The virulence characteristics of Bremia lactucae populations present in Sweden from 1971 to 1976

One hundred and four Swedish isolates of Bremia lactucae were tested between 1971 and 1976 for their virulence phenotypes. Eighty of these isolates showed patterns of virulence previously recorded in other countries while the remainder conformed to seven previously unreported virulence factor combinations. The simplest resistance factor combination that could give resistance to all isolates collected was R2 plus Rl 1. This combination now exists in some recently bred lettuce cultivars. Most of the possible virulence factor combinations occurred in the population at the expected frequency. However, several combinations, which had not been directly selected (by the growing of cultivars carrying the comparable resistance factor combinations) were present at higher or lower than expected frequencies.     

Incomplete specific resistance to Bremia lactucae in lettuce

The complementary pair of dominant genes. Dm 7/1 and Dm 7/2 and the single dominant gene Dm 6 for specific resistance in Bremia lactucae in lettuce, condition incomplete resistance to non-virulent (incompatible) isolates of B. lactucae in both seedling and mature tissue. This incomplete resistance is characterised by a marked reduction in spotophore production compared with fully susceptible (compatible) race/cultivar combinations and by macroscopically visible hypersenstive cell necrosis. The incomplete resistance. In seedling the resistance conditioned by genes giving more complete resistance. In seedlings the resistance conditioned by genes Dm 7/1 and 7/2 was incomplete in outdoor 'crisp' lettuce genotypes and the resistance conditioned by gene Dm 6 was only completely expressed at 20°C.     

New virulence gene combinations in British isolates of Bremia lactucae

Isolates of Bremia lactucae made in 1974 from new Dutch-bred lettuce cultivars carrying the R gene combinations 3, 4, 7 or 3, 4, 7, 8 proved to have the V gene combination 1, 3, 4, 5, 6, 7, 8 predominant. An isolate obtained from cv. Larganda (R2, 7) was found to overcome all ten R genes identified in lettuce cultivars to date and could be designated V 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. The virulence combinations 3, 7, and 2, 3, 7 have not previously been reported in British B. lactucae collections although the former has been found in Holland and both occur in Sweden. The practical implications of these findings are discussed.     

Pathogenic variation and sexual reproduction in Swedish populations of Bremia lactucae

Summary The host-pathogen interaction between lettuce (Lactuca sativa) and downy mildew (Bremia lactucae) is mainly differential and the resistance so far utilized in the host is vertical. As in many other obligate parasites, the introduction of cultivars with new vertical resistance has exerted a strong selection pressure on the pathogen resulting in significant changes in virulence frequencies and in the establishment of races with new combinations of virulence. Genetic diversity in pathogen populations may arise through mutation and gene flow, and new virulence genotypes may then be established through parasexuality and sexual recombination. In Swedish populations of Bremia lactucae, the pattern of variation in the parasite agrees well with that which might be expected in a diploid, outcrossing organism with frequent sexual reproduction. This is supported by: two or more isolates, different in virulence and mating type, may occur together on the same lettuce leaf; zygotes (oospores) are formed in all populations investigated and the frequency varies from 22% to 98%; oospores germinate rather frequently under suitable conditions. To breed for resistance in dynamic host-pathogen systems such as this one is difficult and the program should preferably be based on race-non-specific resistance.     

Resistance to Bremia lactucae in natural populations of Lactuca saligna from some Middle Eastern countries and France

The results of the first detailed screening of a resistance to Bremia lactucae in naturally growing populations of Lactuca saligna are presented here. In total, 146 accessions from 25 populations of L. saligna originating in Israel (N = 136), France (N = 8), Jordan (N = 1) and Turkey (N = 1) were tested at seedling stage for their resistance to 10 highly virulent isolates (races) of B. lactucae from Lactuca sativa (DEG2, Bl:5, Bl:15, Bl:16, Bl:17, Bl:18, Bl:21, Bl:22, Bl:24 and Bl:25). Our study strongly supports the suggestion that L. saligna is indeed generally highly resistant to B. lactucae. However, our results provide evidence that at least at a seedling stage L. saligna may not be a non‐host plant for B. lactucae, as was hypothesised for approximately the last 30 years. Some accessions expressed a differential (i.e. race‐specific) response, which accords with other recently published data for this Lactuca species. Furthermore, some geographical differences in race‐specific resistance were observed, too. Tests performed at an adult‐plant stage, however, did not prove race‐specificity of the respective accessions. To summarise, what is behind the race‐specific character of the responses observed at a seedling stage is still uncertain, as is its comparability with the race‐specific resistance of some other Lactuca species such as L. sativa or L. serriola. The presence of plant stage‐dependent resistance, governed by a combined effect of different quantitative trait loci in young and adult plants of L. saligna, is discussed.     

The genetics of race specific resistance in lettuce (Lactuca sativa) to downy mildew (Bremia lactucae)

Data are presented on the segregation of resistance to five British races and two Dutch races of Bremia lactucae in the F₂ progenies of crosses involving seven resistant and several susceptible lettuce cultivars. These data and also those previously published by other workers are considered in relation to the systematic model proposed by Crute & Johnson (1976) to explain the genetics of race specific resistance to B. lactucae in lettuce. It is shown that, with minor modifications, the model accommodates almost all of the previously published data and correctly predicts the new data, except for one set which cannot at present be interpreted. It is concluded that genetic evidence exists for the presence, among various cultivars of lettuce, of at least four and possibly five different dominant resistance genes of major effect designated Dm₂, Dm₃, Dm₄, Dm₆ and Dm₈; and of a pair of dominant genes with complementary effect designated Dm7/1 and Dm7/2. The resistance conferred by these genes is specified in relation to five British races, five Dutch, three Israeli and one United States race of the fungus. Resistance genotypes are proposed for cultivars Avoncrisp, Avondefiance, Calmar, Great Lakes 659, Kares, Meikoningen, Mildura, Proeftuins Blackpool, Solito, Valverde, Ventura and the USDA line PI 164937.     

Race specific resistance to lettuce downy mildew (Bremia lactucae) in the lettuce cultivar Bourguignonne Grosse Blonde d'Hiver

A stock of lettuce cv. Bourguignonne Grosse Blonde d'Hiver used in previous studies was shown to carry the race specific resistance factor R1 and not R9 as originally thought. Investigations into a commercial stock of cv. Bourguignonne which exhibited a heterogeneous reaction to some B. lactucae isolates showed that it was a mixture of two lines. Each line carried one of two R-factors (referred to as R9A and R9B) in a homozygous condition, although their identification was made difficult by the presence of R5/8. No plants were found which carried both R9A and R9B.     

Specificity of Interactions Between Wild Lactuca spp. and Bremia lactucae Isolates from Lactuca serriola

Specificity of interactions between eight Lactuca species and 8 Bremia lactucae isolates was studied in seedlings and adult plants of 36 Lactuca accessions plus one L. serriola × L. sativa hybrid. Pathogenicity of the isolates and/or plant susceptibility was expressed by sporulation intensity. A highly compatible relationship was observed in all of L. serriola accession/isolate interactions tested. A differential reaction was found in numerous cases testifying to physiological specialization of the pathogen in a wild pathosystem. Nonspecific nondifferential compatibility (quantitative resistance) can be expected in L. serriola PI 281876. Age dependent resistance (seedlings versus adult plants, and vice versa) and heterogeneity of reactions were also recorded. The comparisonof B. lactucae isolates from L. sativa and L. serriola has shown a significant shift of pathogenicity in favour of L. serriola accessions. A high level of resistance was found in the L. serriola × L. sativa hybrid. The existence of basic incompatibility can be expected in L. saligna and L. virosa as well as in such taxonomically remote species as L. viminea, L. squarrosa and L. biennis. Other taxonomically remote species, i.e. L. dentata and L. alpina, exhibited a compatible reaction, although the reaction of the former was differential. A high level of sporulation was recorded in all accession/isolate interactions of L. alpina.     

Further work on the genetics of race specific resistance in lettuce (Lactuca sativa) to downy mildew (Bremia lactucae)

Data are presented on the segregation of resistance to four races of Bremia lactucae in the F2 progenies of crosses involving 15 resistant and various susceptible lettuce cultivars. Most of these data and those recently published by other workers fit the systematic model for the genetics of race specific resistance to B. lactucae proposed by Crute & Johnson(1976). Seven different dominant resistance genes of major effect were found. There was also evidence of a pair of dominant genes with complementary effect, one of which may sometimes be effective on its own. Two of the genes may be linked and another may have two different alleles for resistance at the same locus. The resistance conferred by these genes is specified in relation to two British, two Dutch and four French races of the fungus. Resistance genotypes are proposed for 16 cultivars.     

First Record of Bremia lactucae Infecting Sonchus oleraceus and Sonchus asper in Brazil and its Infectivity to Lettuce

Bremia lactucae is recorded for the first time causing downy mildew on common sowthistle (Sonchus oleraceus) and spiny sowthistle (Sonchus asper) in Brazil. The disease and etiological agent are described. Pathogencity of sporangia obtained from S. oleraceus was tested on 12 species belonging to the Asteraceae, already recorded in the literature as hosts of B. lactucae, and four commonly cultivated varieties of lettuce. All four cultivars of lettuce, S. oleraceus and S. asper showed symptoms of the disease and sporulation of the pathogen 5 days after inoculation. It has been observed that the disease occurs on Sonchus spp. throughout the year in Viçosa (state of Minas Gerais), being more common on these two hosts than on lettuce. Both weeds are very common invaders of vegetable growing areas in Brazil. This indicates that those two hosts for the fungus may be important inoculum reservoirs for disease occurring in lettuce, highlighting the need for control of these weeds as part of the procedures aimed at controlling this disease. These results are also in agreement with the views that B. lactucae is split into host‐specific infra‐specific taxa. Lettuce and sowthistles are regarded as closely related, belonging to the same subfamily and tribe in the Asteraceae (Subfamily Cichorioideae: Tribe Lactuceae).     

Virulence of Bremia lactucae in Sweden and race-specific resistance of lettuce cultivars to Swedish isolates of the fungus

Virulence surveys of Swedish Bremia lactucae populations confirmed that the virulence factors vl to v12 were present in high or very high frequencies. Virulence associated with recently defined new resistance genes was also present. Laboratory tests of lettuce cultivars and Lactuca accessions using different Bremia isolates and field tests with natural inoculum showed that previously undetected virulence factors were present. Due to a lack of highly effective genes for specific resistance and the frequent sexual recombination of virulence genes it is suggested that any future breeding programmes concentrate on non-specific resistance.     

Biodiversity of Lactuca aculeata germplasm assessed by SSR and AFLP markers,and resistance variation to Bremia lactucae

In total, seventy two Lactuca aculeata and three Lactuca serriola samples originating from natural populations of these species in Turkey, Jordan, and Israel were analysed by eight microsatellite and 287 amplified fragment length polymorphism (AFLP) markers. Neighbor–Network and Bayesian clustering were used for visualisation of the differences among the analysed L. aculeata and L. serriola samples, and to confirm hybrid origin (L. aculeata × L. serriola) of three samples (343-8A, 343-8B, 54/07) previously indicated by their morphological traits. Molecular data reflect the geographical origin, i.e., the clustering of samples according to their country of origin. Samples from neighbouring parts of Jordan and Israel expressed similar genetic characteristics, indicating the possibility of migration or artificial introduction of plant material. Forty-one L. aculeata samples were screened for their response to five Bremia lactucae races (Bl: 17, Bl: 18, Bl: 24, Bl: 27, and Bl: 28). Susceptible reactions of L. aculeata prevailed. L. aculeata samples were most frequently susceptible to races Bl: 18, Bl: 24, Bl: 27, Bl: 28; and least susceptible to Bl: 17. No highly efficient source of resistance was detected; however, race-specific reaction patterns were frequently recorded, indicating the possible presence of some race-specific resistance factors/genes in the studied samples of L. aculeata. Conservation and exploitation of this material in lettuce breeding is discussed.     

Further Investigation of the Specificity of Interactions Between Wild Lactuca spp. and Bremia lactucae Isolates from Lactuca serriola

Callus cultures derived from isogenic lines of the tomato cultivars Moneymaker and Craigella, resistant or susceptible to F. oxysporum f. sp. lycopersici, were inoculated with Fusarium oxysporum f. sp. lycopersici race 1. Fungal growth was restricted on callus derived from resistant plants, after inoculation with a conidial suspension, whereas callus derived from susceptible plants was totally overgrown by the fungus within 7 days. The concentration of the phytoalexin rishitin was significantly higher in the callus culture derived from a resistant tomato line compared with the callus culture from a susceptible line, 2 and 3 days after inoculation with mycelium. The results of the experiments were compared with experiments with whole plants. Rishitin production as well as growth of the fungus was comparable with responses in plant-fungus interaction. Therefore callus culture may be useful in studying the interaction between tomato plants and race 1 of F. oxysporum f. sp. lycopersici.     

Linkage Between Virulence Genes, Compatibility Types and Sexual Recombination in the Swedish Population of Bremia lactucae

The host pathogen interaction between Lactuca sativa and Bremia lactucae fits a gene-for-gene model well. Twelve resistance genes of the host are matched by twelve genes for virulence in the pathogen. The evolution of the parasite involves drastic changes in virulence frequencies, and a great diversity in virulence even on a sub-poipulation level. Bremia is a heterothallic, obligate parasite, in which presence of two mating types is needed for sexual reproduction. Sexual recombination probably occurs frequently, indicated by simultaneous occurrence of mating types in commercial lettuce crops, zygote formation, and sufficiently high oospore germination. The pattern of variation agrees well with that of a diploid, out- crossing organism with frequent sexual recombination. Unexpected high frequencies of some of the unnecessary v-genes are probably due to genetic linkage with another "necessary" v-gene.     

Linkage analysis of genes for resistance to downy mildew (Bremia lactucae) in lettuce (Lactuca sativa)

Summary The genetics of specific resistance was studied in F₂ populations which segregated for either one or two resistance genes. The resistance factors 1, 11 and 14 which had not previously been characterized genetically segregated as single dominant genes (Dm). Resistance was determined by three linkage groups; R 1/14, 2, 3, and 6 in the first, R 5/8, and 10 in the second and R 4, 7 and 11 in the third. Cultivars of lettuce commonly used in the differential series to detect virulence to R3 and R10, were demonstrated to carry two tightly linked resistance genes. Implications of this linkage arrangement to the manipulation and characterization of these resistance genes are discussed.     

草地贪夜蛾核型多角体病毒对我国草地贪夜蛾不同地理种群毒力的比较分析

草地贪夜蛾Spodoptera frugiperda(J. E. Smith)入侵我国多个地区,逐渐形成地理种群。在草地贪夜蛾核型多角体病毒Spodoptera frugiperda multiple nucleopolyhedrovirus(SfMNPV)生物农药的生产过程中发现,不同地理来源的宿主对SfMNPV的敏感性和产量存在明显差异,显著影响了SfMNPV的生产效率。为解析其敏感性差异及其产生的原因,本研究首先对云南德宏、广东广州、广西钦州、西藏林芝4个地区草地贪夜蛾种群基因型进行了鉴定,采用生物测定法测试SfMNPV对4龄幼虫的口服毒力,然后,通过向幼虫体内注射草地贪夜蛾核多角体病毒出芽型病毒粒子(budded virions, BVs)的方式,越过口服感染中肠的过程,分析敏感性差异发生的阶段。最后比较了高敏感和低敏感种群中肠肠液pH,并基于16S rDNA测序测定了肠道菌群组成。结果表明,广西种群属于纯合玉米型,其余种群为带有水稻型COI标记的杂合玉米型。广西种群对SfMNPV的口服敏感性最低,西藏种群的敏感性最高,但两者注射BV后死亡率差异无统计学意义,暗示病毒敏感性差异...     

A genetic map of the lettuce downy mildew pathogen,Bremia lactucae,constructed from molecular markers and avirulence genes

The genetic map of Bremia lactucae was expanded utilizing 97 F(1) progeny derived from a cross between Finnish and Californian isolates (SF5xC82P24). Genetic maps were constructed for each parent utilizing 7 avirulence genes, 83 RFLP markers, and 347 AFLP markers, and a consensus map was constructed from the complete data set. The framework map for SF5 contained 24 linkage groups distributed over 835cM; the map for C82P24 contained 21 linkage groups distributed over 606cM. The consensus map contained 12 linkage groups with markers from both parents and 24 parent-specific groups. Six avirulence genes mapped to different linkage groups; four were located at the ends of linkage groups. The closest linkages between molecular markers and avirulence genes were 3cM to Avr4 and 1cM to Avr7. Mating type seemed to be determined by a single locus, where the heterozygote determined the B(2) type and the homozygous recessive genotype determined the B(1) type.