Differential Hosts Research Articles

Intensive and extensive sampling techniques used to measure genetic diversity of Ustilago tritici, using virulence and DNA polymorphism

Assessment of genetic variability within a natural population of a pathogen is dependent on obtaining a genetically diverse collection of isolates for analysis. Collections of isolates of Ustilago tritici are traditionally made by sampling one smutted head of wheat per field from a large number of fields over a large area. However, there is little evidence that extensively sampling one smutted head from each of many fields over a large area yields a more genetically diverse collection of isolates than intensively sampling many smutted heads from each of a few fields. The objective of this study was to compare genetic diversity derived from intensive sampling of U. tritici (80 isolates from 4 fields) with that derived from extensive sampling (81 isolates from 81 fields). Genetic diversity was measured using virulence data and amplified fragment length polymorphism (AFLP). The virulence of the isolates was assessed on five differential hosts, and the AFLP data were obtained using 10 selective primers that yielded 23 polymorphic bands. Genetic diversity was similar within the extensive- and the intensive-sampling collections, when measured using virulence data, but it was more variable within the intensive-sampling collection than within the extensive-sampling collection, when measured using AFLP. In general, the results of AFLP analysis indicated a higher degree of genetic variability than did the virulence data. The results also indicated that host genotypes can strongly influence the genetic variability within populations of an obligate pathogen like U. tritici when this variability is measured on virulence data, but that host genotype has a weaker influence on diversity measured by AFLP.

Virulence spectrum of Puccinia hordei in barley production systems in Ethiopia

Barley leaf rust caused by Puccinia hordei is an important disease of barley in Ethiopia. In the 2003 and 2004 cropping seasons, surveys of P. hordei were conducted on fields in the main rainy, residual moisture and short rainy season‐barley production systems. A total of 381 isolates were analysed on 12 barley differential hosts carrying different Rph resistance genes (Rph1–Rph12). Based on infection phenotypes on leaf rust (Rph) resistance genes, seven pathotypes were identified, namely ETPh7611, ETPh7631, ETPh6611, ETPh7651, ETPh7671, ETPh7653 and ETPh7633, with frequencies of 63·0, 21·5, 6·8, 2·9, 2·6, 2·1 and 1·2%, respectively. ETPh7611 and ETPh7631 were the most common pathotypes found in all the surveyed areas of the three production systems. ETPh7653 was found in the small rainy season production system only. ETPh7671, ETPh7653 and ETPh7633 were the most virulent, but the least frequent, pathotypes. All isolates were virulent on resistance genes Rph1, Rph4, Rph8, Rph9, Rph11 and Rph12. Virulence against Rph3 and Rph7 was absent. Genes Rph2, Rph6+2, Rph5 and Rph10 were effective against 96·3, 88·9, 65 and 2·4% of the rust isolates tested, respectively.

Characterization of Sclerospora graminicola Isolates from Pearl Millet for Virulence and Genetic Diversity

Virulence and genetic diversity were studied using 21 isolates of Sclerospora graminicola, the pearl millet downy mildew pathogen collected from major pearl millet growing areas of India. Variability for virulence was determined by inoculating a set of 10 differential hosts with the S. graminicola isolates in a greenhouse. The isolates varied for latent period (6.4 to 11 days), disease incidence (0 to 98%), virulence index (0 to 18.7) and oospore-production potential (1 to 4). Among the 21 isolates, Sg 139 (Rajasthan) was the most virulent and Sg 110 (Tamil Nadu) the least virulent. Based on virulence index (disease incidence × latent period-1), the 21 isolates were classified into eight virulence groups. Genetic diversity among isolates was studied using AFLP markers. Based on similarity index of banding pattern, the 21 isolates were clustered into eight genotypic groups. The AFLP groupings, however, did not match with that of the virulence groupings, and these two were found independent. The isolate Sg 139 that remained distinct in both pathogenic and genetic groupings indicated its highly virulent nature. Implications of these results in downy mildew resistance breeding are discussed.

Virulence Differentiation of Eight Turnip mosaic virus Isolates Infecting Cruciferous Crops

Turnip mosaic virus (TuMV) is an infectious viral pathogen on the cruciferous crops, predominantly Chinese cabbage (Brassica campestris subsp. pekinensis) and radish (Raphanus sativus). On the basis of the symptom development in selective differential hosts from indicator host species, Chinese cabbage and Korean radish inbred lines, the representative eight isolates of TuMV were divided into two major groups/or six types. Group I includes Th 1, Ca-ad7, and Cj-ca2-1 isolates, while group II includes the other isolates (rg-pfl, r 9-10, Rhcql-2, Stock and Mustard). According to the molecular phylogenetic analysis, these isolates, however, divided into two groups and two independent isolates. Phylogenetic analysis indicated that four isolates (Tu 1, r9-10, Stock and Rh-cql-2) formed a distinct phylogenetic group, and the other two isolates (Ca-ad7 and Cj-ca2-1) also formed another group. Mustard and rg-pfl isolates did not seem to have any relationship with these two groups. Taken together, these results indicated that virulence differentiation on host plants, molecular phylogenetic analysis of the nucleotide and the deduced amino acid of TuMV coat proteins did not show any relationship. The multi-resistant lines, Wonyae 20026 and BP058 in Chinese cabbage represent valuable genetic materials that can be used for crucifer breeding programs on TuMV resistance, but not in Korean radish.

Virulence of Pyrenophora tritici-repentis in the countries of the Silk Road

Tan spot, caused by the fungal pathogen Pyrenophora tritici-repentis, is an important foliar disease of wheat. Eight races of the fungus are presently known based on their virulence on a wheat differential set. In 2001, approximately 80 wheat fields in Azerbaijan, Kyrghyzstan, Kazakhstan, Uzbekistan, and Syria were surveyed for the occurrence of tan spot. These countries lie on the historic Silk Road, which encompasses part of the wheat center of diversity. Single-spore isolates of P. tritici-repentis were recovered from 52 fields, and the virulence of 253 of these isolates was assessed on the differential hosts. The greatest diversity was observed in Azerbaijan, where races 1, 2, 3, 5, 7, and 8 of the pathogen were identified, and in Syria, where races 1, 3, 5, 7, and 8 were detected. However, little variation was found in the virulence of isolates from Kazakhstan, Kyrghyzstan, and Uzbekistan, with only one or two races identified in each of these countries. No isolate of races 4 and 6 was detected. Race 1 was predominant in all countries except Syria, where race 3 was the most common. Isolates of races 1 and 2 were collected mainly from hexaploid wheats, whereas isolates of races 3, 5, 7, and 8 were found predominantly or exclusively on tetraploid wheats. No new virulence pattern was identified, but as races are currently defined on a very limited set of differentials, the complete range of diversity in P. tritici-repentis is likely not being fully assessed.

Molecular Cytogenetic Identification of aWheat-Thinopyron intermedium(Host) Barkworth & DR Dewey Partial Amphiploid Resistant to Powdery Mildew

: Wide cross and molecular cytogenetic methods were used to transfer the powdery mildew resistance gene from Thinopyron intermedium(Host) Barkworth & DR Dewey to wheat. Among the progeny of crossing common wheat (Triticum aestivum L.) Yannong 15 with Th. intermedium, a partial amphiploid E990256, with resistance to powdery mildew, was developed. It had 56 chromosomes and could form 28 bivalents in pollen mother cells at metaphase I of meiosis. Resistance verification by race 15 at the seedling stage and by mixed strains of Erysiphales gramnis DC. f. sp. tritici Em. Marchal at the adult stage showed it was immune to powdery mildew at both stages. Gene postulation via 21 isolates of E. gramnis f. sp. tritici and 29 differential hosts showed it was nearly immune to all the isolates used, and its resistance pattern was different from all the mildew resistance genes used, which indicated it probably contained a new resistance gene to powdery mildew. Biochemical verification showed it might convey different Th. intermedium chromosomes from those of the wheat- Th. intermedium partial amphiploids Zhong 1–5. Genomic in situ hybridization analysis by using St genomic DNA as the probe showed E990256 contained a recombination genome of St and E. (Managing editor: Wei WANG)

The Vh2 and Vh4 scab resistance genes in two differential hosts derived from Russian apple R12740-7A map to the same linkage group of apple

Russian apple R12740-7A is the designation for an accession grown from seed collected in Russia, which was found to be highly resistant to apple scab. The resistance has historically been attributed to a naturally pyramided complex involving three major genes: one race-nonspecific gene, Vr, conditioning resistance to all known races, plus two race-specific genes. The race-nonspecific gene was identified as an independently segregating gene by Dayton and Williams (1968) and is referred to in this paper as Vr-DW. The first researchers to study the scab resistance gene complex in Russian apple never described the phenotype conditioned by the race-nonspecific gene. Later, Aldwinckle et al. (1976) associated the name Vr with a scab resistance gene conditioning distinctive stellate necrotic reactions, which we refer to as Vr-A in order to distinguish it from Vr-DW. We show that the segregation ratios in progenies from the scab differential hosts 2 and 4 that are derived from Russian apple, crossed with susceptible cultivars were consistent with a single gene conditioning resistance in each host. The genes have been named Vh2 and Vh4, respectively. Resistant segregants from host 2 showed stellate necrotic reactions, while those from host 4 showed hypersensitive reactions. Both the phenotypes and the genetic maps for the genes in the respective hosts were very similar to those of the genes previously named Vr-A and Vx, respectively, in an F1 family of Russian apple. We showed that race 2 of V. inaequalis isolated from host 2 was able to infect resistant descendants of the non-differential accession PRI 442-23 as well as host 2. The descendants of PRI 442-23 were expected to carry the race-nonspecific Vr-DW gene, but in fact carry Vr-A. We conclude that the Vh2 gene in host 2 and Vr-A are the same, and that the Vh4 gene in host 4 and Vx are the same. However, a major finding of this study is that the latter gene mapped to linkage group 2 of apple instead of linkage group 10 as suggested from previous research. With the two race-specific genes from Russian apple defined now, we discuss the nature of the race-nonspecific Vr-DW gene in this accession. We also report the identification of a new scab resistance gene, VT57, from either ‘Golden Delicious’ or ‘Red Dougherty’, which conditions chlorotic resistance reactions and is linked to Vh2.

VARIABILITY OF THE PATHOGENICITY OF VENTURIA INAEQUALIS IN EUROPE

In the frame of the DARE (Durable Apple Resistance in Europe) project, a collection of strains of Venturia inaequalis was established. It comprises 319 strains from 8 European countries, isolated from 48 different Malus species and M. x domestica cultivars. The pathogenicity of 39 strains from this collection was tested on a set of 8 differential hosts, under controlled conditions (in a growth-chamber). The results provided information on the virulence of the fungus in Europe, with special attention to virulence to the Vf gene. The most virulent strains originated from Northwest Europe. No significant correlation between aggressiveness and the number of cultivars to which a strain was virulent was found. The results also provided information on the susceptibility of the 8 cultivars of the host range, and permitted the selection of 5 representative strains of the fungus for the test of the apple cultivars and progenies in the greenhouse. The geographical distribution of Vf virulent strains is displayed on a map of Europe, including information from previous publications.

Population Dynamics of Cucumber mosaic virus in Melon Crops and in Weeds in Central Spain

ABSTRACT Understanding epidemiology and evolution of plant viruses requires knowledge of their ecology: obtaining information about the dynamics and genetic structure of viral populations in their various hosts is necessary to analyze host specialization and to identify reservoirs and inoculum sources. We present here a 3-year analysis of the population dynamics of Cucumber mosaic virus (CMV) in Central Spain in weed hosts and melon. CMV infection in weed hosts was detected throughout the year and showed a clear seasonality, with maximum incidence of 20 to 30% in summer and autumn. The dynamics of CMV incidence were different in various weed habitats (i.e., fallow fields, edges, and wastelands), which differed in stability due to the degree of human intervention. CMV incidence in weed habitats was correlated with the amount of vegetation, estimated both as biomass and as soil cover by plants. CMV population dynamics in melon crops was unrelated to that in weed habitats. Genetic characterization by ribonuclease protection assays and biological characterization on differential hosts showed no significant differences in the frequency of genotypes or biotypes for CMV isolates from weeds or melon. Hence, different population dynamics in various hosts and habitats has not resulted in population differentiation, suggesting that migration between hosts and habitats prevents fragmentation of the population.

Characterization of differential coffee tree hosts for Hemileia vastatrix Berk. et Br. with RAPD markers

Eighteen clones of differential coffee tree hosts for Hemileia vastatrix Berk. et Br. were characterized with RAPD markers. The genetic distances were estimated and the genealogical origin of the clones compared to data of marker- based clusters. Thirty-five primers identified 158 polymorphic loci of RAPD markers. The cluster based on the matrix of genetic dissimilarity values was compatible with information on the genealogical origin cited in literature. Specific markers for a number of clones were identified, and a combination of 12 RAPD markers allowed the characterization of the studied clones.

Efeitos na fotossíntese e área foliar de cultivares de alface inoculadas mecanicamente com patótipos do Lettuce mosaic virus e Lettuce mottle virus

Levantamentos realizados no estado de São Paulo indicaram a ocorrência isolada e em infecções mistas do Lettuce mosaic virus (LMV) e do Lettuce mottle virus (LeMoV) em plantas de alface (Lactuca sativa). O presente trabalho teve como objetivo estudar os efeitos da infecção isolada e mista entre o LMV (patótipos II e IV) e o LeMoV, em cultivares de alface suscetível (White Boston) e tolerante (Elisa - gene mol¹) ao LMV patótipo II. As plantas foram inoculadas via extrato vegetal tamponado com isolados de LMV-II, LMV-IV e LeMoV separadamente e em diferentes combinações, com intervalo de 24 h ou simultaneamente com os dois vírus. As plantas infetadas foram analisadas utilizando-se hospedeiras diferenciais para o LMV e o LeMoV, e no caso do LMV pelo teste sorológico de PTA-ELISA. Nas avaliações de peso fresco e seco, área foliar e teor de clorofila, observou-se que a cultivar White Boston foi a mais afetada por ambos os vírus. As infecções mistas e isoladas na cultivar Elisa causaram efeitos semelhantes, provavelmente devido a presença do gene mo1¹ de tolerância ao LMV-II. O isolado LMV-IV foi considerado o mais agressivo nestas cultivares quando comparado ao LMV-II e o LeMoV.

Pathogenic Variation among Isolates of Pyrenophora teres, the Causal Agent of Barley Net Blotch

AbstractIsolates of Pyrenophora teres, the causal agent of net blotch of barley (Hordeum vulgare L.) has been collected from France and Syria. Their virulence spectra were evaluated using 11 barley genotypes as differential hosts. The genotypes exhibited a continuous range of response from highly susceptible to moderately resistant. A mean disease rating of 3.7 is considered as the separation point between avirulent and virulent reactions. The frequency of virulence was highest for isolates S5, R5 and S6‐2 and lowest for R‐ICA31 and R‐HAS‐6. A cluster analysis indicated that the isolates exhibited distinct differential virulence patterns and they were identified into five groups. The French isolates S5, R5 and S6‐2 had a higher mean virulence and a low variance across all genotypes. None of the tested genotypes was highly resistant to all investigated isolates.

Characterisation of a protein from Venturia inaequalis that induces necrosis in Malus carrying the Vm resistance gene

Apple scab (black spot) is caused by the fungus, Venturia inaequalis. Race 1 isolates of this fungus are avirulent on Malus hosts carrying the resistance gene V m . Detached leaves from a V m host (resistant, differential host 5) and ‘Royal Gala’ (susceptible, host 1) were inoculated with a conidial suspension of V. inaequalis. In the resistant reaction, a hypersensitive response (HR), characterised by necrosis and the accumulation of autofluorescent materials in epidermal and mesophyll cells, was observed at the site of fungal penetration. No HR was observed in the susceptible host. V. inaequalis grown in vitro produced an elicitor that induced necrosis, similar to the HR, when infiltrated into leaves of the resistant V m host. No response, however, was observed in the susceptible host. The elicitor was proteinaceous and a fraction with elicitor activity was isolated using ultra-filtration, acetone precipitation and ion-exchange chromatography. The elicitor activity was resistant to boiling but it was abolished by digestion with proteinase K. The protein fraction contained three major proteins all with low isoelectric points (p I 3·0–4·5). The fraction also elicited necrosis in the differential host 4, but not in any of the other resistant hosts tested, including differential hosts 2, 3, and 6. Therefore, the fraction may contain elicitors with more than one host specificity.

Virulence of Canadian isolates of Ustilago tritici: 1964–1998, and the use of the geometric rule in understanding host differential complexity

Knowledge of changes in race distribution and composition in a pathogen population is important for developing host breeding strategies and understanding the biology of the pathogen. The virulence of isolates of Ustilago tritici collected during the period of 1964-1998 from across Canada, but mostly from Manitoba and Saskatchewan, was assessed by inoculation onto a differential host series. Individual isolates of the pathogen were inoculated onto 2 (prior to 1978) or 3 (after 1977) differential hosts if they were collected from durum wheats, or 15 differential hosts if they were collected from hexaploid wheats. A total of 609 isolates were assessed for virulence: 261 collected from durum wheats and 348 from hexaploid wheats. Races T32 and T33 were the most commonly identified from durum wheats and races T5 and T10 the most common from hexaploid wheats. The relative frequencies of durum wheat isolates of U. tritici possessing virulence to the durum wheat differentials ranged from 10% on ‘Pentad’ (TD-11) to 95% on ‘Mindum’ (TD-1) and, for the hexaploid wheat isolates, the relative frequencies of virulence to the hexaploid differentials ranged from 0% on 'Sonop' (TD-14) to 95% on ‘Reward’ (TD-7). During the course of this study, some races were detected, which had not previously been reported in Canada or, in some instances, anywhere. The majority of these races did not represent the detection of new virulences, but rather new virulence combinations. A comparison of the reactions of the different races of U. tritici with the differential host series, using Person's geometric rule, indicated that the durum differential hosts likely contain different single genes for resistance, while the hexaploid differential hosts appear to be multigenic.

Race 3, a new race of Fusarium oxysporum f. sp. lactucae determined by a differential system with commercial cultivars

Pathogenic variation among 26 Japanese isolates of Fusarium oxysporum f. sp. lactucae (FOL) was tested using 21 lettuce cultivars to select commercial lettuce cultivars as race differential indicators. Cultivar Costa Rica No. 4 was resistant to race 1 but susceptible to race 2, consistent with the conventional standard differential line VP1010. Cultivar Banchu Red Fire was susceptible to race 1 but resistant to race 2, which showed an opposite type of reaction as another differential line VP1013. Cultivar Patriot was susceptible to both races. The resistance reactions of the three cultivars under field conditions were identical with that observed in the seedlings. Thus cv. Costa Rica No. 4 and cv. Banchu Red Fire can be used as differential hosts to identify pathogenic races of FOL. This differential system showed that all FOL isolates obtained from diseased butterhead lettuce in Fukuoka, Japan were new races (i.e., pathogenic to three cultivars). We propose that the new race be designated race 3. Isolates of FOL, the pathogen of Fusarium wilt in lettuce, obtained from California showed the same reaction as that of race 1. Furthermore, the Japanese isolate SB1-1 (race 1) and California isolate HL-2 belonged to the same vegetative compatibility group. Our results suggest that both of the fungi are the same forma specialis.

Pathological Characterization and Molecular Analysis of Elsinoe Isolates Causing Scab Diseases of Citrus in Jeju Island in Korea.

Two scab diseases are recognized currently on citrus: (i) citrus scab caused by Elsinoe fawcettii, which has several pathotypes; and (ii) sweet orange scab caused by E. australis. Pathogenicity and cultural characteristics among 36 isolates collected from Jeju Island were investigated. Of 30 isolates from satsuma mandarin, yuzu, and kinkoji, all were E. fawcettii; 27 were similar to the Florida broad host range pathotype and 3 were similar to the Florida narrow host range pathotype by inoculation of differential hosts. Six isolates from natsudaidai were nonpathogenic to satsuma mandarin, rough lemon, sour orange, grapefruit, cleopatra mandarin, and natsudaidai leaves, and were only pathogenic to natsudaidai fruit. Isolates from natsudaidai usually produced unique tomentose colonies on potato dextrose agar compared with isolates from other citrus species. The colonies were relatively fast growing, radially sulcate, larger, and more expansive than the gummy, mucoid colonies of other isolates. Isolates from Florida, Australia, Argentina, and Jeju Island (Korea) were genetically differentiated using random amplified polymorphic DNA markers. E. fawcettii from Korea, Florida, and Australia, E. australis from Argentina, and natsudaidai isolates clustered closely within groups, but were clearly distinguishable among groups.

Identification and Origin of Xanthomonas campestris pv. campestris Races and Related Pathovars

ABSTRACT One hundred sixty-four isolates of Xanthomonas campestris pv. campestris and other X. campestris pathovars known to infect cruciferous hosts (X. campestris pvs. aberrans, raphani, armoraciae, and incanae) were inoculated onto a differential series of Brassica spp. to determine both pathogenicity to brassicas and race. Of these, 144 isolates were identified as X. campestris pv. campestris and grouped into six races, with races 1 (62%) and 4 (32%) being predominant. Other races were rare. The remaining 20 isolates from brassicas and other cruciferous hosts were either nonpathogenic or very weakly pathogenic on the differential series and could not be race-typed. Five of these isolates, from the ornamental crucifers wallflower (Cheiranthus cheiri), stock (Matthiola incana) and candytuft (Iberis sp.), showed clear evidence of pathovar-like specificity to the hosts of origin. A gene-for-gene model based on the interaction of four avirulence genes in X. campestris pv. campestris races and four matching resistance genes in the differential hosts is proposed. Knowledge of the race structure and worldwide distribution of races is fundamental to the search for sources of resistance and for the establishment of successful resistance breeding programs.

Transmisión experimental del virus del mal de Río Cuarto por Delphacodes kuscheli

Entre las enfermedades que afectan al cultivo de maíz (Zea mays) en Argentina, la producida por el virus del mal de Río Cuarto (MRCV) es la más importante. El MRCV pertenece a la familia Reoviridae, género Fijivirus, y su propagación en la naturaleza es realizada por Delphacodes kuscheli (Hemiptera: Delphacidae). La modalidad de transmisión para los miembros de este género de virus es persistente propagativa. Se estableció la necesidad de ajustar un sistema de transmisión eficiente del virus para estudios de caracterización, partiendo de poblaciones libres de virus criadas en laboratorio, para lo cual se ensayaron distintos períodos de adquisición, latencia e inoculación, evaluándose además un rango de hospedantes diferenciales. Se lograron obtener insectos libres de virus en cantidad suficiente para llevar a cabo los trabajos, mediante su cría en fitotrones y cámaras aclimatadas. La transmisión experimental del MRCV se efectuó exitosamente, bajo idénticas condiciones, empleando períodos de adquisición, latencia e inoculación de dos, 10 y uno día respectivamente para los cereales de grano fino y de dos, 10 y dos días para el maíz. Se infectaron de este modo las siguientes especies: maíz, cebada (Hordeum vulgare), avena (Avena sativa), trigo (Triticum aestivum), centeno (Secale cereale), grama rhodes (Chloris gayana) y alpiste (Phalaris canariensis). La detección del virus en las plantas inoculadas se efectuó mediante pruebas serológicas, análisis de dsRNA en electroforesis en gel de poliacrilamida (obteniéndose las 10 bandas típicas de los fijivirus) y microscopía electrónica, detectándose las partículas isométricas de entre 60 y 70 nm de diámetro.

Biological properties of necrotic and non‐necrotic strains of bean yellow mosaic virus in cool season grain legumes

SummaryA collection of 51 bean yellow mosaic virus (BYMV) isolates was transmitted from infected Trifolium subterraneum (subterranean clover) to Lupinus angustifolius (narrow‐leafed lupin) by Myzus persicae (green peach aphid). Depending on isolate and L. angiistifolius genotype used, two distinct responses developed in L. angustifolius plants, either systemic necrosis and plant death or non‐necrotic reactions of varying severity. Ten isolates caused necrosis and plant death in cv. Danja. However, when nine of these were inoculated to breeding line 90L423‐07‐13, seven induced non‐necrotic reactions, while two caused necrosis and plant death. Thirty seven isolates always produced non‐necrotic reactions regardless of genotype of L. angustifolius inoculated. Non‐necrotic and necrotic isolates originally came both from lupins and other species, and the non‐necrotic isolates were no less efficiently transmitted by M. persicae than the necrotic ones. When one isolate of each type was inoculated together to T. subterraneum and nine months later this culture was used as an acquisition source for aphid transmission to L. angustifolius, only the necrotic type was detected. Previous infection of L. angustifolius plants with a non necrolic isolate prevented subsequent infection by a necrotic one. All necrotic and non‐necrotic isolates reacted with BYMV antiserum in ELISA but only two cross‐reacted with antiserum to clover yellow vein virus (CYVV). When selected necrotic and non‐necrotic isolates were inoculated to differential hosts, all behaved like BYMV and not CYVV. When three isolates of each type were transmitted to 11 other cool season grain legume species, except in Cicer arietinum (chickpea), there were no necrotic reactions, but symptom severity varied with the isolate and species inoculated. The two isolates that caused necrosis in C. arietinum did not do so in L. angustifolius. The six isolates from Vicia faba (faba bean) all caused non‐necrotic reactions in L. angustifolius cv. Danja and 90L423‐07‐13. These and two necrotic isolates readily infected five genotypes of V. faba always causing severe symptoms. However, three non‐necrotic isolates from L. angustifolius and a further necrotic isolate were poorly infectious on V. faba in which they generally induced mild symptoms.These results show that at least three strain groups of BYMV can be distinguished by their reactions in different L. angustifolius genotypes, one causing necrosis and death in cv. Danja and 90L423‐07‐13, one causing necrosis and death in Danja but not 90L423‐07‐13, and one causing non‐nccrotic reactions in both. These strain groups could not be distinguished when representative necrotic and non‐necrotic isolates were inoculated to other grain legume species. However, inoculation to V. faba distinguished two other BYMV strain groupings differing in severity of symptoms and ability to infect this species.

Occurrence of a New Pathotype of Lettuce mosaic virus on Lettuce in Brazil.

Since 1970 lettuce mosaic has not been an important disease of lettuce in Brazil, due to the growing of cultivars that contain gene g, derived from cv. Gallega de Invierno, for tolerance. Recently, however, a widespread, serious outbreak of lettuce mosaic occurred in the State of Sao Paulo. Both lettuce cultivars that lack and those that contain gene g have been affected, suggesting the emergence of a new pathotype of Lettuce mosaic virus (LMV). Commercial lettuce fields were visited, and 20 samples were collected for virus identification by bioassay on differential hosts, serological tests, and electron microscopy. Of these, 12 were infected only by LMV, and 8 contained mixed infections of LMV and a possible new sequivirus, Lettuce mottle virus. Seedlings of susceptible cv. White Boston were inoculated with LMV from tolerant cultivars, and plants were allowed to flower. Seeds were collected and sown, and seedlings with mosaic symptoms were recovered. Twenty monolesional isolates, obtained by passing the virus three times through Chenopodium amaranticolor, were inoculated individually on lettuce differential cvs. Salinas and White Boston (susceptible) and cvs. Salinas 88, Vanguard 75, Ithaca, Malika, and Gallega de Invierno (tolerant) to previously described pathotypes of LMV (1). Considering the susceptibility of all test differentials, we concluded that a LMV pathotype IV exists that has overcome tolerance conferred by genes m and g and is responsible for the new outbreak of LMV in Brazil. This is the first report of LMV-IV in Latin America. Reference: (1) D. A. C. Pink et al. Plant Pathol. 41:5, 1992.