Set Of Differential Hosts Research Articles

Virulence of Puccinia triticina and Puccinia tritici-duri on durum wheat in southern Spain, from 2020 to 2022

Leaf rust is a major wheat disease in southern Spain, where durum wheat is an important crop. Until the 2019/2020 season, this disease was effectively managed, as the most widely planted cultivars in southern Spain had effective resistance genes. A problem arose in the spring of 2020, when every farm field and durum wheat trial examined displayed leaf rust symptoms. Leaves had few but large uredinial pustules, different from those of the normal leaf rust caused by Puccinia triticina, and telia developed rapidly after only a few days. The symptoms clearly fitted with P. tritici-duri, another wheat leaf rust species already reported in the western Mediterranean Basin. This species is not new in southern Spain but has never been observed at such high severity on almost every durum wheat cultivar grown in that region. Leaf rust severity was assessed in durum wheat field trials in the 2020, 2021 and 2022 growing seasons in four provinces of southern Spain. During 2020 and 2021, 13 single pustule isolates of leaf rust were also collected from different cultivars of durum wheat. Inoculation of the isolates on a differential host set showed that four different races were present, two being of P. tritici-duri. Only cultivar Calero showed consistent resistance to the races of P. tritici-duri employed in this study.

Field apple scab susceptibility of a diverse Malus germplasm collection identifies potential sources of resistance for apple breeding

BackgroundBreeding for resistance to apple scab (caused by Venturia inaequalis), the most devastating fungal disease of apples, relies on genetic resources maintained in germplasm collections.MethodsTo identify new sources of scab resistance, we evaluated 177 Malus accessions, including 27 primary and 13 hybrid Malus species from diverse geographical origins, in an orchard at Geneva, New York. We also screened a differential host set for 2 years to monitor for changes in the effectiveness of ten known scab resistance genes, which allowed us to confirm the presence of virulent pathogen races in the orchard.ResultsWe found that ~ 37% of the wild Malus accessions and domesticated cultivars were resistant to apple scab in the field. Several of these accessions were unrelated to sources of previously known resistance genes and are promising for apple scab genetic research and resistance breeding. Cultivars carrying the Rvi6 (Vf) gene from Malus floribunda clone 821, e.g. ‘Liberty’ or ‘Florina’, remained resistant despite the breakdown of Rvi6. ‘Demir’, a Malus hybrid from Turkey, and ‘Chisel Jersey’, a traditional English hard cider cultivar, showed fewer symptoms than the Rvi6 resistant cultivar ‘Prima’. Races 1 to 7 and 9 of V. inaequalis were present in the orchard, but no scab was observed on the indicator host accessions for races 11 and 12.ConclusionsDetailed and systematic screening of Malus germplasm identified resistant and moderately resistant donor accessions based on resistance reaction types. These accessions are promising for use in future genetic studies to identify novel sources of scab resistance alleles for apple breeding to develop cultivars with durable apple scab resistance.

Pathotype diversity of Fusarium oxysporum f. sp. mungcola causing wilt on mungbean (Vigna radiata)

Fusarium wilt, caused by Fusarium oxysporum f. sp. mungcola (Fom), is an increasingly serious disease of mungbean (Vigna radiata (L.) R.Wilczek) in China. Pathogenic variability has been observed among Fom isolates; however, there are no reports describing Fom races or pathotypes. Thus, this study was conducted with the aim of developing a set of pathotype differentials to reveal Fom pathotype diversity by assessing virulence variability of Fom isolates. First, 105 mungbean cultivars were screened against a standard virulent Fom isolate (F08). Eleven of the 105 cultivars were selected as candidate differentials of Fom according to resistance phenotype and genetic background. Second, the resistance of the 11 candidate differential cultivars was tested against 30 Fom isolates from different geographical origins in China. Highly significant differences were observed among isolate × cultivar interaction patterns, indicating that pathotype differentiation exists in Fom isolates. Based on the different reaction patterns combining with genetic background, seven of the 11 cultivars were selected to constitute a set of differential hosts of Fom pathotype, used to distinguish pathotypes of 84 Fom isolates from different geographical regions by evaluating the virulence reaction pattern. The results showed that the 84 Fom isolates were defined as 12 pathotypes. Finally, we tried to confirm whether the 12 Fom pathotypes could be distinguished by a PCR-based diagnostic method based on the two genes (SIX6 and SIX11) reported to be associated with Fom pathogenicity. However, the Fom pathotype could not be distinguished by variation of the PCR products or their resulting sequences of the two genes. This is the first study to develop a set of Fom pathotype differential hosts and identify 12 Fom pathotypes, which provides important information for resistance breeding and disease control.

Status of research, breeding and protection of cucurbits in relation to cucurbit downy mildew: their limits and perspectives

Knowledge of the recent distribution, host range, virulence, DNA profiles, and resistance to fungicides of Pseudoperonospora cubensis (Berk. & M.A. Curtis) Rostovzev, and sources of host resistance to this pathogen are reviewed. P. cubensis is widely distributed in nearly all continents, with a high capacity to adapt to changing environmental conditions and new regions. Its host-plant range includes more than 60 species and 20 genera of Cucurbitaceae. It exhibits clear host specialization, with shifts in host range and an increasing spectrum of new host species. Temporal shifts to higher virulence levels and more complex virulence patterning within pathogen populations have been recorded in the Czech Republic, Israel (in 2002), and the USA (in 2004). Pathotypes have been described, and a triple-tetrad code proposed for their denomination. A differential host set for racial determination and denomination is under development. Forty-five different reaction patterns were recorded from the interaction of 115 Cucumis melo accessions to 8 isolates of P. cubensis with different levels of virulence; most C. melo accessions were highly susceptible to all isolates; but PI 315410 (VIR 5682) was resistant to 5 isolates. Because of race-specificity in host-pathogen interactions, resistance breeding is rather difficult and resistance typically not stable. In vitro tests of fungicide efficacy in 2001-2014 documented high levels of efficacy in fosetyl-Al (Aliette 80 WP) and propamocarb (Previcur 607 SL) to Czech strains of P. cubensis, with increased resistance of some strains, inefficacy of metalaxyl, metalaxyl-M and cymoxanil (Ridomil Plus 48 WP, Ridomil Gold MZ 68 WP, Curzate K), and inefficacy of dimethomorph (Acrobat MZ) with a temporal shift from resistant to tolerant and sensitive isolates. Changes in the genetic structure of populations of 67 P. cubensis isolates collected in the Czech Republic and other European countries during 1995-2012 were detected by Multilocus Sequence Analysis (MLSA) of four mitochondrial and two nuclear DNA regions. These changes may be associated with recently reported shifts in virulence. Data on host-range changes and spatio-temporal variability in virulence and fungicide resistance confirm the high micro-evolutionary potential of P. cubensis populations and more detailed research is required for a better understanding of these host-pathogen interactions as well as for more efficient disease management.

Pseudoperonospora cubensis virulence and pathotype structure in Kazakhstan

Severe epidemics of cucurbit downy mildew caused by Pseudoperonospora cubensis have been observed in recent years in major cucumber‐producing regions in Kazakhstan. However, information on the pathogenic variability of P. cubensis that could facilitate screening of candidate cucumber varieties and hybrids prior to deployment for commercial production is currently not available. A total of 112 isolates were collected from 2014 to 2016 during surveys in three major cucumber‐producing regions of Kazakhstan. Single lesion isolates were tested for their compatibility with a differential host set composed of 12 genotypes. Most isolates were compatible with Cucumis sativus and C. melo subsp. melo, while none were compatible with Citrullus lanatus. Isolates differed primarily by their virulence to C. pepo subsp. pepo and Benincasa hispida. Based on the patterns of compatibility with the differential host set, 29 pathotypes were identified. Of these, 18 were associated with a specific production region, six occurred in two regions and five were present in all three regions. The number of virulence factors (VF) ranged from 2 to 10, with 65% of the pathotypes having eight or more VFs. Indices of diversity showed that virulence was very diverse within the pathogen populations. Clustering of pathotypes based on similarity in virulence showed the existence of two main clusters. The high pathogenic variability observed in this study suggests that newly developed cucumber varieties and hybrids with resistance to cucurbit downy mildew need to be evaluated against a wide range of pathotypes prior to their deployment for commercial production in Kazakhstan.

Apple cultivar Regia possessing both Rvi2 and Rvi4 resistance genes is the source of a new race of Venturia inaequalis

The apple cultivar Regia, bred in Dresden-Pillnitz, Germany, is one of the few cultivars originating from a different scab (Venturia inaequalis) resistance background than Malus floribunda 821, the source of Rvi6. Cv. Regia is a descendant of Russian apple R12740-7A and has been proved to contain the two scab resistance genes Rvi2 and Rvi4. The cultivar itself has been grown in Dresden-Pillnitz since the early 1970s, while seedling populations derived from cv. Regia have been raised in fungicide-free plots since at least 1982. In 2011, small scab lesions were found for the first time on leaves of cv. Regia trees in an experimental unsprayed orchard in Dresden-Pillnitz. Single spore isolate Regia2 (R2) was cultured from these scab lesions and propagated, and then inoculated on scions of the VINQUEST scab differential host set comprising host (0) to host (15), host (17) and cv. Regia, grafted on rootstock M9, in a greenhouse. Strong sporulation was observed on hosts (0), (1), (2), (8), (9), (10) and cv. Regia, and weak sporulation on hosts (3), (4), (13) and (17), suggesting that the new isolate possesses the respective virulences. The implications of these results are discussed in relation to the natural infection of the differential hosts in the orchard and the artificial inoculation of the grafted differential hosts with inoculum gained from in vitro culture of R2.

Evidence and Consequence of a Highly Adapted Clonal Haplotype within the Australian Ascochyta rabiei Population.

The Australian Ascochyta rabiei (Pass.) Labr. (syn. Phoma rabiei) population has low genotypic diversity with only one mating type detected to date, potentially precluding substantial evolution through recombination. However, a large diversity in aggressiveness exists. In an effort to better understand the risk from selective adaptation to currently used resistance sources and chemical control strategies, the population was examined in detail. For this, a total of 598 isolates were quasi-hierarchically sampled between 2013 and 2015 across all major Australian chickpea growing regions and commonly grown host genotypes. Although a large number of haplotypes were identified (66) through short sequence repeat (SSR) genotyping, overall low gene diversity (Hexp = 0.066) and genotypic diversity (D = 0.57) was detected. Almost 70% of the isolates assessed were of a single dominant haplotype (ARH01). Disease screening on a differential host set, including three commonly deployed resistance sources, revealed distinct aggressiveness among the isolates, with 17% of all isolates identified as highly aggressive. Almost 75% of these were of the ARH01 haplotype. A similar pattern was observed at the host level, with 46% of all isolates collected from the commonly grown host genotype Genesis090 (classified as “resistant” during the term of collection) identified as highly aggressive. Of these, 63% belonged to the ARH01 haplotype. In conclusion, the ARH01 haplotype represents a significant risk to the Australian chickpea industry, being not only widely adapted to the diverse agro-geographical environments of the Australian chickpea growing regions, but also containing a disproportionately large number of aggressive isolates, indicating fitness to survive and replicate on the best resistance sources in the Australian germplasm.

Virulence Structure Within Populations of Pseudoperonospora cubensis in the United States.

Cucurbit downy mildew (CDM), caused by the obligate oomycete Pseudoperonospora cubensis, has resurged around the world during the past three decades. A new pathotype or genetic recombinant of P. cubensis have been suggested as possible reasons for the resurgence of CDM in the United States in 2004. In total, 22 isolates collected between 2004 and 2014, mainly in the eastern United States, were tested for their compatibility with a set of 15 cucurbit host types. The virulence structure within these isolates was evaluated on a set of 12 differential genotypes from eight genera. All isolates were highly compatible with the susceptible cultivar of Cucumis sativus, whereas the least compatibility was observed with Luffa cylindrica and Momordica charantia. Based on the compatibility with the differential host set, five pathotypes (1, 3, 4, 5, and 6) were identified among the 22 isolates examined. Pathotypes 1 and 3 had not been previously described in the United States and isolates of these two new pathotypes were also compatible with 'Poinsett 76', a cultivar of C. sativus known to be resistant to CDM prior to 2004. Virulence within the pathogen population was expressed based on virulence factors, virulence phenotypes, and virulence complexity. The number of virulence factors ranged from two to eight, indicating a complex virulence structure, with 77% of the isolates having five to eight virulence factors. Thirteen virulence phenotypes were identified; the mean number of virulence factors per isolate and mean number of virulence factors per virulence phenotype was 5.05 and 5.23, respectively, indicating that complex isolates and phenotypes contributed equally to the complex virulence structure of P. cubensis. Gleason and Shannon indices of diversity were 3.88 and 2.32, respectively, indicating a diverse virulence structure of P. cubensis within the United States population. The diverse virulence and high virulence complexity within the pathogen population indicate that host resistance alone in available cucurbit cultivars will not be effective to control CDM. An integrated approach involving a combination of fungicide application and introduction of cultivars with new resistance genes will be required for effective management of CDM.

Virulence Characterization of Venturia inaequalis Reference Isolates on the Differential Set of Malus Hosts.

A set of differential hosts has recently been identified for 17 apple scab resistance genes in an updated system for defining gene-for-gene (GfG) relationships in the Venturia inaequalis-Malus pathosystem. However, a set of reference isolates characterized for their complementary avirulence alleles is not yet available. In this paper, we report on improving the set of differential hosts for h(7) and propose the apple genotype LPG3-29 as carrying the single major resistance gene Rvi7. We characterized a reference set of 23 V. inaequalis isolates on 14 differential apple hosts carrying major resistance genes under controlled conditions. We identified isolates that were virulent on at least one of the following defined resistance gene hosts: h(1), h(2), h(3), h(4), h(5), h(6), h(7), h(8), h(9), h(10), and h(13). Sixteen different virulence patterns were observed. In general, the isolates carried one to three virulences, but some of them were more complex, with up to six virulences. This set of well-characterized isolates will be helpful for the identification of additional apple scab resistance genes in apple germplasm and the characterization of new GfG relationships to help improve our understanding of the host-pathogen interactions in the V. inaequalis-Malus pathosystem.

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 3. Physiological races ofPhytophthora infestans

A survey of races ofPhytophthora infestans was made in 1985 and 1986 on the western slopes of the volcano Nevado de Toluca, Mexico. Isolates were collected from infected plants of wildSolanum species found in five long (7–10 km) and some short (300–500 m) transects. The study are of 15 000 ha was between 2900 and 3900 m altitude. Isolates were collected fromSolanum demissum (35 isolates),verrucosum (11),iopetalum (2),brachycarpum (7),stoloniferum (2), r-genotype (9) and from differential hosts (11). No isolate was identified as race 0. Ten virulence genes were identified with a set of differential hosts and the most common were numbers 2 and 4. Isolates with eight virulence genes were the most common and those with ten were found on eight occasions in the natural habitat and on three in the experimental fields.

Rooted leaf cuttings as an aid in screening for resistance to Plasmodiophora brassicae

SUMMARYRooted leaf cuttings were used to screen a range of genotypes and cultivars from Brassica spp. and Raphanus sativus for resistance to Plasmodiophora brassicae. The optimum conditions for obtaining equivalent disease incidence and disease severity on leaf cuttings and on seedlings were investigated.In the method used leaves were cut near the base of the petiole before the lamina was fully expanded. They were kept in a mist propagator for 6 to 18 days and then transferred to a clubroot test bench and individually inoculated with a suspension of resting spores. There was an optimum period for each cultivar between cutting and inoculation which gave the maximum incidence of infection: c. 15 days for Doon Major (swede); 10–15 days for Civasto (stubble turnip) and Giant Rape (forage rape); c. 22 days for Maris Kestrel (marrow stem kale) and c. 10 days for Slobolt (fodder radish). Gall size on rooted leaf cuttings gave a quantitative measure of disease severity equivalent to that on seedlings in tests using the European Clubroot Differential host set, the five above‐mentioned cultivars and four swede cultivars. There was little callus production except on some B. campestris hosts and on cuttings with shoot tissue; microscopic examination was used to confirm the presence or absence of infection in galls.Hormone rooting powder did not increase the rate of root production in two cultivars of forage rape. Root development was less extensive in B. campestris than in B. oleracea or B. napus. Apparently normal shoots developed on 96% of leaf cuttings (inoculated and uninoculated) in which a fragment of axillary bud tissue was included. The results are discussed in relation to screening for resistance in whole plants and in tissue culture.

Study of physiologic specialization in Plasmodiophora brassicae: Proposals for attempted rationalization through an international approach

A new set of differential hosts is described and proposed for research into physiologic specialization in Plasmodiophora brassicae , and methods are suggested for the performing of differentiation tests. The differential set comprises five hosts each of Brassica campestris, B. napus and B. oleracea . Thirty-four European P. brassicae collections were differentiated. A modification of the binary notation system of nomenclature is adopted for coding P. brassicae populations, the latter term being preferred to physiologic race. The coding system described is amenable to considerable expansion.

The Viruses Causing Mosaic of Cowpeas and Azuki Beans, and Their Transmissibility through Seeds

Three kinds of viruses were isolated from mosaic diseased plants of cowpeas (Vigna catjang and V. sesquipedalis). One of them is a seed-borne virus in cowpea plants, which is transmitted by sap and by at least three species of aphids. Its host range is restricted to several genera of Leguminosae, and Chenopodium. Infectivity of the crude sap was lost by heating at 50-60°C for 10 minutes, and by aging at room temperature for 5 to 6 days, and by diluting at 1:104-1:105. The electron microscopy of virus preparations by dip method always revealed a small amount of elongated virus particles of about 750mμ in length. Thus, according to Anderson's classification scheme (1955), the virus in question was considered to be a member of the Group 2 among Vigna and Crotalaria viruses. To avoid the present confusion in virus names of the Group 2, the authors proposed to use a name of cowpea aphid-borne mosaic virus (CAMV) (Lovisolo and Conti, 1966) for all members of the Group 2. The remaining two viruses isolated from cowpea plants were cucumber mosaic virus (CMV) and subclover mottle virus. The latter is considered to be a virus belonging to broad bean wilt virus group. Neither of these two viruses is transmitted through seeds of cowpea. More than fifty collections of cowpea plants showing mosaic symptoms from commercial fields in Kanto district, were tested for identification of the causal virus by sap inoculations to a set of differential hosts. The results showed that CAMV was found predominant. Viruses causing mosaic of azuki bean (Phaseolus angularis) were also investigated. Azuki bean mosaic virus (AzMV), CMV, and CAMV were isolated from azuki bean plants showing mosaic symptoms. AzMV is a seed-borne virus in azuki bean, Whereas both CMV and CAMV are not transmitted through seeds of azuki bean. Host range of AzMV is restricted to leguminous plants. The virus in the crude sap is inactivated by heating at 55-60°C for 10 minutes, and by aging at room temperature for 2 to 7 days. The dilution end point lies between 1:104 and 1:105. AzMV preparations examined under the electron microscope by dip method always showed a small amount of elongated virus particles of about 750mμ in length. AzMV seems to be closely related to CAMV, because the former virus protects azuki bean plants from infection by the latter virus. Fifty collections of mosaic plants of azuki bean from commercial plantings in Kanto district, and identification of the causal virus involved was undertaken by sap inoculations to a set of differential hosts. The results showed that the percentages of infection by CMV, CAMV, and AzMV were 55, 26, and 32%, respectively. It was also noted that some of azuki bean plants were infected simultaneously with CMV and either of CAMV and AzMV.

Testing clubroot-resistance of turnips in the netherlands and the physiologic specialization of plasmodiophora brassicae

1. Clubroot is the main disease of fodder turnips in the Netherlands. Growing resistant varieties is the only practical means of control. 2. Sowing seeds in the first half of July is more efficient for selection for clubroot resistance than sowing in the normal period, early in August. 3. Artificial infestation of a field soil may have advantages over a naturally infested field in selecting for resistance. “Furrow infestation” i.e. putting soil from heavily diseased farm fields into the furrows before sowing the turnips, seems a practicable method. 4. Ten strains of Plasmodiophora brassicae were distinguished in the Netherlands and Belgium on the basis of the infection spectrum in a large set of differential hosts. Seven of them originate from farm fields where turnips are frequently grown. Each strain represents a group of clubroot isolates with similar infection spectra. 5. Turnip varieties included in the 39 th Dutch Descriptive List of Varieties of Field Crops (1964) can be divided into three main groups based on their reaction to clubroot strains: 1) the V-group, comprising varieties which are very susceptible on nearly all diseased fields, 2) the M-group, varieties which are fairly resistant on most infested fields, and 3) the E-group, varieties which are fairly to very resistant on nearly all infested fields in the Netherlands; however, they are just as susceptible as the V-and M-group on a number of fields in Belgium. In addition a fourth group may be distinguished, viz. the L-group, comprising some varieties not mentioned in the “List”. These are moderately resistant on all clubroot fields in the Netherlands. 6. Races of the fungus which are aggressive to the M-group apparently have been prevalent in certain clubroot fields for many years. In other fields these races gradually increase during some years when varieties of the M-group are grown. A mixture of different races is likely to occur in every infested field. Each of the above mentioned strains also has to be regarded as a mixture of races. 7. Turnip varieties found resistant abroad usually turn out to be susceptible in the Netherlands. Only some Belgian varieties proved to be very resistant in our country. A few varieties which are susceptible in the Netherlands were found to be very resistant in some other countries. Clubroot races occurring in turnip fields in foreign countries, therefore, generally appear to be different from those found in the Netherlands.