The influence of cultivar and isolate on the development of gangrene (Phoma exigua var. foveata) in potato tubers

In studies on the infection of potato cultivars with different field isolates of Phoma exigua var. foveata, cultivar × isolate interactions, although sufficiently large to influence the relative pathogenicities of isolates on different cultivars, were not substantial enough to affect cultivar ranking order. Cultivar rank was markedly influenced by pathogen isolate only when both field and culture collection isolates were compared. This suggests that the complications of cultivar × isolate interactions can be avoided in cultivar screening tests by the use of recent field isolates of high pathogenicity. It was found that cultivar was considerably more important than isolate in determining gangrene lesion size. The need to consider both lesion diameter and depth when estimating rot size following point inoculation of different cultivars was confirmed.     

Pathogenicity and its alternation with both morphological and genetic variation in Plasmopara halstedii (sunflower downy mildew)

Morphological, pathogenic and genetic variation was studied in seven Plasmopara halstedii (sunflower downy mildew) isolates of several races using five singlezoosporangium isolates per pathogen isolate. Aggressiveness criteria were analysed in one sunflower inbred line showing a high level of quantitative resistance. Genetic relationships were detected between the single zoosporangium isolates using 12 expressed sequence tags (EST)-derived markers. Analysis of the five single zoosporangium isolates for P. halstedii isolates showed variability within pathogen isolates for all aggressiveness criteria, but not for all pathogen isolates. Isolates of races 100 and 3xx were characterised with shorter latent period and higher sporulation density than the isolate of races 7xx. All pathogen isolates showed high percentage infection values and caused a large reduction in seedling size except for one isolate involved in dwarfing. There was no relation between zoosporangia form or size and race virulence profiles or aggressiveness criteria. There was no intra-genetic variation for all pathogen isolates, but it was observed an important genetic variation between single zoosporangium isolates of all races. No correlation was detected between pathogenicity traits and EST genotypes.     

Responses of different geographic populations of two potato tuber moth species to genetic variants of Phthorimaea operculella granulovirus

Phthorimaea operculella granulovirus (PhopGV) belongs to the genus Betabaculovirus of the arthropod‐infecting Baculoviridae. PhopGV is able to infect several gelechiid species. Among them are the potato tuber moths Phthorimaea operculella Zeller and Tecia solanivora Povolny (both Lepidoptera: Gelechiidae). In various South American countries, PhopGV‐based biopesticides are used to control either P. operculella or T. solanivora. Many trials have indicated that a particular viral isolate can exhibit very distinct pathogenicity when infecting different host species or different populations of one host species. In this study, we compared host–pathogen interactions using various PhopGV isolates and various populations of P. operculella and T. solanivora. Virus isolates from P. operculella were more pathogenic against their original host species than against T. solanivora. A PhopGV isolated from T. solanivora was less efficient against P. operculella. In addition, virus isolates differed in pathogenicity toward their hosts (i.e., lethal concentrations of isolates ranged from low to high). Unexpectedly, we also found that host populations of one species from distinct geographic origins did not differ significantly in susceptibility to the same PhopGV isolate. This was the case for both host species and for five PhopGV isolates. Comparative restriction fragment length polymorphism (RFLP) analyses of 11 isolates including those used in bio‐assays indicated three main regions of variation in the genome of PhopGV, corresponding to the regions of open reading frame PhopGV046, gene PhopGV129 (egt), and repeat 9 (located between open reading frames PhopGV083 and PhopGV084). Comparison of the nucleotide sequences of the insertions/deletions present in these regions were carried out for the most variable isolate, JLZ9f. The results are discussed in the context of the production and use of PhopGV as a biological agent against these two pest species.     

Verticillium wilt of pea (Pisum sativum)

A wilt disease of garden pea (Pisum sativum) caused by Verticillium dahliae is described and the range of pathogenicity of the isolate investigated. It is pathogenic to potato, sweet pea, antirrhinum and broad bean and isolates of V. dahliae from potato, lucerne and sweet pea and V. albo-atrum from lucerne are pathogenic to pea. Since the most common disease symptoms, acropetal progression of chlorosis and necrosis of the leaves followed by premature defoliation are indistinguishable from natural senescence, it is probable that disease and senescence symptoms are confused in the field. The premature defoliation results in marked reduction in green leaf area, leaf dry weight and pod yield.     

Variation in pathogenicity among isolates of Elsinoe phaseoli from Phaseolus species

Expanding primary leaves of 11 cultivars of Phaseolus vulgaris, and one each of Phaseolus coccineus, Phaseolus lunatus and Vigna unguiculata were inoculated with 23 isolates of Elsinoe phaseoli from Phaseolus spp. and one from Vigna unguiculata. On the basis of the macroscopic and microscopic appearance of the lesions, symptoms were placed into five classes. Spore yields from the lesions in each class were used to classify the types of lesions as representing resistance or susceptibility to each isolate. There was a differential response of cultivars to isolates. According to the reaction of five cultivars of P. vulgaris to infection, isolates were placed in four pathogenicity groups. In addition to the differential reaction of cultivars, there was some evidence of host specificity among the isolates. Thus, P. lunatus was susceptible to most of the P. vulgaris isolates but resistant to the ones from P. coccineus. Vigna unguiculata was susceptible only to the V. unguiculata isolate and was resistant to all the isolates from Phaseolus spp. The isolate from V. unguiculata failed to infect any of the bean cultivars or the other Phaseolus spp. The need to select suitable isolates for challenging cultivars in a breeding programme is stressed.     

Host Specialization among Vegetative Compatibility Groups of Verticillium dahliae in Relation to Verticillium longisporum

A collection of 24 isolates of Verticillium dahliae and 10 isolates of Verticillium longisporum originating from nine different host plants and from several geographic regions was tested for host specificity on 11 economically important crops such as potato, tomato, strawberry, linseed, three legumes and four Brassica species. In order to reveal host specificity the potential of each isolate to induce disease and affect plant yield was recorded for all isolate–host combinations. The collected data were statistically processed by means of a cluster analysis. As a result, the host range of individual isolates was found to be more dependent on the vegetative compatibility group (VCG) of the isolate than on its original host plant provenance. Twenty‐two out of 24 V. dahliae isolates belonged to either VCG 2B or 4B. VCG 2B isolates showed specificity for legumes, strawberry, potato and linseed, whereas VCG 4B was specifically virulent on potato, strawberry and linseed. Subgroups within VCG 2B and 4B almost lacking any host preference were designated 2B* and 4B*. Three isolates from VCG 2B*, however, severely attacked tomato which is a host outside the authentic host range of VCG 2B. The pathogenicity of V. longisporum isolates was restricted to cruciferous hosts. Conversely, cruciferous plants were not affected by isolates from VCGs 2B and 4B of V. dahliae. This lack of cross‐infectivity of certain subpopulations of V. dahliae and of V. longisporum may be useful in the management of this soil‐borne wilt disease.     

Paracoccidioides brasiliensis isolated from armadillos is virulent to Syrian hamsters

Isolates of Paracoccidioides brasiliensis may vary in virulence according to time of in vitro subcultivation. The present study compared the morphology and pathogenicity to hamsters of two P. brasiliensis isolates: one obtained from human lesions and maintained in the laboratory for several years (Pb-18) and the other isolate recovered from hamsters inoculated with organ homogenates from armadillos (Pb-T). The microscopic morphology of Pb-18 and Pb-T showed yeast cells with similar diameter. However, Pb-T produced a significantly higher number of buds per mother cell than Pb-18. Besides, the mycelial form of Pb-T developed abundant sporulation during 8 weeks of culture which was absent in the Pb-18 isolate. Virulence studies demonstrated that mortality rates, antibody levels, fungal load and extent of lesions in the organs were significantly higher in animals infected with Pb-T. The results demonstrated that Pb-T recently isolated from an animal was more virulent than Pb-18. These differences between the two P. brasillensis isolates may be indicators of virulence attenuation in this fungal species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Identification of Xanthomonas campestris pv. Juglandis from (Persian) English Walnut Nursery Trees in South Africa

Bacterial isolates producing yellowish colonies on Nutrient Agar were recovered from symptoms of suspect walnut blight disease on leaves of nursery trees in the southwestern Cape Province of South Africa. The isolates were identified by pathogenicity tests on leaves of walnut and plum trees in the greenhouse. Fifteen isolates from four cultivars at two nurseries produced typical lesions of blight on walnut and one isolate. typical lesions of bacterial spot disease on plum leaves. Cluster analysis was done on 28 characteristics recorded from colony growth. colour. form. and elevation on four different culture media, and starch hydrolysis on a semi-selective medium for the isolation of Xanthomonas campestris pv. juglandis. Total DNA of the isolates was digested with restriction endonuclease Spel and resolved by contour-clamped homogeneous electric field (CHEF) electrophoresis. Two phenotypic clusters were distinguished among the 15 South African and one reference strain of X.c.pv. juglandis at the 54%S_sm level. The isolate which induced disease symptoms on plum grouped with reference strains of Xanthomonas campestris pv. pruni in a third cluster. Two-thirds of the isolates were not characterized on the semi-selective medium for X.c. pv. juglandis. DNA restriction fragment banding patterns were similar for most isolates of X.c.juglandis in the same phenotypic cluster. However, DNA banding patterns were non-distinct for some isolates with similar phenotypic characters. Phenotypic characteristics and DNA restriction fragment banding patterns of the isolates were not correlated with geographical origin or cultivar specificity.     

Comparative pathogenicity of Fusarium graminearum isolates from China revealed by wheat coleoptile and floret inoculations

Fusarium head blight (FHB) or scab caused by Fusarium species is an economically important disease on small grain cereal crops worldwide. Accurate assessments of the pathogenicity of fungal isolates is a key obstacle toward a better understanding of the Fusarium-wheat scab system. In this study, a new laboratory method for inoculation of wheat coleoptiles was developed, which consists of cutting off the coleoptile apex, covering the cut apex with a piece of filter paper soaked in conidial suspension, and measuring the lengths of brown lesions 7 days post inoculation. After coleoptile inoculation, distinct brown lesions in the diseased stems were observed, in which the presence of the fungus was verified by PCR amplification with F.␣graminearum Schwable-specific primers. Coleoptile inoculation of six wheat varieties indicated that a highly susceptible wheat variety was more suitable as a differentiating host for the pathogenicity assay. Analysis of the coleoptiles inoculated with a set of 58 different isolates of F. graminearum showed a significant difference in the lengths of the lesions, forming the basis by which pathogenicity of the isolates was assessed. Field inoculation of florets of three wheat varieties over 2 years revealed significant differences in pathogenicity among the 58 isolates, and that the highly resistant and highly susceptible wheat varieties were more appropriate and stable for pathogenicity assessment in field trials. Comparative analyses of eight inoculation experiments of wheat with 58 F. graminearum isolates showed significant direct linear correlations (P<0.001) between coleoptile and floret inoculations. These results indicate that the wheat coleoptile inoculation is a simple, rapid and reliable method for pathogenicity studies of F.␣graminearum in wheat.     

Biochemical characterisation of Pythium spp. involved in cavity spot of carrots in France

The pathogenicity and growth rate in vivo were assessed on 27 isolates of Pythium spp. recovered from cavity spot lesions on carrots grown in various parts of northwest France. Polyacrylamide gel electrophoresis of isoesterases was used to identify the Pythium spp. involved. Slow-growing isolates were more aggressive than fast-growing ones when inoculated on carrot tap roots. Isoesterase patterns identified the slow-growing isolates as P. violae and P. sulcatum; P. ultimum and P. intermedium were identified among the less aggressive fast-growing isolate group, in which some isolates were also classed as P. sylvaticum or P. irregulare, which have similar electrophoretic profiles. The incidence of Pythium spp. associated with the disease in France is discussed in regard to cavity spot in other countries.     

Comparisons of Isolates of Fusarium avenaceum from White Lupin and Other Crops by Pathogenicity Tests,DNA Analyses and Vegetative Compatibility Tests

Isolates of Fusarium avenaceum, mostly from crops of white lupin or wheat, were tested for pathogenicity on white lupin and wheat plants and compared by DNA tests and, in a limited study, vegetative compatibility. Most of the 80 isolates were pathogenic on both plant species after inoculation on shoot bases. Disease severity was greater at higher incubation temperatures that ranged from 15/10°C to 25/20°C (day/night temperatures). Isolates from lupin crops tended to be more pathogenic, on average, on lupins than on cereals. Polymerase chain reaction (PCR)‐restriction fragment length polymorphism (RFLP) analysis of the internal transcribed spacer region of the rDNA distinguished two groups of isolates that occurred in different proportions among isolates from lupins and cereal crops. Random amplified polymorphic DNA (RAPD)‐PCR analyses indicated considerable genetic variation among isolates, but there was some similarity among groups of isolates from populations in the same field. Genetic diversity was confirmed by a high degree of vegetative incompatibility among 20 isolates using nitrate nonutilizing mutants. There were no relationships among pathogenicity, RFLP group, RAPD group and vegetative compatibility group.     

Development of Pathogenicity and AFLP to Characterize Fusarium oxysporum f.sp. momordicae Isolates from Bitter Gourd in China

Bitter gourd (Momordica charantia L.) cultivated in China is regarded as an important vegetable crop and is of considerable economic importance. However, it is susceptible to fusarium wilt, which causes heavy economic losses. Forty‐eight isolates were isolated from diseased bitter gourd plants that displayed typical fusarium wilt symptoms. Based on the morphological features, the rDNA internal transcribed space (ITS) sequences, pathogenicity and host biotypes, all of the isolates tested were pathogenic to the susceptible bitter gourd plants species (cv. ‘Guinongke No. 2’) and were identified as Fusarium oxysporum f. sp. momordicae (FOM). Our results classified different isolates as slightly, moderately or highly virulent. Among the isolates tested, 43 isolates slightly infected bottle gourd (Lagenaria siceraria var. clavata), whereas they did not infect other species from the family Cucurbitaceae. Genetic diversity among 48 isolates was characterized using amplified fragment length polymorphism (AFLP) analysis. The number of bands amplified by each primer pairs ranged from 41 to 66, with sizes ranging from 200 to 500 bp. A total of 366 bands were observed, out of which 363 were polymorphic (99.14%). The Nei's genetic identity of the six geographical populations varied from 0.7362 to 0.9707. The mean Nei's gene diversity index (H = 0.2644) and the mean Shannon's information index (I = 0.4071) at species level were higher than ones at populations level, indicated that the variation within populations was greater than that among populations. The Nei's GST (0.5103) and gene flow (Nm = 0.4923) revealed that genetic differentiation was mainly among populations and few gene exchanges. The dendrogram obtained from AFLP marker showed that there was a good correlation between isolates from different geographical locations and their pathogenicity. The AFLP marker effectively distinguished the high virulent isolates from the less virulent isolates. The highly virulent isolates were distinctly separated in different clusters, which indicated a significantly high correlation with the geographical origin in the AFLP dendrogram. The pathogenicity and molecular marker analysis confirmed the presence of variation in virulence as well as genetic diversity among the FOM isolates studied.     

Oxidative Enzymes of Ceratocystis fimbriata Isolates Differing in Host Specificity

Eight isolates of Ceratocystis fimbriata differing in pathogenicity on sweet potato roots were compared in vitro for peroxidase, catalase and polyphenol oxidase activities. Pathogenic isolates were generally lower in specific activities of peroxidase and catalase than the nonpathogenic isolates. Cultures of pathogenic isolates were distinguished by black culture liquid compared to the yellow color of nonpathogenic isolates. No consistent differences among the isolates were apparent when the specific activity of polyphenol oxidase preparations were compared on various phenolic substrates. The peroxidase of C. fimbriata had an approximate molecular weight of 81,000 when compared with known protein standards on a column of Sephadex G-100 dextran gel. Its substrate specificity differed from horseradish peroxidase in that it was nonreactive with guaiacol.     

Isolates of Microdochium nivale and M. majus Differentiated by Pathogenicity on Perennial Ryegrass (Lolium perenne L.) and in vitro Growth at Low Temperature

Pink snow mould is a serious disease on grasses and winter cereals in cold and temperate zones during winter. To better understand the basis for the variation in pathogenicity between different isolates of Microdochium nivale and M. majus and to simplify selection of highly pathogenic isolates to use when screening for resistance to pink snow mould in perennial ryegrass, we sought traits correlated with pathogenicity. Isolates of M. nivale were more pathogenic on perennial ryegrass than isolates of M. majus, as measured by survival and regrowth of perennial ryegrass after infection and incubation under simulated snow cover. Pathogenicity as measured by relative regrowth was highly correlated with fungal growth rate on potato dextrose agar (PDA) at 2°C. Measuring fungal growth on PDA therefore seems to be a relatively simple method of screening for potentially highly pathogenic isolates. In a study of a limited number of isolates, highly pathogenic isolates showed an earlier increase and a higher total specific activity of β‐glucosidase, a cell wall‐degrading enzyme, compared with less pathogenic isolates. None of the M. majus isolates was highly pathogenic on perennial ryegrass. Our results indicate biological differences between M. nivale and M. majus and thus strengthen the recently published sequence‐based evidence for the elevation of these former varieties to species status.     

Biological characterization of a clinical and an environmental isolate of <Emphasis Type="Italic">Acanthamoeba polyphaga</Emphasis>: analysis of relevant parameters to decode pathogenicity

Acanthamoeba spp. consists of free-living amoebae, widespread in nature, which occasionally can cause human infections including granulomatous amoebic encephalitis and amoebic keratitis. Acanthamoeba pathogenesis is not entirely known and correlations between pathogenic potential and taxonomy are complex issues. In order to decipher the definition of a pathogenic amoeba, the objective of this work was to decipher the definition of pathogenic amoeba by characterizing two isolates of Acanthamoeba polyphaga obtained from different origins (a keratitis patient and freshwater), looking for differences among them. The clinical isolate grew faster in Peptone-yeast extract-glucose (PYG) medium, transformed more rapidly from a trophozoite to cyst and exhibited increased cytopathic effect on cultured cells. Morphological differences were also noted, since freshwater amoebae presented more acanthopodia than the clinical isolate. Moreover, actin labeling demonstrated that microfilament organization varies between isolates, with the presence of locomotory structures as lobopodia and lamellipodia in the keratitis isolate, which were less adherent on plastic. Zymography demonstrated that the keratitis isolates presented higher proteolytic activity and also were more able to invade collagen matrices. Altogether, we conclude that a group of stable physiological characteristics exist in Acanthamoeba that can be related to pathogenicity.     

The Variability of Pyrenochaeta terrestris Isolates Based on Isozyme Polymorphism, Cultural Characteristics and Virulence on Differential Onion Breeding Lines

Isolates of Pyrenochaeta terrestris from South Africa and the United States of America were evaluated for pathogenicity, isozyme polymorphism and cultural characteristics. Isozyme polymorphism indicated that the isolates of this pathogen, even from the same field, are highly variable. Based on variability in the enzymes: alcohol dehydrogenase, aldolase, malate dehydrogenase and sorbitol dehydrogenase, the isolates could be divided into five electrophoretic types. The virulence of isolates to onion breeding lines resistant, moderately resistant and susceptible to pink root was variable and differentiation depended on the isolate used in the evaluation. The isolates showed extensive variation in both the ability to form setose pycnidia in culture and in the cultural pigmentation.     

Analysis of Cultural and Genetic Diversity in Alternaria helianthi and Determination of Pathogenic Variability Using Wild Helianthus Species

The study aims at assessment of morphological, molecular and pathogenic variability of Alternaria helianthi, incitant of leaf blight of sunflower. Morphological characteristics determined for 26 isolates of A. helianthi from India revealed variations in shape of culture, pigmentation, conidial measurements, number of septa and colony growth. The conidia of isolate Ah‐7 were long, while conidia of isolate Ah‐15 were short. Based on cultural characters, isolates were classified into four groups. Genetic variability of the isolates was assessed by random amplified polymorphic DNA analysis. Good polymorphism was observed and cluster analysis indicated presence of six genetically distinct groups among the isolates. The isolates Ah‐1, Ah‐7 and Ah‐14 were genetically distinct. Resistant sources are not available in cultivated sunflower, while wild Helianthus species possess resistance to multiple stresses. We evaluated reaction of wild Helianthus species to isolates of A. helianthi. Among wild Helianthus species, H. tuberosus followed by H. occidentalis showed moderately to highly resistant reaction to all the isolates and recorded less disease incidence. The species H. argophyllus followed by H. laevigatus showed more disease incidence. The cultivated sunflower recorded susceptible reaction to most of the isolates and recorded high disease incidence. The isolates differed significantly for pathogenic reaction and were grouped into three pathogenicity groups; low, medium and high. Six isolates induced <20% disease incidence and were included in the low pathogenicity group. Majority of isolates were in the medium pathogenicity group. Six isolates i.e. Ah‐9, Ah‐10, Ah‐18, Ah‐20, Ah‐24 and Ah‐26 induced more than 50% disease incidence and were considered high pathogenicity group. Our results demonstrate the existence of considerable variation in resistance of Helianthus species to A. helianthi and also in morphological and genetic characters of A. helianthi isolates prevalent in India.     

Common resistance in red raspberry to Botrytis cinerea and Didymella applanata, two pathogens occupying the same ecological niche

Comparisons were made between Botrytis cinerea and Didymella applanata, which occupy the same ecological niche on canes of red raspberry. Isolates of B. cinerea from diverse localities within the British Isles were pathogenic to the SCRI selection M30 when internodal wounds on young canes were inoculated. A single inoculation frequently caused the buds at several nodes to fail the following spring. Differences in the lengths of the stem lesions that formed indicated differences in isolate pathogenicity, but these were not related to isolate origin. Bud suppression and lateral shoot failure also occurred when petioles of cvs Mailing Orion, Mailing Jewel and Glen Clova were wound inoculated with B. cinerea up to late August. The relative resistance of seven cultivars to three isolates of each pathogen was determined. Principal components analysis of data from five estimates of resistance to each pathogen showed that 40% of the variation described a common resistance to the two diseases. Further analysis showed that cvs Chilcotin and Meeker had the strongest common resistance and that cvs Glen Prosen and Mailing Jewel had the weakest. The remaining variation described cultivar differences in relative bud length after petiole inoculation with either pathogen, and differences in the relative importance of spring and autumn symptoms. Only 7% of the variation indicated independent resistance to the two pathogens and this was not influenced by cultivar differences.     

Characterization and Sensitivity to Fungicides of Rhizoctonia spp. Recovered from Potato Plants in Bolu,Turkey

Isolates of Rhizoctonia spp. associated with stem canker and black scurf disease of potato were examined for their anastomosis group, sequence variations in the ITS‐5.8S rDNA region, pathogenicity and sensitivity to fungicides. A total of 92 isolates were obtained from diseased tuber, stolon and sprouts of the potato plants, collected from five districts of Bolu province, Turkey. Based on the anastomosis group and the similarity of the nucleotide sequence of the ITS‐5.8S rDNA, most of the isolates (81.5%) were identified as AG 3 PT. Other isolates belonged to AG 2‐1 (1.08%), AG 2‐2 IV (1.08%), AG 4 HG II (8.07%), AG 5 (2.17%), binucleate Rhizoctonia AG A (1.08%) and AG K (4.35%). Pathogenicity tests showed that isolates of AG 3 PT, AG 4 HG II and AG 5 caused similar degrees of disease severity on 45‐day‐old potato seedlings, whereas AG 2‐1 was moderately virulent. AG 2‐2 IV and binucleate Rhizoctonia spp. were weakly pathogenic or non‐pathogenic on potato seedlings. In this study, anastomosis groups of Rhizoctonia spp. isolates associated with potato in Turkey were characterized for the first time using molecular techniques and classified at the level of subgroups. Furthermore, the effect of selected fungicides was evaluated on disease development caused by soil‐borne inoculums of different anastomosis groups (AGs). Flutolanil and Bacillus subtilis QST 713 were found to be most effective against the Rhizoctonia isolates tested. These results revealed significant differences among the fungicides on disease development resulted from the different AGs.     

Analysis of SDS-dissociated proteins of pathogenic and nonpathogenicFusarium species

SDS — dissociated proteins from eight isolates belonging to threeFusarium species were assessed and compared using polyacrylamide gel electrophoresis. Intra and inter specific variation in banding patterns were analyzed both qualitatively and quantitatively. Special emphasis was placed on variability of protein banding pattern between two nonpathogenic strains ofF. oxysporum (C5 and C14). Protein profiles from host-associated isolates were distinct, and each isolate showed a uniquely characteristic profile. Attempts were made to provide information on the genetic system controlling pathogenicity, compared to nonpathogenicity, at the protein level. The data obtained from electrophoresis support the potential use of this experimental approach to help distinguish between differentFusarium isolates.