Understanding pathogen population structure and virulence variation for efficient resistance breeding to control cucurbit powdery mildews

Cucurbit powdery mildew (CPM) is caused most frequently by well-differentiated obligate erysiphaceous ectoparasites Golovinomyces orontii and Podosphaera xanthii, which vary in their ecology and virulence. All economically important cucurbit crops host both of these CPM species. Breeding of cucurbits for CPM resistance is highly important worldwide, but adequate knowledge of CPM species determination, as well as virulence structure, population dynamics, and spatiotemporal variation of these pathogens, has not yet been achieved. New tools have been developed to enhance research on CPM virulence variation for more efficient breeding and seed and crop production. A set of differential genotypes of Cucumis melo, with high differentiation capacity, may contribute substantially to understanding of variation in CPM virulence at both individual and population levels. Long-term observations (2001–2012) of CPM pathogens in the Czech Republic were used to analyse virulence variation within and among annual CPM populations and demonstrate the utility of recently developed tools for studying species variability and virulence variation of CPM pathogens worldwide. Detailed analyses of diversity and spatiotemporal fluctuations in the composition of CPM populations provide crucial information for shaping breeding programmes and predicting the most effective sources of race-specific resistance. The primary aim of this work was to create a uniform framework for determination of CPM species structure and diversity, virulence phenotypes, virulence and phenotype frequencies, phenotype complexity, dynamics, and variation within and among CPM populations. In addition, practical advice is presented on how to select the most relevant data and interpret them for use in cucurbit resistance breeding.     

Morphological and molecular characterization of <Emphasis Type="Italic">Oidium</Emphasis> subgenus <Emphasis Type="Italic">Reticuloidium</Emphasis> (powdery mildew) newly occurred on cucumber in Japan

In 2002, a powdery mildew with catenate conidia lacking fibrosin bodies was found on cucumber in a greenhouse in Kanagawa Prefecture, Japan. Morphological observation revealed that the fungus belongs to Oidium subgenus Reticuloidium, anamorph of the genus Golovinomyces. Molecular phylogenetic analyses of the nucleotide sequences of the rDNA ITS regions and D1/D2 domains of the 28S rDNA indicated that the fungus belongs to the clade of G. orontii with other Golovinomyces fungi from a wide range of host plants, suggesting that the fungus was newly transported from abroad. Because there has been no prior report of cucumber powdery mildew caused by Reticuloidium, further research on the physiology, epidemiology, control and resistant cucumber varieties is required.     

Evaluation and QTL mapping of resistance to powdery mildew in lettuce

Lettuce (Lactuca sativa) is the major leafy vegetable that is susceptible to powdery mildew disease under greenhouse and field conditions. Quantitative trait loci (QTLs) for resistance to powdery mildew under greenhouse conditions were mapped in an interspecific population derived from a cross between susceptible L. sativa cultivar Salinas and the highly susceptible L. serriola accession UC96US23. Four significant QTLs were detected on linkage groups LG 1 (pm‐1.1), LG 2 (pm‐2.1 and pm‐2.2) and LG 7 (pm‐7.1), each explaining between 35 to 42% of the phenotypic variation. The four QTLs are not located in the documented hotspots of lettuce resistance genes. Alleles for the disease resistance at the four QTLs originated from both parents (two from each), demonstrating that even highly susceptible accessions may provide alleles for resistance to powdery mildew. These QTLs appeared to operate during limited periods of time. Results of the field trials with F_2:3 and F_3:4 families derived from a Soraya (moderately resistant) × Salinas cross demonstrated effective transfer of resistance to powdery mildew in this material. An integrated rating approach was used to estimate relative levels of resistance in 80 cultivars and accessions tested in a total of 23 field and greenhouse experiments. Generally, very low resistance was observed in crisphead‐type lettuces, while the highest relative resistance was recorded in leaf and butterhead types. Comparison of two disease assessment methods (percentage rating and the area under the disease progress steps, AUDPS) for detection of QTLs shows that the two approaches complement each other.     

A new resistance gene to powdery mildew identified in <Emphasis Type="Italic">Solanum neorossii</Emphasis> has been localized on the short arm of potato chromosome 6

A new resistance (R) gene to powdery mildew has been identified and characterized in a population derived from the wild potato species, Solanum neorossii under natural infection in the greenhouse. The segregation of resistance has revealed that this R gene is controlled by a single monogenic and dominant gene designated Rpm-nrs1. Analysis of the DNA sequence on an internal transcribed spacer (ITS) region of the pathogen genome suggests that the pathogen causing the powdery mildew disease is either Golovinomyces orontii or G. cichoracearum. The resistance locus was localized to the short arm of chromosome 6 where several disease R genes already identified in potato and tomato are known to reside. The resistance locus cosegregated in 96 progeny with three AFLP markers and one PCR marker. The sequences of the two cosegregating AFLP markers are highly homologous to Mi-1 conferring resistance to nematode, potato aphid and whitefly and Rpi-blb2 conferring resistance to late blight. The results in this study will facilitate the cloning of this gene conferring resistance to powdery mildew.     

Fungicide resistance in Czech populations of cucurbit powdery mildews

A total of 108 isolates of cucurbit powdery mildew (CPM) fungi, 78Golovinomyces cichoracearum (Gc) and 30Podosphaera xanthii (Px), originating from nine Czech regions and 22 districts of the Czech Republic were collected in the years 2001–2004. These isolates were screened for tolerance and/or resistance to the three frequently used fungicides (fenarimol, dinocap, benomyl). Fungicide sensitivity was determined by a modified leaf-disc bioassay with five concentrations. Fungicide efficacy differed significantly: fenarimol was the most effective and all isolates of both CPM were controlled by the recommended concentration (36μg a.i. ml⁻¹). Some isolates expressed resistance (profuse sporulation) or tolerance (limited sporulation) to lower concentrations (9.6 and 18μg a.i. ml⁻¹). Specific temporal variation in tolerance/resistance was observed, with some isolates ofGc from 2002 evincing tolerant or resistant reactions to these low fenarimol concentrations, but isolates with similar reactions were not detected during the years 2003–2004. Dinocap showed decreasing efficacy during all 3 years. A shift to more tolerant reactions in the CPM populations was detected forGc in 2001–2002, and forPx in 2001 and 2004. Benomyl was found to be ineffective, because the majority of screened isolates (88%Gc and 97%Px) belonged to the highly resistant strains, with resistant reaction on the recommended concentration (250μg a.i. ml⁻¹) and tolerance or resistance on higher concentrations (500 and 1000μg a.i. ml⁻¹). Sensitivity differed between the CPM species. Whereas practically allPx isolates (except one from 2003) were resistant, 12% ofGc isolates from the years 2001–2003 showed sensitive and/or tolerant reactions. In 2004, only benomyl-resistantGc strains were detected. Variation in tolerance/resistance was detected to all screened fungicides during the course of this study at some repeatedly sampled locations. http://www.phytoparasitica.org posting June 12, 2008.     

Effect of temperature on infection and development of powdery mildew on cucumber

Podosphaera xanthii and Golovinomyces orontii are the causal agents of cucurbit powdery mildew. The effect of temperature on conidial germination, infection and sporulation was studied under controlled conditions. Conidia were inoculated on cucumber leaf discs, and incubated at six constant temperatures (from 10 to 35 °C in 5 °C steps) for 3 to 72 h to evaluate conidial germination and infection, and for 6–15 days to evaluate sporulation intensity. Germination took place at all tested temperatures, but was close to zero at 35 °C. The longest germ tubes measured in this experiment were 141.74 μm for the secondary germ tube of P. xanthii at 20 °C after 48 h of incubation, and 67.92 μm for G. orontii for the primary germ tube at 20 °C after 48 h of incubation. The optimal temperatures for conidial germination, infection and sporulation were 24.4, 25.7 and 22.3 °C, respectively, for P. xanthii, and 17.9, 17.3 and 14.9 °C, respectively, for G. orontii. Equations were developed to describe conidial germination with a coefficient of determination (R²) of 0.85 and 0.90 for P. xanthii and G. orontii, respectively. Infection equations resulted in R² of 0.94 and 0.93 for P. xanthii and G. orontii, respectively; and for sporulation, R² of 0.75 and 0.76 for P. xanthii and G. orontii respectively, as a function of temperature. These results can be used to develop models for the risk of cucurbit powdery mildew under field conditions.     

Host range expansion in a powdery mildew fungus (<Emphasis Type="Italic">Golovinomyces</Emphasis> sp.) infecting <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>: <Emphasis Type="Italic">Torenia fournieri</Emphasis> as a new host

Since 2003, Torenia fournieri plants grown for experimental purposes were repeatedly infected by powdery mildew in a laboratory in Hungary. Based on morphological characteristics, the pathogen belonged to the mitosporic genus Oidium subgen. Reticuloidium, the anamorph stage of Golovinomyces. The rDNA ITS sequence was identical to that of two other powdery mildew fungi, infecting Arabidopsis and Veronica, respectively, in different parts of the world. According to a previous phylogenetic analysis of ITS and 28S rDNA sequences, those two powdery mildews belong to a recently evolved group of Golovinomyces characterized by multiple host range expansions during their evolution. Both the ITS sequence and the morphological data indicate that the powdery mildew anamorph infecting Torenia also belongs to this group. It is likely that the powdery mildew infections of the experimental T. fournieri plants, native to south-east Asia, were the result of a very recent host range expansion of a polyphagous Golovinomyces because (i) T. fournieri is absent from our region, except as an experimental plant grown in the laboratory, (ii) the powdery mildew fungus infecting this exotic plant belongs to a group of Golovinomyces where host range expansion is a frequent evolutionary scenario, (iii) cross-inoculation tests showed that this pathogen is also able to infect other plant species, notably A. thaliana and tobacco, and (iv) no Golovinomyces species are known to infect T. fournieri anywhere in the world. Although host range expansion has often been proposed as a common evolutionary process in the Erysiphales, and also in other biotrophic plant pathogens, this has not been clearly demonstrated in any case studies so far. To our knowledge, this is the first convincing case of a host range expansion event in the Erysiphales.     

甘肃省黄芩白粉病病原鉴定及对黄芩叶片生理特性的影响

为探讨黄芩白粉病的发病症状、病原种类及对黄芩叶片生理特性的影响，从甘肃省定西市陇西县黄芩主产区采集黄芩白粉病病叶，通过病原菌无性和有性形态特征观察并结合rDNA-ITS序列分析对其进行种类鉴定。结果表明：引起甘肃省黄芩白粉病的病原菌为琉璃草高氏白粉菌(Golovinomyces cynoglossi)，该病菌能引起室内黄芩白粉病，发病黄芩叶片电解质渗漏电导率升高，叶绿素含量降低；发病黄芩植株叶片总灰分、Zn、Mn、Cu、Fe、Ca、Mg、全P和全B的含量升高，而蛋白质、总糖、可溶性糖、全N、全K的含量降低。在国内首次报道了琉璃草高氏白粉菌(G.cynoglossi)引起黄芩白粉病，明确了白粉病对黄芩叶片生理特性的影响。