作用观察

文章对湖北小麦白粉病菌有性时期对自生麦苗侵染情况进行观察.结果显示:侵染自生麦苗的主要菌源是病残体上的闭囊壳释放的子囊孢子,且闭囊壳量大、阴湿条件下自生麦苗发病重,而闭囊壳量少、阴湿条件下则自生麦苗发病轻,干燥、阳光直射条件不利于闭囊壳向自生麦苗传病.田间病残体上的闭囊壳无法与秋苗形成菌源衔接,即不能直接为秋苗提供菌源,只是侵染自生麦苗的重要菌源,在小麦白粉病侵染循环中起着间接越夏的重要作用.导致麦苗发病的闭囊壳必须在土表以上,埋于土壤中的闭囊壳一般不会导致麦苗发病,土表以上病残体或闭囊壳量愈大,发病的可能性也更大.     

Structure and evolution of a population of Erysiphe graminis f.sp. hordei

Samples of single colony isolates of Erysiphe graminis f.sp. hordei were collected in Cambridge on three dates in 1985, and tested for the presence of 12 virulence alleles and resistance to two fungicides, triadimenol and ethirimol. The frequency of the virulence V-(La) fell during 1985, while the frequency of V-h. virulence on cv. Triumph and higher levels of resistance to each fungicide and combined resistance to both fungicides rose. Two phenotypes, both of which possessed virulence on cv. Triumph and three unnecessary virulence alleles and had similar sensitivities to ethirimol, but differed in their level of resistance to triadimenol, accounted for 35.9% of all isolates. The high frequency of these phenotypes accounted for most of the observed gametic phase disequilibria between pathogenicity characters. Most individuals with these two phenotypes may be members of the same clone. It was estimated that 25% of the spore population which initiated the autumn epidemic of E.g. f.sp. hordei originated from ascospores formed by sexual reproduction in the summer. It is proposed that genetic drift followed by hitch-hiking selection, due to intense selection for a clone virulent on a newly-introduced cultivar, is a major factor influencing the frequency of unnecessary virulence alleles.