Evidence of host‐range expansion from new powdery mildew (Blumeria graminis) infections of triticale (×Triticosecale) in France

This study aimed to determine whether powdery mildew caused by Blumeria graminis is an endemic pathogen of triticale (×Triticosecale: Triticum × Secale), emerging as a result of recent changes in its pathogenicity, or whether it is a new pathogen, possibly resulting from hybridization between ff. spp. tritici and secalis. A secondary aim was to consider breeding practices that may have favoured this emergence. Phylogenetic analyses based upon six genes revealed the close relatedness of the novel entity and the ff. spp. tritici and secalis, but the IGS marker finally grouped together the isolates collected on triticale and on wheat, supporting the scenario of a recent host‐range expansion from wheat to triticale. Pathotype analyses concluded that virulence spectra of B. graminis infecting triticale were new in comparison to those observed for other reference formae speciales, and lack of fungicide resistance in triticale isolates strengthens the hypothesis of no or little genetic exchange between wheat and triticale populations of powdery mildew. This adaptation may follow the breakdown of plant resistance genes, which are probably not very diverse in current triticale cultivars since this criterion was not considered as a major one until recent years. Moreover, the complex selection and genetics of this hybrid cereal makes it difficult to predict the transmission of powdery mildew resistance genes.     

小麦品种温度互作中抗条锈微效基因的表达

初步研究了不同温度下小麦品种抗条锈性的表达机制 ,结果分析表明 :品种间、温度间及品种×温度间差异均达到了极显著水平。试验进一步证明京核 1号小麦温敏微效基因的存在。在高温潜育发病条件下有利于微效基因抗性的表达。接种前高温处理 1～ 2d ,在常温下潜育发病 ,对温敏基因抗性的表达没有显著影响 ;接种后高温处理 1～ 2d ,在常温下潜育发病 ,对温敏基因抗性的表达有一定的诱导作用。而接种后在常温下处理 1～ 2d ,再于高温下潜育发病 ,更有利于微效基因抗性表达     

A rapid virulence assay for the Dutch elm disease fungus Ophiostoma novo‐ulmi by inoculation of apple (Malus × domestica ‘Golden Delicious’) fruits

Large‐scale virulence tests using trees or saplings are expensive, time‐consuming and require a considerable amount of space. The suitability of using ‘Golden Delicious’ apples as a rapid screen for identifying Ophiostoma novo‐ulmi transformants with reduced virulence was thus evaluated. When a collection of O. novo‐ulmi field isolates belonging to subspecies novo‐ulmi or americana was inoculated to apples, members of subsp. novo‐ulmi induced, on average, larger necrotic lesions than subsp. americana isolates. The size of the lesions on apples was not correlated with mycelial growth rate of isolates on nutrient agar. Insertional mutants from O. novo‐ulmi subsp. novo‐ulmi isolate H327 were inoculated to ‘Golden Delicious’ apples and Ulmus parvifolia × U. americana saplings in parallel experiments. Results clearly indicated that the O. novo‐ulmi transformants included several exhibiting significantly altered levels of virulence. Variability among replicates within a treatment was reduced in apple inoculation data compared to elm sapling data. Overall, the ‘Golden Delicious’ apple assay was found to be an excellent means for rapidly assessing the virulence level of O. novo‐ulmi isolates.     

Effect of cold‐induced changes in physical and chemical leaf properties on the resistance of winter triticale (×Triticosecale) to the fungal pathogen Microdochium nivale

This study showed that several mechanisms of the basal resistance of winter triticale to Microdochium nivale are cultivar‐dependent and can be induced specifically during plant hardening. Experiments and microscopic observations were conducted on triticale cvs Hewo (able to develop resistance after cold treatment) and Magnat (susceptible to infection despite hardening). In cv. Hewo, cold hardening altered the physical and chemical properties of the leaf surface and prevented both adhesion of M. nivale hyphae to the leaves and direct penetration of the epidermis. Cold‐induced submicron‐ and micron‐scale roughness on the leaf epidermis resulted in superhydrophobicity, restricting fungal adhesion and growth, while the lower permeability and altered chemical composition of the host cell wall protected against tissue digestion by the fungus. The fungal strategy to access the nutrient resources of resistant hosts is the penetration of leaf tissues through stomata, followed by biotrophic intercellular growth of individual hyphae and the formation of haustoria‐like structures within mesophyll cells. In contrast, a destructive necrotrophic fungal lifestyle occurs in susceptible seedlings, despite cold hardening of the plants, with the host epidermis, mesophyll and vascular tissues being digested and becoming disorganized as a result of the low chemical and mechanical stability of the cell wall matrix. This work indicates that specific genetically encoded physical and mechanical properties of the cell wall and leaf tissues that depend on cold hardening are factors that can determine plant resistance against fungal diseases.     

Bacterial leaf blight of strawberry (Fragaria (x) ananassa) caused by a pathovar of Xanthomonas arboricola, not similar to Xanthomonas fragariae Kennedy & King. Description of the causal organism as Xanthomonas arboricola pv. fragariae (pv. nov., comb. nov.)

A new bacterial disease of strawberry is described. This disease, called bacterial leaf blight of strawberry, is characterized by dry, brown necrotic leaf spots and large brown V-shaped lesions along the leaf margin, midrib and major veins. Symptoms are different from angular leaf spot of strawberry caused by the bacterium Xanthomonas fragariae . Strains of the bacterial leaf blight pathogen were characterized in a polyphasic approach by biochemical tests, fatty acid analysis, protein electrophoresis, serology, PCR, pigment analysis, ice-nucleation activity, AFLP analysis, DNA:DNA hybridization, pathogenicity and host range tests, and compared with a number of reference strains of X. fragariae and other Xanthomonas species. Bacterial leaf blight strains formed a homogeneous group in all tests, completely different from X. fragariae . They were the only strains causing leaf blight of strawberry upon artificial inoculation into strawberry. Fatty acid and protein electrophoretic analysis showed that the strains belong to the phenon X. campestris ( sensu latu , including pathovars now classified as belonging to X. arboricola ). AFLP analysis and DNA:DNA hybridization further clarified their taxonomic position as belonging to X. arboricola. The name X. arboricola pv. fragariae is proposed for the bacterium causing leaf blight of strawberry with strain PD2780 (LMG 19145) as pathovar type strain. Criteria for routine identification are given and the taxonomic status is discussed.     

Application of cycleave PCR to the detection of a point mutation (F167Y) in the β2‐tubulin gene of Fusarium graminearum

BACKGROUND: Resistance of Fusarium graminearum to the benzimidazole fungicide carbendazim is caused by point mutations in the β₂‐tubulin gene (FGSG_06611.3). The point mutation at codon 167 (TTT → TAT, F167Y) occurs in more than 90% of field isolates in China. It is important to find a suitable method for rapid detection and quantification of this point mutation in the F. graminearum populations. RESULTS: A pair of primers, Codon167F/Codon167R, were designed to amplify a fragment containing the mutation site, and two cycling probes labelled with different fluorescent reporters were used to detect whether the mutation was present. A cycleave real‐time PCR method was developed for rapid determination of the frequency of this point mutation in 282 F. graminearum perithecia collected from different fields in Jiangsu Province, China. The mutation frequency in ascospores from the perithecia to carbendazim by a spore germination assay was 6.0%, while the frequency of the point mutation at codon 167 by the cycleave real‐time PCR assay was 3.9%. CONCLUSION: The cycleave real‐time PCR method is suitable for accurate detection of the codon 167 point mutation. The frequency of this mutation in the β₂‐tubulin gene represents the resistance frequency in F. graminearum populations to carbendazim. Copyright © 2011 Society of Chemical Industry     

Transmission of ‘Candidatus Phytoplasma pyri’ by naturally infected Cacopsylla pyri to peach,an approach to the epidemiology of peach yellow leaf roll (PYLR) in Spain

Peach orchards in the northeast of Spain were severely affected in 2012 by a previously unreported disease in this area. The symptoms included early reddening, leaf curling, decline, abnormal fruits, and in some cases death of the peach trees. All the infected peach samples were positive for ‘Candidatus Phytoplasma pyri’, but none were infected by the ‘Ca. Phytoplasma prunorum’. In this work, potential vectors able to transmit ‘Ca. Phytoplasma pyri’ from pear to peach and between peach trees were studied and their infective potential was analysed at different times of the year. Transmission trials of the phytoplasma with potential vectors to an artificial feeding medium for insects and to healthy peach trees were conducted. Additionally, isolated phytoplasmas were genetically characterized to determine which isolates were able to infect peach trees. Results showed that the only insect species captured inside peach plots that was a carrier of the ‘Ca. Phytoplasma pyri’ phytoplasma was Cacopsylla pyri. Other insect species captured and known to be phytoplasma transmitters were present in very low numbers, and were not infected with ‘Ca. Phytoplasma pyri’ phytoplasma. A total of 1928 individuals of C. pyri were captured in the peach orchards, of which around 49% were phytoplasma carriers. All the peach trees exposed to C. pyri in 2014, and 65% in 2015, were infected by ‘Ca. Phytoplasma pyri’ 1 year after exposure, showing that this species is able to transmit the phytoplasma to peach. Molecular characterization showed that some genotypes are preferentially determined in peach.