Structure and variation in the wild-plant pathosystem: <Emphasis Type="Italic">Lactuca serriola–Bremia lactucae</Emphasis>

Over the past decade, extensive research on the wild-plant pathosystem, Lactuca serriola (prickly lettuce)–Bremia lactucae (lettuce downy mildew), has been conducted in the Czech Republic. Studies focused on pathogen occurrence and distribution, host range, variation in symptom expression and disease severity, interactions of B. lactucae with another fungal species (Golovinomyces cichoracearum) on L. serriola, variation in resistance within natural populations of L. serriola, the structure and dynamics of virulence within populations of B. lactucae, sexual reproduction of B. lactucae, and a comparison of virulence structure and changes in B. lactucae populations occurring in wild (L. serriola) and crop (L. sativa) pathosystems. The incidence of B. lactucae on naturally growing L. serriola and other Asteraceae was recorded. Lactuca serriola was the most commonly occurring host species, followed by Sonchus oleraceus. Over the duration of these studies, the incidence of B. lactucae in L. serriola populations varied between 45–87%. Disease incidence and disease prevalence were partly related to the size, density and different habitats of L. serriola populations. In addition to B. lactucae infection, infection by the lettuce powdery mildew fungus (Golovinomyces cichoracearum) was quite common, including co-infection. Variation in resistance to B. lactucae was studied by using ten isolates (NL and BL races with known virulence patterns) at a metapopulation level, i.e. 250 L. serriola samples representing 16 populations from the Czech Republic (CZ). Our comparisons revealed broad variation in host resistance among host populations and also intrapopulation variability. In the CZ populations, 45 resistance phenotypes were recorded, the most frequent were race-specific reaction patterns. Structural and temporal changes in virulence variation of B. lactucae populations on L. serriola were studied during 1998–2005. Altogether, 313 isolates of B. lactucae originating from the Czech Republic were examined for the presence of 32 virulence factors (v-factors), and 93 different virulence phenotypes (v-phenotypes) were recorded. A study of v-factor frequency showed that common v-factors in B. lactucae populations match some of the race-specific resistance genes/factors (Dm genes or R-factors) originating from L. serriola. The highest frequency was recorded by v-factors v7, v11, v15–17, and v24–30. In contrast, v-factors (e.g. v1–4, 6, and 10) matching Dm genes originating from L. sativa were very rare. This demonstrates the close adaptation of B. lactucae virulence to the host (L. serriola) genetic background. Temporal changes in virulence frequencies over the period were recorded. In many v-factors (v11, v14, v16, and v25–28), fluctuations were observed, some (v14 and v17) shifting to higher frequencies, and others (v5/8 and v23) decreasing. The occurrence of mating types was studied (1997–1999) in a set of 59 B. lactucae isolates. Both compatibility types (B1 and B2) were recorded; however the majority of the isolates (96%) were type B2. A comparative study of B. lactucae virulence variation between the wild (L. serriola) and crop (L. sativa) pathosystems showed major differences. Migration and gene flow between both pathosystems and the potential danger of wild B. lactucae populations for cultivated lettuce are discussed. This paper summarizes comprehensive and unique research on an oomycete pathogen (B. lactucae) that is shared between a crop (lettuce, L. sativa) and its close wild relative (prickly lettuce, L. serriola). The data demonstrate clear evidence about race-specific interactions, variation and changes in virulence, and coevolutionary relationships in the wild pathosystem L. serriola–B. lactucae. Conclusions contribute to the broadening and better understanding of gene-for-gene systems in natural host–pathogen populations and their relationships to crop pathosystems.     

Sexual compatibility types of Bremia lactucae isolates originating from Lactuca serriola

Results are given on the occurrence of sexual compatibility types of seven isolates ofBremia lactucae originating fromLactuca serriola (prickly lettuce). It is concluded that the isolates studied are heterothallic. Both compatibility types (B1 en B2) were determined, but type B2 was prevalent. Sexual recombination ofB. lactucae isolates originating from wild and cultivated lettuce may occur.Samenvatting Zeven isolaten vanBremia lactucae, afkomstig vanLactuca serriola in Tsjechoslowakije, zijn onderzocht op hun sexuele compatibiliteitstype door ze te combineren met Nederlandse fysio's vanB. lactucae, afkomstig van cultuursla (L. sativa), waarvan het compatibiliteitstype (B1 of B2) bekend is. Alle isolaten vanL. serriola bleken heterothallisch te zijn, waarbij type B2 meer werd aangetroffen dan type B1. Sexuele recombinatie vanBremia-isolaten van wildeLactuca-soorten en cultuursla blijkt goed mogelijk te zijn.     

Distribution of race-specific resistance against <Emphasis Type="Italic">Bremia lactucae</Emphasis> in natural populations of <Emphasis Type="Italic">Lactuca serriola</Emphasis>

A metapopulation approach was applied to population studies of a common weed, Lactuca serriola (prickly lettuce). Seedlings grown from seed samples collected from 752 individual L. serriola plants in 50 populations occurring along an east-to-west transect across four European countries (Czech Republic, Germany, Netherlands and United Kingdom) were screened for resistance to 10 common races of Bremia lactucae. Based on the recorded reaction patterns, host individuals were characterized into specific resistance (R-) phenotypes. Diversity of R-phenotypes, their variation and distribution among and within European populations, was evaluated at different spatial scales, i.e. from a metapopulation involving the entire European study area to individual plants occurring in local populations. Generally, European populations of L. serriola have been shown to be highly susceptible to B. lactucae. However, large variation in L. serriola resistance was found both among and within individual countries. There was a clear gradient of increasing uniformity of race-specificity moving from central to western Europe, as well as a slight decrease in the diversity of R-phenotypes. Populations in the United Kingdom were the most divergent in terms of resistance structure from other geographic regions, and also were the most homogeneous, most likely a consequence of the relatively greater degree of spatial isolation from other regions. Metapopulation, inter- and intra-population variation in host resistance is discussed from the viewpoint of occurrence of race-specific interactions in this wild plant pathosystem.     

Virulence patterns of<Emphasis Type="Italic">Bremia lactucae</Emphasis> in Israel

The variation and distribution of virulent phenotypes ofBremia lactucae Regel, the causal agent of lettuce downy mildew, were studied during 2002–2003 in lettuce fields (Lactuca sativa) in Israel. A total of 21 isolates ofB. lactucae were collected from nine locations in three regions of Israel: Galilee, the Coastal Plain, and the Shefela. The isolates were examined for the presence of 21 virulence factors (v-factors) and their combinations with differential sets of lettuce lines/varieties. There were clear differences in v-factors, and a broad diversity of v-phenotypes among the isolates was found. Although 17 different v-phenotypes and 20 v-factors were detected, a composite of similar v-phenotypes generally occurred between isolates within the three regions. They differed mostly in the presence or absence of only a few v-factors. The Coastal Plain region averaged the highest virulence complexity (0.63), significantly different from that of the Shefela (0.45) and of Galilee (0.4). Comparison of the IsraeliB. lactucae isolates that were tested in this study with data of other countries showed that factor v18, which did not occur in the Israeli populations, was detected only in Czech and German pathogen populations. http://www.phytoparasitica.org posting Dec. 21, 2006.     

Phylogenetic investigations in the downy mildew genus <Emphasis Type="Italic">Bremia</Emphasis> reveal several distinct lineages and a species with a presumably exceptional wide host range

Bremia lactucae is one of the most devastating and widespread pathogens in lettuce production worldwide. Despite its economical importance, uncertainty prevails about the species delimitation in the genus Bremia. Commonly, Bremia is considered to be monotypic, containing only Bremia lactucae, while taxonomists have described additional species, and molecular phylogenetic studies have shown significant sequence divergence between accessions from different hosts. Here, we report that several previously described species are genetically highly distinct from Bremia lactucae parasitic to Lactuca sativa. These include Bremia lapsanae, Bremia sonchicola, and Bremia taraxaci. In addition to these host-specific species, a plurivorous species is revealed, which infects hosts from three different tribes in the Asteraceae subfamilies Asteroideae and Carduoideae. The broad host range of clade 1 is exceptional for downy mildews and only paralleled by Pseudoperonospora cubensis, which infects a broad range of Cucurbitaceae. The taxonomic status of Bremia cirsii and of Bremia centaureae remains unresolved, as the accessions from Cirsium and Centaurea, respectively, did not form a monophylum but were partly contained in the plurivorous clade 1. Bremia lactucae was found to be restricted to Lactuca sativa and Lactuca serriola. Thus, it can be assumed that Bremia infections on weeds apart from Lactuca species do not pose a significant risk for lettuce production. However, it is unlikely that breeding resistance genes from Lactuca serriola into Lactuca sativa will result in durable resistance of lettuce to downy mildew disease, because the current study provides additional evidence that Bremia accessions from both hosts form a population continuum.     

A new race of Fusarium oxysporum f. sp. lactucae of lettuce

Fusarium oxysporum f. sp. lactucae, the causal agent of fusarium wilt of lettuce (Lactuca sativa), occurs in most countries in which lettuce is grown and causes serious economic losses. Three races (1, 2 and 3) of the pathogen have previously been identified on the basis of their ability to cause disease on differential lettuce cultivars, as well as by means of molecular tools developed to characterize different races of this pathogen. Only race 1 has been detected in Europe so far. In this study, two isolates of F. oxysporum, obtained from lettuce plants grown in the Netherlands showing symptoms of wilt, have been characterized by combining the study of pathogenicity with differential cultivars of lettuce and molecular assays to determine whether the isolates are different from the known races of F. oxysporum f. sp. lactucae. This study reports the presence of F. oxysporum f. sp. lactucae for the first time in the Netherlands. The causal pathogen has been identified, using the IRAP‐SCAR technique, as a new race of F. oxysporum f. sp. lactucae. Specific primers have been designed to identify this new race.     

成都蒿笋霜霉病菌初侵染及寄主范围研究初报

成都萵笋霜霉病菌孢子囊寿命很短,生活力只能維持7—10天。在病叶、茎及种子中,均未发現卵孢子。其初侵染来源可能是种子或病組織中潛藏的菌絲。調查結果表明,萵笋霜霉病菌可以藉气流或水流終年侵染,不在活体外休眠。 萵笋霜霉病菌寄主范围小,在6种菊科植物上接种未获成功,表現出明显的专化現象,品种間抗病性亦有显著的差異。     

Post infection application of DL-3-amino-butyric acid (BABA) induces multiple forms of resistance against <Emphasis Type="Italic">Bremia lactucae</Emphasis> in lettuce

DL-3-amino-butyric acid (BABA) induces local and systemic resistance against disease in numerous plant species. In a recent study we showed that preventive application of BABA to lettuce (Lactuca sativa) plants induced resistance against downy mildew caused by the oomycete Bremia lactucae by callose encasement of the primary infection structures of the pathogen. Now we show that post-infection application of BABA to the foliage or the roots, even at progressive stages of disease development, is highly protective against B. lactucae. Resistance induced by BABA is manifested in multiple microscopic forms, depending on the time of its application. When applied at 1 day post inoculation (dpi) BABA induced HR in penetrated epidermal cells; at 2 dpi it caused massive encasement with callose of the primary haustoria; and, at 3 or 4 dpi it enhanced the accumulation of H₂O₂ in the developing mycelia runners and altered their colour to red. The pronounced change in the colour of the mycelium was visually apparent to the naked eye. In all cases the pathogen failed to sporulate on the treated plants. This is the first indication that an immunizing compound may be protective at advanced stages of disease development.     

Variation and distribution of virulence phenotypes of Bremia lactucae in natural populations of Lactuca serriola

During 1997–2000 the virulence variation and distribution of virulence phenotypes of Bremia lactucae (lettuce downy mildew) in natural populations of Lactuca serriola (prickly lettuce) were studied. Altogether 139 isolates of B. lactucae originating from the Czech Republic, France and Germany were examined for the presence of 27 virulence factors (v-factors) and their combinations. In the Czech population, 37 different v-phenotypes (P1–P37) of B. lactucae were found to occur on L. serriola . Most v-phenotypes were characterized by v-factors that match resistance ( Dm genes/R-factors) carried by L. serriola . A wide diversity of v-phenotypes was recorded every year, but most were rare and did not reappear in other populations of B. lactucae . The three v-phenotypes P21, P28 and P29 were most frequent and widely distributed. There was variation both between spatially isolated populations and within populations of the pathogen. Geographic differences in virulence were found for the southern parts of Moravia, where the phenotypic composition of pathogen populations was completely different from the remaining part of the area investigated. However, some populations had v-phenotypes similar to those of spatially distant populations in Moravia.     

Virulence characteristics of Bremia lactucae populations in Norway

Use of resistant cultivars represent an efficient control measure for lettuce downy mildew (Bremia lactucae), although the durability of presently deployed resistance genes remains uncertain. Our objective was to document the pathogenic diversity of B. lactucae isolates in Norway. A total of 69 isolates of B. lactucae were collected between 2001 and 2006 from 65 commercial fields and four greenhouses in southeastern and southwestern Norway and tested for the presence of one or more of 19 virulence factors (v-factors). Phenotypic diversity was calculated based on presence or absence of v-factors, and as an overall comparison of v-phenotypes for each isolate. Disease severity varied over the years of the study, and epidemics were most consistently severe in southeastern Norway. The most commonly occurring v-factors, in order of frequency, were v5/8, v7, v2, v18, v4, v13, v6, v11, v12, v1 and v10. Virulence factor v17 was not found, while v36 was found in one isolate only. A total of 44 different v-phenotypes were identified within the population represented by the 69 isolates, yielding an incidence of unique virulence types of 63 %; a relatively high level of pathogen diversity. Four of the identified v-phenotypes were identical to races Bl:17, 18, 22 and 24, which have been previously reported in European populations of B. lactucae. The variability of the Norwegian B. lactucae populations verifies the genetic flexibility of this pathogen and its great ability to adapt to changes in host plants and surrounding conditions.     

Uptake, translocation and metabolism of aminocyclopyrachlor in prickly lettuce, rush skeletonweed and yellow starthistle

BACKGROUND: Aminocyclopyrachlor is a new herbicide proposed to control broadleaf weeds and shrubs in non‐crop and rangeland systems. To gain a better understanding of observed field efficacy, the uptake and translocation of foliar‐applied aminocyclopyrachlor (DPX‐MAT28) and aminocyclopyrachlor methyl ester (DPX‐KJM44) were evaluated in two annuals, prickly lettuce (Lactuca serriola L.) and yellow starthistle (Centaurea solstitialis L.), and one perennial, rush skeletonweed (Chondrilla juncea L.). RESULTS: Absorption and translocation varied between species. While absorption of DPX‐KJM44 was greater than absorption of DPX‐MAT28, rush skeletonweed absorbed the most, followed by yellow starthistle and prickly lettuce. Overall, the total translocation of either herbicide was highest in yellow starthistle, followed by rush skeletonweed and prickly lettuce. Proportional herbicide movement between species was similar, with the majority translocating to developing shoots. However, in rush skeletonweed, early translocation was directed to root tissue. In rush skeletonweed, no DPX‐MAT28 metabolism occurred, while DPX‐KJM44 was rapidly de‐esterified and translocated as DPX‐MAT28. CONCLUSION: Aminocyclopyrachlor absorption and translocation are dependent on active ingredient structure and species sensitivity. Highly sensitive species such as prickly lettuce absorb and translocate less material than relatively less sensitive species such as rush skeletonweed. De‐esterification of DPX‐KJM44 appears to delay translocation of the resulting acid in yellow starthistle and rush skeletonweed. Copyright © 2011 Society of Chemical Industry     

Effects of inoculum density,leaf wetness duration and nitrate concentration on the occurrence of lettuce leaf spot

In Ehime Prefecture, Japan, lettuce leaf spot (Septoria lactucae) caused huge losses in marketable lettuce yields. To explore potential measures to control disease outbreaks, the effects of inoculum density, leaf wetness duration and nitrate concentration on the development of leaf spot on lettuce (Lactuca sativa) were evaluated. Conidia were collected from diseased plants in an infested field by single-spore isolation and were used to inoculate potted lettuce plants with different conidial concentrations. Lesions developed on inoculated lettuce plants at inoculum concentrations from 10⁰ to 10⁶ conidia/ml. The disease was more severe when the inoculum exceeded 10² conidia/ml, and severity increased with increasing concentrations. Assessment of the relationship between disease development and the duration of postinoculation leaf wetness revealed that symptoms appeared when the inoculated plants remained wet for 12 h or longer. The number of lesions and total nitrogen content in the lettuce leaves both increased when nitrate was applied.     

Classical and molecular genetics of <Emphasis Type="Italic">Bremia lactucae</Emphasis>, cause of lettuce downy mildew

Lettuce downy mildew caused by Bremia lactucae has long been a model for understanding biotrophic oomycete–plant interactions. Initial research involved physiological and cytological studies that have been reviewed earlier. This review provides an overview of the genetic and molecular analyses that have occurred in the past 25 years as well as perspectives on future directions. The interaction between B. lactucae and lettuce (Lactuca sativa) is determined by an extensively characterized gene-for-gene relationship. Resistance genes have been cloned from L. sativa that encode proteins similar to resistance proteins isolated from other plant species. Avirulence genes have yet to be cloned from B. lactucae, although candidate sequences have been identified on the basis of motifs present in secreted avirulence proteins characterized from other oomycetes. Bremia lactucae has a minimum of 7 or 8 chromosome pairs ranging in size from 3 to at least 8 Mb and a set of linear polymorphic molecules that range in size between 0.3 and 1.6 Mb and are inherited in a non-Mendelian manner. Several methods indicated the genome size of B. lactucae to be ca. 50 Mb, although this is probably an underestimate, comprising approximately equal fractions of highly repeated sequences, intermediate repeats, and low-copy sequences. The genome of B. lactucae still awaits sequencing. To date, several EST libraries have been sequenced to provide an incomplete view of the gene space. Bremia lactucae has yet to be transformed, but regulatory sequences from it form components of transformation vectors used for other oomycetes. Molecular technology has now advanced to the point where rapid progress is likely in determining the molecular basis of specificity, mating type, and fungicide insensitivity.     

Molecular and biological characterization of two potyviruses infecting lettuce in southeastern France

Several potyviruses affect lettuce (Lactuca sativa) and chicory (Cichorium spp.) crops worldwide and are important constraints for production because of the direct losses that they induce and/or because of their seed transmission. Here, the molecular and biological properties are described of two potyviruses that were recently isolated from lettuce plants showing mosaic or strong necrotic symptoms in an experimental field in southeastern France. The first potyvirus belongs to the species Endive necrotic mosaic virus and is present in a large number of wild plant species, especially Tragopogon pratensis. It is unable to infect lettuce cultivars with a resistance to Turnip mosaic virus that is present in many European cultivars and probably conferred by the Tu gene. The second potyvirus belongs to the tentative species lettuce Italian necrotic virus and was not observed in wild plants. It infected all tested lettuce cultivars. Wild accessions of Lactuca serriola, Lactuca saligna, Lactuca virosa and Lactuca perennis were identified as resistant to one or the other potyvirus and could be used for resistance breeding in lettuce. No resistance against these two potyviruses was observed in the tested Cichorium endivia cultivars. In contrast, all tested Cichorium intybus cultivars or accessions were resistant.     

First report of Rhizomonas sp. causing corky root of lettuce in Europe

Typical symptoms of corky root were observed on iceberg lettuce (Lactuca sativa L.) in the Netherlands and England, on prickly lettuce (Lactuca serriola L.) in Spain, and on sowthistle (Sonchus oleraceus L.) in Greece. Slow-growing bacteria with similar colonies as strains ofRhizomonas suberifaciens or otherRhizomonas species were isolated from soil surrounding plants with corky root symptoms using lettuce seedlings as bait. Crude lysate from all strains was tested for DNA homology with DNA fromR. suberifaciens strain CA1 (R. sub. CA1) andRhizomonas sp. strain WI4 (R. sp. WI4). Strains that had homology values higher than that ofR. sp. WI4 orR. sub. CA1 were tested for pathogenicity on 1-wk-old lettuce seedlings, cv. Salinas, Two strains from the Netherlands induced typical symptoms of corky root on lettuce. These strains tested negative with monoclonal antibody MAb-Rs1 specific forRhizomonas suberifaciens in an enzyme-linked immunosorbent assay (ELISA). They had low DNA homology withR. sub. CA1 (4–9%) and low to moderate DNA homology withR. sp. WI4 (4–17%). Some nonpathogenic strains had moderate to high levels of DNA homology withR. sub. CA1 orR. sp. WI4 (19–84%). All strains had fatty acid profiles similar to those ofRhizomonas species. This is the first report ofRhizomonas sp. causing corky root of lettuce in Europe.     

Seed transmission of<Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f.sp.<Emphasis Type="Italic">lactucae</Emphasis>

Twenty-seven seed samples belonging to the lettuce cultivars most frequently grown in Lombardy (northwestern Italy), in an area severely affected by Fusarium wilt of lettuce, were assayed for the presence ofFusarium oxysporum on a Fusarium-selective medium. Isolations were carried out on subsamples of seeds (500 to 1500) belonging to the same seed lots used for sowing, and either unwashed or disinfected in 1% sodium hypochloride. The pathogenicity of the isolates ofF. oxysporum obtained was tested in four trials carried out on lettuce cultivars of the butterhead type, very susceptible to Fusarium wilt. Nine of the 27 samples of seeds obtained from commercial seed lots used for sowing in fields affected by Fusarium wilt were contaminated byF. oxysporum. Among the 16 isolates ofF. oxysporum obtained, only one was isolated from disinfected seeds. Three of the isolates were pathogenic on the tested cultivars of lettuce, exhibiting a level of pathogenicity similar to that of the isolates ofF. oxysporum f.sp.lactucae obtained from infected wilted plants in Italy, USA and Taiwan, used as comparison. The results obtained indicate that lettuce seeds are a potential source of inoculum for Fusarium wilt of lettuce. The possibility of isolatingF. oxysporum f.sp.lactucae, although from a low percent of seeds, supports the hypothesis that the rapid spread of Fusarium wilt of lettuce observed recently in Italy is due to the use of infected propagation material. Measures for prevention and control of the disease are discussed. http://www.phytoparasitica.org posting Dec. 16, 2003.     

Phytogard® and DL-β-amino Butyric Acid (BABA) Induce Resistance to Downy Mildew (Bremia Lactucae) in Lettuce (Lactuca sativa L)

Downy mildew of lettuce (Bremia lactucae) is a serious disease. An alternative to chemicals is the application of disease resistance inducers. The aim of this study was to test whether DL--amino butyric acid (BABA) and Phytogard® (K₂HPO₃) could induce resistance in downy mildew susceptible plants. Aqueous solutions of BABA (0, 10, 20, 30, 50, 80, 100mM) and Phytogard® (0.0, 5.8, 29.0, 40.6, 58.0 and 87.0ppm) were sprayed on seven-day-old seedlings 0, 3, 7 and 15 days before or 1–3 days after inoculation with B. lactucae. Results obtained showed that Phytogard®- and BABA-induced resistance was dose-dependent. At 40.6ppm for Phytogard® and 10mM for BABA, complete protection was obtained. Both compounds had a curative effect and the induced resistance lasted for at least 15 days. It was also shown that both compounds induced systemic resistance in lettuce against downy mildew. Phytogard® at 40.6ppm completely inhibited spore germination while BABA at 20mM did not. Pathogenesis related (PR) protein analysis showed that BABA induced weak accumulation of PR-2, but not PR-1, PR-5 and PR-9. Phytogard® induced none of these proteins. The use of these two compounds to protect lettuce from B. lactucae is discussed.     

Activity of carboxylic acid amide (CAA) fungicides against Bremia lactucae

Four carboxylic acid amide (CAA) fungicides, mandipropamid (MPD), dimethomorph (DMM), benthiavalicarb (BENT) and iprovalicarb (IPRO) were examined for their effects on various developmental stages of Bremia lactucae, the causal agent of downy mildew in lettuce, in vitro and in planta. Spore germination in vitro or on leaf surfaces was inhibited by all CAA fungicides (technical or formulated). MPD was more effective in suppressing germination than DMM or BENT, whereas IPRO was least effective. CAA induced no disruption of F-actin microfilament organisation in germinating spores of B. lactucae. CAA applied to germinating spores in vitro prevented further extension of the germ tubes. When applied to germinated spores on the leaf surface they prevented penetration. Preventive application of CAA to intact plants inhibited infection. MPD was more effective in suppressing infection than DMM or BENT, whereas IPRO was least effective. Curative application was effective at ≤3 h post-inoculation (hpi) but not at ≥18 hpi. CAA (except IPRO) applied to upper leaf surfaces inhibited spore germination on the lower surface and hence reduced infection. CAA suppressed sporulation of B. lactucae on floating leaf discs and when sprayed onto infected plants two days before onset of sporulation. BENT and DMM were more effective in suppressing sporulation than MPD or IPRO. Epidemics of downy mildew in shade-house grown lettuce were suppressed by CAA. A single spray applied to five-leaf plants before transplanting controlled the disease for 50 days. The results suggest that CAA are effective inhibitors of spore germination and therefore should be used as preventive agents against downy mildew of lettuce caused by B. lactucae.     

Phenotypic and histological expression of different genetic backgrounds in interactions between lettuce, wild Lactuca spp., L. sativa × L. serriola hybrids and Bremia lactucae

Phenotypic and histological responses of cultivated lettuce (Lactuca sativa) and wild relatives L. saligna, L.␣virosa as well as interspecific crosses derived from L. sativa × L. serriola to two races of Bremia lactucae (CS2, CS9) were investigated. With the exception of L. sativa genotypes, all accessions and hybrids expressed incomplete or complete resistance to both pathogen races, with slight differences at seedling and adult plant stages, respectively. Histological features of the interactions (development of pathogen infection structures and host hypersensitive response to attempted infection) were studied on leaf discs 48 h after inoculation. Interactions with similar phenotypic expression of resistance were characterized by significant variation in rate of development of pathogen infection structures and hypersensitive reactions. Differences found within eight Lactuca spp. accessions and hybrids challenged by two distinct pathogen races are interpreted and discussed.     

Widespread distribution of resistance to triazole fungicides in Brazilian populations of the wheat blast pathogen

Fungicides have not been effective in controlling the wheat blast disease in Brazil. An earlier analysis of 179 isolates of Pyricularia oryzae Triticum lineage (PoTl) sampled from wheat fields across six populations in central-southern Brazil during 2012 discovered a high level of resistance to strobilurin fungicides. Here we analysed azole resistance in the same strains based on EC₅₀ measurements for tebuconazole and epoxiconazole. All six Brazilian populations of PoTl exhibited high resistance to both azoles, with in vitro EC₅₀ values that were at least 35 to 50 times higher than the recommended field doses. We sequenced the CYP51A and CYP51B genes to determine if they were likely to play a role in the observed azole resistance. Although we found five distinct haplotypes in PoTl carrying four nonsynonymous substitutions in CYP51A, none of these substitutions were correlated with elevated EC₅₀. CYP51B was sequenced for nine PoTl isolates, three each representing low, medium, and high tebuconazole EC₅₀. Both PoTl CYP51A and CYP51B could complement yeast CYP51 function. All PoTl CYP51A-expressing yeast transformants were less sensitive to triazoles than the PoTl CYP51B ones. Transformants expressing PoTl CYP51A haplotype H1 carrying the R158K substitution were not more resistant than those expressing PoTl CYP51A haplotype H5, which is synonymous to haplotype H6, found in triazole-sensitive P. oryzae Oryza isolates from rice blast. Therefore, the reduced triazole sensitivity of wheat blast isolates compared to rice blast isolates appears to be associated with a non-target-site related resistance mechanism acquired after higher exposure to triazoles.