Identification of a bacterium isolated from galls on carrot and weeds

In 2004, bacterial galls were found on the roots of carrots in Shizuoka Prefecture, Japan. Galls were about 0.1–2 cm in diameter, light brown in color and had rough surfaces. In 2005, similar galls were found on the roots of three weeds: henbit (Lamium amplexicaule L.), Persian speedwell (Veronica persica Poir.) and leaf mustard (Brassica juncea L.). A bacterium that forms white, rough colonies was isolated from the carrot and weeds galls. The bacterial isolates had properties identical with Rhizobacter dauci Goto and Kuwata. Phylogenetic analysis based on 16S rDNA sequences showed that the carrot isolate had the highest homology (similarity of 100%) with that of the type strain of R. dauci. Rep-PCR genomic fingerprinting using BOX A1R primer showed that the carrot and weeds isolates were nearly identical. Pathogenicity of the isolates was confirmed by inoculating the roots of carrots and the weeds. After 2–5 weeks, they formed galls on the roots of the original host species and on other plant species tested. The galls were indistinguishable from those formed naturally, and the inoculated bacterium was reisolated. Thus, the causal bacterium of carrot and weeds gall was identified as R. dauci, and the bacterium was found to have a wider host range than previously known. These weed hosts may serve as inoculum sources for carrot bacterial gall disease.     

Characterization of fungal pathogens (Diaporthe angelicae and D. eres) responsible for umbel browning and stem necrosis on carrot in France

A collection of 102 Diaporthe isolates was compiled from lesions on carrot, parsley and wild Apiaceae species in France from 2010 to 2014. Molecular typing based on ITS rDNA sequences resulted in the identification of 85 D. angelicae and 17 D. eres isolates. Based on sequences of the 3′ part of the IGS rDNA, intraspecific variability was analysed for 17 D. angelicae and 13 D. eres isolates from diverse plant species, locations in France, and plant tissues. The genetic diversity was greater for D. angelicae isolates than D. eres isolates. In vitro sensitivity of five D. angelicae and four D. eres isolates to each of nine fungicides was similar for isolates of both species, with a marked variation in fungicide sensitivity depending on the active ingredient. To assess the pathogenicity of D. angelicae and D. eres isolates on carrot, one isolate of each species was inoculated onto umbels in a controlled environment. Typical lesions were observed for both isolates. Carrot crop debris collected from a seed production field in France and placed in controlled conditions produced perithecia and ascospores typical of Diaporthe, that were further characterized molecularly as belonging to D. angelicae. Detection of Diaporthe species on seed lots from three carrot production fields in France was investigated. Both species were detected on seeds by conventional PCR assay, with a greater frequency for D. angelicae than D. eres (67% vs 33%, respectively). Overall, the results highlighted that umbel browning in carrot seed crops in France was mainly caused by D. angelicae.     

Susceptibility of wild carrot (Daucus carota ssp. carota) to Sclerotinia sclerotiorum

Sclerotinia soft rot, caused by Sclerotinia sclerotiorum, is a severe disease of cultivated carrots (Daucus carota ssp. sativus) in storage. It is not known whether Sclerotinia soft rot also affects wild carrots (D. carota ssp. carota), which hybridise and exchange genes, among them resistance genes, with the cultivated carrot. We investigated the susceptibility of wild carrots to S. sclerotiorum isolates from cultivated carrot under controlled and outdoor conditions. Inoculated roots from both wild and cultivated plants produced sclerotia and soft rot in a growth chamber test. Two isolates differed significantly in the ability to produce lesions and sclerotia on roots of both wild carrots and cv. Bolero. Flowering stems of wild carrots produced dry, pale lesions after inoculation with the pathogen, and above-ground plant weight was significantly reduced 4 weeks after inoculation in a greenhouse test. Wild and cultivar rosette plants died earlier and fewer plants survived when inoculated with the pathogen under outdoor test conditions. Cultivar plants died earlier than wild plants, but survived as frequently. Plants inoculated in the crown died earlier and at a lower frequency than plants inoculated on leaves. Wild carrots may thus serve as a host of S. sclerotiorum and thus eventually benefit from any uptake of resistance genes, among them transgenes, via introgression from cultivated carrots.     

Variability in aggressiveness of Quambalaria pitereka isolates

Quambalaria shoot blight, caused by the fungal pathogen Quambalaria pitereka, is a serious disease of eucalypt plantations in Australia. The aggressiveness of four Q. pitereka isolates was compared on a range of host genera, species, provenances and clones. Isolates differed substantially in their aggressiveness, with two consistently showing higher levels of aggressiveness based on incidence and severity of disease and lesion size. Isolates derived from Corymbia citriodora subsp. variegata (Ccv) and C. torelliana were shown to have a relatively restricted host range, with lesions but no sporulation found on Eucalyptus species, Angophora species other than A. costata and Corymbia species other than Ccv, the host of origin. The level of aggressiveness toward the different provenances of spotted gum and C. torelliana varied between isolates and there was evidence of some isolate × host interaction within provenances of Ccv. The two methods of inoculation used in this study, spray and spot inoculation, gave similar results. However, the fact that the spot inoculation method was labour‐intensive was a disadvantage limiting the numbers of isolates and hosts that can be tested.     

The threat of wild habitat to scab resistant apple cultivars

Evaluations of plant resistance to pathogens are rarely made using isolates from wild habitats, although the heterogeneity of such habitats may generate pathogen diversity which could be a source of new virulence in cultivated habitats. The aim of this study was to investigate whether scab resistance factors, identified and characterized in apples using isolates of Venturia inaequalis from a cultivated habitat, remained effective against isolates from a wild habitat. Three V. inaequalis core collections originating from the cultivated apple Malus × domestica and from two wild species, M. sieversii and M. sylvestris, were established to maximize pathogen diversity. For each core collection, 10 isolates were inoculated in mixtures onto 51 genotypes from an apple progeny segregating for two qualitative resistance genes and six quantitative resistance loci (QRL). On each apple genotype, isolates that contributed to the scab symptoms were identified within the mixture using microsatellite markers. The most frequently detected isolates were inoculated singly to compare their aggressiveness according to their host origin. The results showed that isolates from a wild habitat were able to infect the susceptible apple genotypes. However, these isolates were never more aggressive than isolates from the cultivated habitat on the resistance factors tested. It can therefore be concluded that the resistance factors used in this study, identified with V. inaequalis isolates from a cultivated habitat, remained effective against isolates from M. sylvestris and M. sieversii.     

Contrasting species boundaries between sections Alternaria and Porri of the genus Alternaria

The fungal genus Alternaria comprises a large number of asexual taxa with diverse ecological, morphological and biological modes ranging from saprophytes to plant pathogens. Understanding the speciation processes affecting asexual fungi is important for estimating biological diversity, which in turn affects plant disease management and quarantine enforcement. This study included 106 isolates of Alternaria representing five phylogenetically defined clades in two sister sub‐generic groups: section Porri (A. dauci, A. solani and A. limicola) and section Alternaria (A. alternata/tenuissima and A. arborescens). Species in section Porri are host‐specific while species in section Alternaria have wider host ranges. For each isolate, DNA sequences of three genes (Alt a1, ATPase, Calmodulin) were used to estimate phylogenies at the population and species levels. Three multilocus haplotypes were distinguished among A. dauci isolates and only one haplotype among A. solani and A. limicola isolates, revealing low or no differentiation within each taxon and strong clonal structure for taxa in this section. In contrast, 37 multilocus haplotypes were found among A. alternata/tenuissima isolates and 21 multilocus haplotypes among A. arborescens isolates, revealing much higher genotypic diversity and multiple clonal lineages within taxa, which is not typical of asexual reproducing lineages. A species tree was inferred using a Yule Speciation model and a strict molecular clock assumption. Species boundaries were well defined within section Porri. However, species boundaries within section Alternaria were only partially resolved with no well‐defined species boundaries, possibly due to incomplete lineage sorting. Significant association with host specificity seems a driving force for speciation.     

Genetic diversity and pathogenicity of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> isolated from wilted rocket plants in Italy

Thirty-six isolates of Fusarium oxysporum originated from Eruca vesicaria and Diplotaxis tenuifolia together with eight reference strains belonging to the formae speciales raphani, matthioli and conglutinans, typical on the Brassicaceae family, were tested for pathogenicity on two species of rocket plants (E. vesicaria L., syn. E. sativa, cv. ‘Rucola coltivata’; and D. tenuifolia cv. ‘Winter’) cultivated in the glasshouse. The results showed that different isolates were slightly, moderately or highly virulent. The strains were examined for differences in the nucleotide sequence of the ribosomal DNA (rDNA) intergenic spacer (IGS) region, about 2.5 kb long. The phylogenetic (neighbor-joining) analysis performed on the isolates enabled identification of four different groups, named I, II, III and IV. Thirty-one isolates out of 36 clustered in group I and were genetically similar to F. oxysporum f.sp. raphani. By considering the pathogenicity of the strains included in Group I, a partial host specialization could be observed: the average disease index of the isolates from D. tenuifolia was higher on wild rocket, whereas the average disease index of the isolates from E. vesicaria was higher on cultivated rocket. Moreover, isolates from cultivated rocket showed, on average, a higher degree of aggressiveness than the isolates from wild rocket. Concerning Group I, the sequence analysis confirmed the homogeneity of the population, with only five parsimony-informative SNPs and five haplotypes. Twenty-six out of 31 isolates belonged to haplotype 1. Groups II and III were genetically similar to strains of F. oxysporum f.sp. matthioli. Three other strains, not pathogenic or with a medium level of virulence, clustered together in Group 4, but their sequence was distant from that of other formae speciales. The pathogenicity and IGS analysis confirmed the presence of virulence variation and genetic diversity among the F. oxysporum isolates studied. To our knowledge, this is the first report of differentiation of formae speciales of F. oxysporum on rocket plants by IGS analysis.     

Assessing host specialization of Botrytis cinerea on lettuce and tomato by genotypic and phenotypic characterization

This study tested the hypothesis that Botyrtis cinerea shows host specialization on tomato and lettuce, using phenotypic and genotypic tools. Strains were isolated from tomato and lettuce grown together in the same greenhouse. Forty‐four lettuce strains and 42 tomato strains were investigated for their genetic diversity and their aggressiveness. Both gene diversity and allelic richness were significantly higher in lettuce strains than in tomato strains (P = 0·01). Cluster analysis revealed a clear division of the strains under study into two clusters. However, this structure did not separate the strains according to their host of origin. Tomato strains were significantly more aggressive than lettuce strains when inoculated on tomatoes (P = 0·001), but no significant differences in aggressiveness were observed when the strains were inoculated on lettuce (P = 0·17) or on apple (P = 0·87). The results suggest an absence of clear host specialization of B. cinerea on tomato and lettuce.     

Influence of rate of soil fertilization on alternaria leaf blight (Alternaria dauci) in carrots

The possibility of suppressingAlternaria dauci (Kühn) Groves & Skolko, the causal agent of Alternaria leaf blight in carrot, by excess application of fertilizer was examined in greenhouse and field experiments. Reducing the rate of fertilization by one half from the optimal rate (100 ppm N, 19 ppm P and 74 ppm K) resulted in a 23–30% increase in the severity of Alternaria leaf blight. However, doubling the rate of fertilization resulted in only a 10–15% decrease in disease severity. Inoculating with different concentrations ofA. dauci spores (10³ or 10⁴ spores/ml) did not alter the response of the plants to the fertilization rate, although significantly higher disease severity was observed in plants inoculated with the higher spore concentration. These results were corroborated in the field, where neither disease severity nor harvested yield was significantly affected by tripling the amount of soil fertilization. Application of foliar fungicides, on the other hand, had substantial effects on both disease and yield. Therefore, it was concluded that carrot crops should be fertilized and maintained for optimum yield, and thatA. dauci should be managed by properly timed applications of fungicides during the growing season. Contribution no. 533/99 from the Inst. of Plant Protection, Agricultural Research Organization.     

Biological and molecular variation amongst Australian Turnip mosaic virus isolates

Turnip mosaic virus (TuMV) causes crop losses worldwide. Eight Australian TuMV isolates originally obtained from five different species in two plant families were inoculated to 14 plant species belonging to four families to compare their host reactions. They differed considerably in virulence in Brassicaceae crop species and virus indicator hosts belonging to three other families. The isolates infected most Brassica species inoculated, but not Raphanus sativus, usually causing systemic mosaic symptoms, so they resembled TuMV biological host type [B]. Whole genome sequences of seven of the Australian isolates were obtained and had lengths of 9834 nucleotides (nt). When they were compared with 37 non‐recombinant TuMV genomes from other continents and another whole genome from Australia, six of them formed an Australian group within the overall world‐B phylogenetic grouping, while the remaining new genome sequence and the additional whole genome from Australia were part of the basal‐B grouping. When the seven new Australian genomes and the additional whole genome from Australia were subjected to recombination analysis, six different recombination events were found. Six genomes contained one or two recombination events each, but one was non‐recombinant. The non‐recombinant isolate was in the Australian grouping within the overall world‐B group while the remaining recombinant isolates were in the basal‐B and world‐B phylogenetic groups.     

Molecular identification of Fusarium spp. associated with bakanae disease of rice in Italy and assessment of their pathogenicity

Between 2006 and 2008, 146 isolates of Fusarium spp. were obtained from bakanae‐diseased rice plants and seeds from the major rice‐growing regions of Italy. These isolates were identified based on translation elongation factor (EF‐1α) sequence and pathogenicity tests were used to assess their aggressiveness against the susceptible rice cultivar Galileo. Use of the EF‐1α sequence gave reliable identification and showed that Fusarium fujikuroi, the causal agent of bakanae disease, was the most abundant Fusarium spp. isolated. These data were confirmed by inoculation of the isolates to rice seeds which were then germinated in the greenhouse, showing that only F. fujikuroi isolates were able to cause bakanae disease. Pathogenic isolates were identified with different levels of aggressiveness. Phylogenetic analysis based on EF‐1α sequences generated a tree which separated the various Fusarium species into different clusters with high bootstrap values.     

PCR‐based identification of Pythium spp. causing cavity spot in carrots and sensitive detection in soil samples

On the basis of ITS sequences PCR primers were designed for the identification of the five Pythium species found to be most important for the development of carrot cavity spot in Norway: P. intermedium, P. sulcatum, P. sylvaticum, P. violae and P. ‘vipa’. The P. ‘vipa’ isolates had a unique ITS sequence, differed morphologically from all other Pythium isolates, and thus probably represent a new species. The PCR primers were species‐specific with no cross‐reaction to other Pythium species or to fungal isolates from carrot tested. The detection limits varied for the different primer pairs. The two most sensitive assays allowed detection of as little as 5 fg DNA. All five Pythium species could be detected in lesions from diseased carrots. Weak positive signals were obtained from some carrot samples without symptoms. PCR assays allowed detection of pathogens in soil. In samples of soil known to produce cavity spots on cropped carrots, strong signals were obtained. In several soil samples more than one of the five Pythium species could be detected. The utilization of this diagnostic PCR assay in analysis of field soil and carrot tissue might in the future be exploited to reduce the incidence of this serious carrot disease.     

Most Colletotrichum species associated with tree tomato (Solanum betaceum) and mango (Mangifera indica) crops are not host‐specific

An important constraint for crop production in Colombia is the high incidence of anthracnose caused by Colletotrichum species. Although several studies have focused on these fungi, the relationship between the different fungal species within the genus and their hosts and whether they display any host preference or host specificity has yet to be examined. In Colombia, diseases caused by Colletotrichum species are particularly severe in mango (Mangifera indica) and tree tomato (Solanum betaceum) crops. In a previous investigation, the Colletotrichum phylogenetic species attacking these crops were identified. The present study aimed to determine whether isolates collected from tree tomato and mango showed host preference or host specificity by assessing aggressiveness, spore density, latent period, and fitness of each strain on the two hosts. In the departments of Cundinamarca and Tolima, Colombia, isolates were collected from plants that presented typical anthracnose symptoms and were identified as C. acutatum, C. asianum, C. boninense, C. gloeosporioides, C. tamarilloi and C. theobromicola. Inoculation of conidia of each isolate onto both hosts showed isolates had no host preference and only the C. gloeosporioides isolate showed host specificity. However, in general, isolates produced a higher spore density when inoculated on the alternate host, which may indicate a difference in the degree of adaptation to each host. Statistical analyses of the assessed parameter values revealed that isolates use different infection strategies when infecting each host. In light of these results, the implications of using quantitative estimations of fitness when studying fungal pathogens are discussed.     

Species composition,toxigenic potential and pathogenicity of Fusarium graminearum species complex isolates from southern Brazilian rice

This study aimed to assess the extent and distribution of Fusarium graminearum species complex (FGSC) diversity in rice seeds produced in southern Brazil. Four species and two trichothecene genotypes were detected among 89 FGSC isolates, based on a multilocus genotyping assay: F. asiaticum (69·6%) with the nivalenol (NIV) genotype, F. graminearum (14·6%) with the 15‐acetyldeoxynivalenol (ADON) genotype, and F. cortaderiae (14·6%) and F. meridionale (1·1%), both with the NIV genotype. Seven selected F. asiaticum isolates from rice produced NIV in rice‐based substrate in vitro, at levels ranging from 4·7 to 84·1 μg g^?1. Similarly, two F. graminearum isolates from rice produced mainly 15‐ADON (c. 15–41 μg g^?1) and a smaller amount of 3‐ADON (c. 6–12 μg g^?1). One F. meridionale and two F. cortaderiae isolates did not produce detectable levels of trichothecenes. Two F. asiaticum isolates from rice and two from wheat (from a previous study), and one F. graminearum isolate from wheat, were pathogenic to both crops at various levels of aggressiveness based on measures of disease severity in wheat spikes and rice kernel infection in a greenhouse assay. Fusarium asiaticum and the reference F. graminearum isolate from wheat produced NIV, and deoxynivalenol and acetylates, respectively, in the kernels of inoculated wheat heads. No trichothecene was produced in kernels from inoculated rice panicles by any of the isolates. These findings constitute the first report of FGSC composition in rice outside Asia, and confirm the dominance of F. asiaticum in rice agroecosystems.     

The endophytic fungus Piriformospora indica protects wheat from fusarium crown rot disease in simulated UK autumn conditions

The root endophytic fungus Piriformospora indica (Sebacinacea) forms mutualistic symbioses with a broad range of host plants, increasing their biomass production and resistance to fungal pathogens. This study evaluated the effect of P. indica on fusarium crown rot disease of wheat, under in vitro and glasshouse conditions. Interaction of P. indica and Fusarium isolates under axenic culture conditions indicated no direct antagonistic activity of P. indica against Fusarium isolates. Seedlings of wheat were inoculated with P. indica and pathogenic Fusarium culmorum or F. graminearum and grown in sterilized soil‐free medium or in a non‐sterilized mix of soil and sand. Fusarium alone reduced emergence and led to visible browning and reduced root growth. Roots of seedlings in pots inoculated with both Fusarium isolates and P. indica were free of visible symptoms; seed emergence and root biomass were equivalent to the uninoculated. DNA was quantified by real‐time polymerase chain reaction (qPCR). The ratio of FusariumDNA to wheat DNA rose rapidly in the plants inoculated with Fusarium alone; isolates and species were not significantly different. Piriformospora indica inoculation reduced the ratio of Fusarium to host DNA in the root systems. The reduction increased with time. The ratio of P. indica to wheat DNA initially rose but then declined in root systems without Fusarium. With Fusarium, the ratio rose throughout the experiment. The absolute amount of FusariumDNA in root systems increased in the absence of P. indica but was static in plants co‐inoculated with P. indica.     

Pathogenic and molecular comparison of Puccinia kuehnii isolates and reactions of sugarcane varieties to orange rust

Sugarcane orange rust, a disease caused by Puccinia kuehnii, was first reported in Brazil in 2009. There are no studies comparing the Brazilian P. kuehnii collections and the reaction of important sugarcane varieties under controlled conditions. This work compared the reaction of seven sugarcane varieties inoculated with six different P. kuehnii isolates from Brazilian sugarcane areas and verified the pathogenic and genetic variability of these isolates. The incubation (I) and latency (L) disease periods, disease severity (SEV), total number of lesions (TNL), total number of sporulating lesions (TNSL), and percentage of sporulating lesions (%SL) were evaluated. Furthermore, ITS1 and IGS ribosomal sequences of all P. kuehnii isolates used in this study were compared with pathogen sequences from 13 different countries. The disease incubation ranged from 7 to 10 days and the latency ranged from 10 to 21 days. SEV and TNL showed large variations and few significant differences between the reaction of the varieties to P. kuehnii, in contrast with the variables TNSL and %SL. The P. kuehnii isolates did not compose different virulent races, but the isolate from one site (Araras) was a more aggressive race. The ITS1 and IGS ribosomal sequences of six P. kuehnii isolates were identical with each other and to most P. kuehnii American sequences deposited at GenBank. The studied sequences of P. kuehnii isolates differed from the sequences from Asia, Tahiti and Oceania.     

Heterogeneity of Pseudomonas savastanoi populations infecting Myrtus communis in Sardinia (Italy)

Genetic, phenotypic and host range diversity among Pseudomonas savastanoi isolates from Myrtus communis were investigated. Thirty‐one isolates from six Sardinian commercial myrtle orchards and three isolates from plants growing spontaneously on the island of Rhodes (Greece) were compared with reference strains of P. savastanoi from olive, oleander, ash and myrtle. Multilocus sequence analysis (MLSA) indicated the presence of a monomorphic population with a very low level of variability. Conversely, Biolog phenotypic fingerprinting and phytohormone production analyses showed a considerable metabolic diversity, as bacteria obtained from single infected tissue differed more than bacteria obtained from different orchards. When pathogenicity tests were carried out on myrtle plants, different types of symptoms were induced: knots, canker lesions with or without tissue proliferations and, occasionally, wilting of the inoculated twig, a symptom never reported before for P. savastanoi. Comparable symptoms were also observed in the natural environment both on spontaneous and cultivated plants. Moreover, the host range of the myrtle population was heterogeneous and not well defined. Some isolates showed a wide host range whilst others were pathogenic only to their natural host. Overall these findings suggest that the diversity of the P. savastanoi population from myrtle does not depend so much on the locality or the natural host and does not allow the Sardinian and Greek isolates, together with previously characterized myrtle strains, to be ascribed to a known pathovar of P. savastanoi, nor to propose their belonging, as a whole, to a new pathovar.     

Molecular phylogenetic,morphological, and pathogenic analyses reveal a single clonal population of Septoria lycopersici with a narrower host range in Brazil

Septoria leaf spot, caused by Septoria lycopersici, is considered one of the most important diseases of tomato in Brazil. Despite its importance, the disease agent is still poorly studied. Septoria isolates collected from different production regions of Brazil were characterized by molecular, morphological, and pathogenic methods. A set of 104 isolates was sequenced for the DNA Tub, Cal, and EF1-α loci. Ten isolates were selected, according to geographical region of origin and type of leaf lesion (typical or atypical), for morphological characterization and for evaluation of aggressiveness on tomato cultivar Santa Clara. To evaluate the pathogen host range, cultivated and wild Solanaceae plants were inoculated with four selected isolates. The results showed that all isolates grouped with the type isolate of S. lycopersici in maximum likelihood and Bayesian inference trees. The isolates were morphologically similar. All isolates selected for pathogenicity testing on tomato were able to induce typical symptoms of the disease, but differed in their aggressiveness. A total of eight species of Solanaceae were also identified as potential alternative hosts for S. lycopersici. This information will provide a more accurate assessment of the risks involved with the introduction of new crops, especially of the genus Solanum, in areas where the species is already present. In addition, it will provide the basis for the establishment of more efficient methods in the management of Septoria leaf spot of tomatoes in natural conditions and in the different production systems.     

Infection of potato by Rhizoctonia solani: effect of anastomosis group

Glasshouse and field experiments showed that the pathogenicity and disease type on potato varied between different anastomosis groups (AGs) of Rhizoctonia solani. For example, severe stem and stolon disease developed in plants inoculated with a single isolate of AG3PT and AG5. Severe root disease was observed with single isolates of AG8 and to a lesser extent AG3PT, but rarely with single isolates of the other AGs tested. In both field and glasshouse experiments the AG2‐1 isolate (X81) produced only small lesions (<5 mm). However, this was not representative of two other AG2‐1 isolates. When AG2‐1 isolates of the three different rDNA IGS1 types were tested in a glasshouse trial, one caused more severe stem and stolon infection than AG3PT. In the field experiment, the yield of tubers, by weight, was significantly less (P < 0·05) in all inoculated plants than for uninoculated (control) plants. Yield losses were greatest and tuber numbers smallest in plots inoculated with an AG8 isolate, suggesting that root infection is important in determining quantitative yield loss. The incidence of black scurf was greatest in the progeny tubers in plots inoculated with AG3PT (83·9%), whereas only very small amounts of black scurf developed on tubers from plants infected with AG2‐1 (510 bp) or AG5 isolates. This is supported by laboratory tests, where isolates of AG3PT produced significantly more sclerotia on potato dextrose agar than isolates of AGs 2‐1, 4, 5 and 8.     

Aggressiveness of eight Venturia inaequalis isolates virulent or avirulent to the major resistance gene Rvi6 on a non‐Rvi6 apple cultivar

For sustainable management of scab‐resistant apple cultivars, it is necessary to understand the role of aggressiveness in the adaptation of Venturia inaequalis populations and particularly the costs to the organism of acquiring additional virulence. The aims of the present study were (i) to identify the quantitative variables that are most important in determining the differences in aggressiveness among groups of V. inaequalis isolates, and (ii) to ascertain whether virulent and avirulent isolates of V. inaequalis differ significantly in aggressiveness. The aggressiveness of eight isolates that differed in their virulence to the major resistance gene Rvi6 was compared on the non‐Rvi6 apple cv. Gala. Three components of aggressiveness, namely lesion density, the number of spores per square centimetre of leaf area, and the number of spores per lesion, were evaluated 21 days after inoculation, and the kinetics of lesion density over time were analysed in terms of maximum lesion density, length of latent period and rate of lesion appearance. On the second youngest but fully developed leaf at the time of inoculation, maximum lesion density in the virulent group was 20% lower and the latent period 7% longer, than in the avirulent group. However, the alternative hypothesis, namely that isolates had adapted to quantitative resistance present in cv. Gala depending on their cultivar of origin, could not be rejected. The analysis of the kinetics of lesion density by a non‐linear mixed‐effect model proved useful in the assessment of aggressiveness.