Frequency and geographical distribution of specific virulence factors in Bremia lactucae populations in England from 1973 to 1975

Samples of lettuces infected with Bremia lactucae were collected from seven areas of England during 1973-5. These were tested under controlled environmental conditions on a range of differentially resistant cultivars to determine the frequency of virulence (V) factors in the pathogen population. In most cases the results could be accommodated by the relationship advanced by Crate & Johnson (1976). A virulence phenotype was determined for each isolate and individual virulences within isolates were identified as present at high or low frequencies. The most common V factors were V3, V4, V6 and V8 which reflected the resistance (R) factor complement of lettuce cultivars currently in commercial production. There were few gross differences in the geographical distribution of virulence factors between the seven areas. In the West Central region, however, which encompasses the Evesham and Lancashire production areas, results indicated that the use of cultivars containing resistance (R) factors 2 and 8 respectively would have provided protection from the disease during 1974. There were differences in virulence combinations at the holding or locality level within each large area, which could be exploited by growers providing testing was done in their particular locality. Universally susceptible cultivars and those with only limited resistance factors were often found to be infected with B. lactucae populations which contained large numbers of V-factors. It might have been expected that these pathogen populations would have been restricted to cultivars with large numbers of R-factors. This finding conflicts with the suggestion that pathogen populations with complex virulence patterns are ‘unfit’ as compared with those with more simple virulence components.     

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.     

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.     

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.     

Occurrence of Bremia lactucae in Natural Populations of Lactuca serriola

In the period 1996–2001 the natural occurrence of Bremia lactucae (lettuce downy mildew) on Asteraceae plants was studied in the Czech Republic. Lactuca serriola (prickly lettuce) is the most common naturally growing host species of B. lactucae. Infection of plants was recorded during the whole vegetation season with the first occurrence in April and last in November. Bremia lactucae was found on host plants in all developmental stages. High percentages of naturally infected populations of L. serriola were recorded. Host plants exhibited broad variation in phenotypic expression of disease symptoms and degree of infection, however, the intensity of infection was rather low in the majority of populations. Geographic distribution of B. lactucae was studied in the two main parts of Czech Republic, central and southern Moravia, and eastern, northern and central Bohemia. Bremia lactucae was recorded in all these areas. Nevertheless, in the warmest parts of the Czech Republic (southern Moravia) only sporadic occurrence of the pathogen was recorded. Bremia lactucae infection on L. serriola and disease severity was judged also in relation to the type of habitat, and the size and density of host plant populations. However, no substantial differences among various habitats were found; only host plants growing in urban areas were frequently free of infection and the degree of infection was very low. Nevertheless, these plants were commonly infected with powdery mildew (Golovinomyces cichoracearum), which is most aggressive pathogen of this type of habitat.     

Two Classes of Chromosome-Sized Molecules Are Present in Bremia lactucae

Francis, D. M., and Michelmore, R. W. 1993. Two classes of chromosome-sized molecules are present in Bremia lactucae. Experimental Mycology, 17, 284-300. The size and number of chromosomes from Bremia lactucae have been estimated using pulsed-field gel electrophoresis. The karyotype consists of a minimum of seven chromosomes between 3.0 and at least 8 Mb and a set of polymorphic molecules between 300 kb and 1.6 Mb. Genetic and hybridization analyses confirmed the distinction between the large chromosomes and the smaller polymorphic molecules. The polymorphic molecules are linear, nonribosomal, and located in the nucleus; they are related in sequence and do not segregate in a Mendelian fashion. The polymorphic molecules are therefore either B chromosomes or large linear plasmids. Chromosomes carrying two avirulence genes and several restriction fragment length polymorphism markers have been identified.     

Linkage between genes for resistance to downy mildew (Bremia lactucae) in lettuce

Data are presented for the joint segregation of the gene combinations: Dm-2/Dm-3, Dm-3/Dm-6, Dm-2/Dm-6 and Dm-6/Dm-8, following inoculation with five races of B. lactucae. It is postulated that Dm-2, 3 and 6 comprise a tight linkage group but that Dm-6 and Dm-8 are not linked as has been proposed previously. Information on the reaction of some resistant lettuce cultivars to certain B. lactucae races and some new postulated host genotypes is also presented.     

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.     

The influence of seedling age and development on the infection of lettuce by Bremia lactucae

Laboratory methods are described for the culture and observation of Bremia lactucae on lettuce. Mycelial growth of the fungus within the cotyledons of the cultivar Cheshunt Early Giant progressed to a minimum density at which sporulation occurred, and then continued to a maximum density coinciding with peak sporulation. The timing and intensity of sporulation was influenced by the concentration of the inoculum. Seedlings often became systemically infected and this, together with intensity of sporulation, was a good indicator of fungal growth and hence host susceptibility. The severity of disease development on four susceptible lettuce cultivars (Cheshunt Early Giant, Paris White, Trocadero Improved and Crackling Ice) decreased with age and development of the seedlings at inoculation. The reasons for this are 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.     

The hypersensitive reaction, membrane damage and accumulation of autofluorescent phenolics in lettuce cells challenged by Bremia lactucae

The expression of resistance to Bremia lactucae determined by the resistance genes Dm5/8 and Dm7 in lettuce was examined; incompatibility involved the hypersensitive reaction (HR) which occurred only within penetrated cells at early and late stages of fungal development, respectively. Autofluorescence observed under UV and blue light excitation in cells undergoing the HR was associated with the accumulation of ester-linked syringaldehyde and caffeic acid on plant cell walls. Two phases of phenolic deposition were identified. The first was highly localized around penetration points and occurred during incompatible and compatible interactions. The second and major phase was only activated after the occurrence of irreversible membrane damage in the penetrated cell and was reduced by inhibitors of mRNA synthesis. Fungal structures, primary and secondary vesicles, intercellular hyphae and haustoria also became autofluorescent during incompatible interactions. Changes in the fluorescence due to preformed phenolics located in the plant cell vacuole were found just before plasma membrane damage became irreversible during the HR. In addition to localized deposition of phenolics, increases in the concentrations of the major free phenolic esters identified as dicaffeoyl tartaric and chlorogenic acids also occurred during incompatible interactions. The results suggest that membrane damage in penetrated cells occurs at different rates in resistance controlled by Dm5/8 and Dm7 and indicate an important role for irreversible membrane damage in lettuce as a key signalling event leading to widespread activation of defence responses in surrounding cells.     

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).     

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.