Penicillium digitatum Suppresses Production of Hydrogen Peroxide in Host Tissue During Infection of Citrus Fruit

ABSTRACT During the infection of citrus fruit by Penicillium digitatum there is little evidence of a host defense response. This suggests that P. digitatum has the ability to suppress host defenses. The current study demonstrates that P. digitatum suppresses a defense-related hydrogen peroxide (H(2)O(2)) burst in host tissue. In contrast, the nonhost pathogen, Penicillium expansum, triggers production of a significant amount of H(2)O(2) in citrus fruit exocarp. Using laser scanning confocal microscopy, we demonstrated that P. digitatum suppressed an elevation in H(2)O(2) up to 42 h after inoculation. Nevertheless, H(2)O(2) levels around wounds inoculated with P. expansum increased by 63-fold above the control. P. digitatum continued to suppress H(2)O(2) production in citrus fruit exocarp up to 66 h postinoculation and H(2)O(2) levels were actually threefold below that of noninoculated controls. In contrast, the H(2)O(2) level was still about 11-fold above the control value in wound sites inoculated with P. expansum. Studies on the effect of organic acids (as pH modulators) on the response of citrus fruit to compatible and noncompatible pathogens indicated that pathogenicity was enhanced only when host-tissue acidification was accompanied by the suppression of H(2)O(2). Additionally, pathogenicity of both P. digitatum and P. expansum on citrus fruit was significantly enhanced by the H(2)O(2)-scavenging enzyme catalase. Based on our study and previous reports regarding the potential involvement of citric acid and catalase in green mold pathogenesis, we suggest that these compounds are strongly associated with the virulence of P. digitatum.     

An Analysis of Host Adaptation and Its Relationship with Virulence in Cucumber mosaic virus

ABSTRACT The host range of a pathogen can have special consequences on its evolution and the evolution of its virulence. For generalists, adaptation to different hosts may be conditioned by different trade-offs in the pathogen's life history and be affected by evolutionary processes that shape pathogen populations. We have examined adaptation of Cucumber mosaic virus (CMV) to different hosts, and analyzed the relationship between host adaptation and virulence. For this, six CMV isolates from central Spain from three different hosts were compared for the ability to multiply and to affect host growth. These analyses were done before and after an experimental evolution process consisting of 10 serial passages in the original host of the isolate. The differential capacity to infect different hosts was compatible with host adaptation. However, the capacity to multiply in different hosts did not provide evidence of host adaptation and was not improved after 10 passages, suggesting that fitness of the natural population of CMV was at, or near to, its maximum. No relationship was found between capacity of multiplication and virulence in any of the three different hosts. These results suggest that the "trade-off" model for the evolution of virulence may not apply to CMV.     

大青沟木本植物果实类型组成与环境演变研究

通过对大青沟分布的49种木本植物,其中包括23种乔木,26种灌木的果实类型组成和所占的比例及其水分生态型分析,结果表明:大青沟木本植物水分生态型以中生植物为主,占87.76%;果实类型主要是蒴果、浆果、核果和翅果,分别占26.53%、22.45%、16.33%和16.33%,其次为坚果和梨果,分别占6.12%、6.12%,瘦果、荚果和骨突果所占比例最小,分别占2.04%、2.04%和2.04%。木本植物群落的垂直分布决定植物对水资源利用方式不同,以地下水为利用资源的木本植物,其果实主要是通过动物来传播的,而以地表水为利用资源的木本植物,其果实主要是通过风来传播的。由此根据现有木本植物果实类型组成和生态型谱初步推断,大青沟植物群落的形成过程是以风为媒介,使植物果实扩散和定居而形成的,之后又以动物为媒介,使植物果实通过动物扩散和定居,从而形成目前不同生态型的植物群落,而不是大青沟作为植物避难所,原有地带性植被的残存。     

Sensitivity of Penicillium digitatum and P. italicum to Postharvest Citrus Fungicides in California

ABSTRACT Penicillium digitatum isolates (326), collected in California citrus groves and packinghouses, were assayed qualitatively for their sensitivity to imazalil, thiabendazole, and o-phenylphenol. Eighteen typical triple-resistant isolates, acquired in each of 3 years (1988, 1990, and 1994), were assayed quantitatively for their sensitivity to each of the three fungicides. No significant differences were found in the mean sensitivity of the isolates collected in different years. However, the proportion of isolates that were resistant to all three fungicides increased from 43% in 1988 to 77% in 1990 and 74% in 1994. Imazalil-resistant biotypes of P. digitatum were isolated frequently in California packinghouses, while resistant P. italicum was rare. No fungicide-resistant biotypes of either species were collected from citrus groves. Wild-type P. italicum was slightly less sensitive than wild-type P. digitatum to all three fungicides. The concentration of imazalil producing 50% growth inhibition (EC(50)) was three times greater when the age of the P. digitatum assay inoculum was increased from 12 to 24 h. Activity of imazalil increased with pH of the assay medium in the range pH 5.1 to 5.9, reflecting the greater concentration of dissociated imazalil at the higher pH value.     

河北省红富士苹果青霉病病菌种类鉴定

青霉病在苹果储藏期引起苹果腐烂,造成较大的经济损失.对采自河北省保定、沧州、石家庄等地冷库红富士苹果病果进行分析,共分离到42个青霉菌菌株.根据病原菌的形态特征,将42个菌株划分为4个类型.依据柯赫氏法则对分离到的菌株进行研究,4个类型青霉均为致病菌,分别为扩展青霉、冰岛青霉、壳青霉和圆弧青霉.根据分离频率和致病性数据分析,扩展青霉为优势菌株.     

Variation in Virulence Within Meloidogyne chitwoodi, M. fallax, and M. hapla on Solanum spp

ABSTRACT The virulence of Meloidogyne hapla, M. chitwoodi, and M. fallax was studied on genotypes of Solanum spp. in a greenhouse. Juveniles of 11 M. hapla race A isolates, 3 M. hapla race B isolates, and 5 mono-female lines of a M. hapla race A isolate were inoculated on S. chacoense, S. hougasii, and S. sparsipilum. Juveniles of eight M. chitwoodi isolates, five M. fallax isolates, and six mono-female lines of a M. chitwoodi isolate were inoculated on S. bulbocastanum, S. chacoense, S. hougasii, S. stoloniferum, and S. tuberosum. Virulence was expressed as nematode reproduction 8 weeks after inoculation. Nematode reproduction was estimated by the number of egg masses and, in one experiment, by the number of hatched second-stage juveniles per inoculated juvenile. Considerable variation in virulence and resistance was observed among M. hapla isolates and plant genotypes, respectively. The M. hapla isolate-plant species interaction was highly significant. The response to M. chitwoodi ranged from susceptible (S. tuberosum and S. chacoense) to highly resistant (S. bulbocastanum and S. hougasii). S. tuberosum was susceptible to M. fallax, whereas all four wild species were resistant. In contrast to M. hapla, no significant isolate-plant genotype interaction was obtained for M. chitwoodi or M. fallax, indicating no or little intraspecific variation in virulence. M. chitwoodi juveniles in species mixtures with M. fallax isolates appeared to be able to break the resistance of S. bulbocastanum and S. hougasii. Significant differences among mono-female lines of M. hapla and M. chitwoodi were observed, indicating heterogeneity of pathogenicity within meiotic parthenogenic Meloidogyne populations.     

Identification and Pathogenicity of Rhizoctonia spp. Isolated from Apple Roots and Orchard Soils

ABSTRACT Rhizoctonia spp. were isolated from the roots of apple trees and associated soil collected in orchards located near Moxee, Quincy, East Wenatchee, and Wenatchee, WA. The anastomosis groups (AGs) of Rhizoctonia spp. isolated from apple were determined by hyphal anastomosis with tester strains on 2% water agar and, where warranted, sequence analysis of the rDNA internal transcribed spacer region and restriction analysis of an amplified fragment from the 28S ribosomal RNA gene were used to corroborate these identifications. The dominant AG of R. solani isolated from the Moxee and East Wenatchee orchards were AG 5 and AG 6, respectively. Binucleate Rhizoctonia spp. were recovered from apple roots at three of four orchards surveyed and included isolates of AG-A, -G, -I, -J, and -Q. In artificial inoculations, isolates of R. solani AG 5 and AG 6 caused extensive root rot and death of 2- to 20-week-old apple transplants, providing evidence that isolates of R. solani AG 6 can be highly virulent and do not merely exist as saprophytes. The effect of binucleate Rhizoctonia spp. on growth of apple seedlings was isolate-dependent and ranged from growth enhancement to severe root rot. R. solani AG 5 and AG 6 were isolated from stunted trees, but not healthy trees, in an orchard near Moxee, WA, that exhibited severe symptoms of apple replant disease, suggesting that R. solani may have a role in this disease complex.     

Characterization of Phytophthora spp. Causing Outbreaks of Citrus Brown Rot in Florida

ABSTRACT Epidemics of citrus brown rot from 1994 to 1997 in the south-central and east-coast citrus areas of Florida were characterized and the causal Phytophthora spp. identified. Two species of Phytophthora, P. palmivora and P. nicotianae, were consistently associated with brown rot. Epidemics caused by P. palmivora appeared to be initiated on immature fruit dropped on the orchard floor. The soilborne fungus infected and sporulated on these fruit and was then disseminated to fruit above 1 m in the canopy. In contrast, infection by P. nicotianae, the common cause of root rot, was confined to the lowest 1 m of the canopy. Fruit infected by P. palmivora produced large amounts of ellipsoidal sporangia available for splash dispersal, whereas those infected by P. nicotianae produced far fewer spherical sporangia. Isolates from brown rot epidemics were compared with P. nicotianae from citrus in Florida and Texas, P. citrophthora in California, P. palmivora, and selected Phytophthora spp. from other hosts. Brown rot symptoms produced by the different pathogenic citrus isolates on inoculated fruit were indistinguishable. Morphology, mating behavior, and isozyme patterns of brown rot isolates from 1988 to 1997 matched P. palmivora from citrus roots, other host plants, and other locations, but were different from characterized isolates of P. citrophthora in California and P. nicotianae in Florida and Texas. Cellulose acetate electrophoresis of the isozyme glucose-6-phosphate isomerase rapidly identified the causal citrus pathogen from infected fruit and soil isolation plates. Although P. palmivora is an aggressive pathogen of citrus roots, bark, and fruit, populations in orchard soils were low compared with P. nicotianae.     

人工卵赤眼蜂对玉米害虫的控害效果

研究表明,用人工卵繁殖的松毛虫赤眼蜂和螟黄赤眼蜂对夏玉米田２～３代玉米螟均有较好的控害效果。放蜂区校正卵粒寄生率为６５．４４％～６８．１６％;百株玉米蛀孔数和雄穗被害较对照分别减少６６．６７％～７０．３７％和７３．３３％～８０．００％。防治玉米雌穗上棉铃虫的效果,螟黄赤眼蜂要明显好于松毛虫赤眼蜂（２０．０％）,虫口减退率达７１．１％。另外还研制了适合人工卵赤眼蜂田间放蜂的放蜂器,出蜂率达９０％以上。     

The Relationship of Host Range, Physiology, and Genotype to Virulence on Cantaloupe in Pseudomonas syringae from Cantaloupe Blight Epidemics in France

ABSTRACT In 1993, a bacterial blight caused important losses of cantaloupe (Cucumis melo var. cantalupensis) in southwestern France and has now been reported in all cantaloupe-growing regions of France. The causal agent of this blight is Pseudomonas syringae, although on a worldwide basis this bacterium has not been a major pathogen of melon for over 50 years. To identify the pathovar of the cantaloupe pathogen, we employed biochemical tests, plasmid and chromosomal profiling, and host range studies for 23 strains from cantaloupe and 47 reference strains of 14 pathovars of P. syringae. Numerical analysis of 119 traits, serological typing, syringomycin production, and BOX-polymerase chain reaction profiles did not allow us to differentiate among pathovars related to P. syringae pv. syringae. Host range studies of cantaloupe and references strains on 18 plant species showed that virulence to sugar beet was a common feature of strains virulent on cantaloupe, but was not common to strains avirulent on cantaloupe. Virulence to other species of plants varied among strains, but the overall extent of the host range was proportional to aggressiveness to cantaloupe. We propose that the strains attacking cantaloupe in France be considered P. syringae pv. aptata and that adequate host range testing may reveal that this pathovar is the cause of cantaloupe blight reported in other parts of the world.     

Relationship Between Production of the Phytotoxin Prehelminthosporol and Virulence in Isolates of the Plant Pathogenic Fungus Bipolaris sorokiniana

A gas chromatographic method was developed to quantify the phytotoxin prehelminthosporol, which is a sesquiterpene metabolite of the plant pathogen Bipolaris sorokiniana. The toxin was extracted from mycelium or culture filtrates, pre-cleaned using solid phase extraction, and analyzed by gas chromatography as a trimethylsilyl-derivative. The detection limit of the method was 5ngl^–1 (signal to noise ratio 4:1) which corresponds to ca. 15ng prehelminthosporol per mg dry weight of mycelium or 15ng prehelminthosporol per ml culture filtrate. The total amount of prehelminthosporol (mycelium plus culture filtrate) increased with cultivation time when examined in six isolates of B. sorokiniana after 6, 9, 12 and 15 days of incubation. The screening experiment of 17 isolates for prehelminthosporol production after 8 days of incubation revealed significant differences in the toxin production between the isolates. The isolates with low toxin production had lower virulence towards barley roots compared to those with higher production of the toxin. However, the virulence did not increase with prehelminthosporol level among the high producing isolates. Prehelminthosporol was also analyzed in a number of related Bipolaris and Drechslera species. In addition to B. sorokiniana, three out of six Bipolaris species (B. setariae, B. zeicola, B. victoriae) produced prehelminthosporol, which indicates that ability to produce prehelminthosporol is conserved among closely-related Bipolaris species.     

柑橘采前落果原因调查及防治建议

2008年,由于受到四川广元柑橘大实蝇事件的影响,柑橘在全国范围内均滞销,我省也不例外.与以往相比,2009年柑橘采前落果现象十分严重.因此,为深入了解落果原因,笔者于2008年11月上旬对落果较重的柑橘园进行了调查与统计.     

Efficacy of Althaea officinalis leaf extract in controlling Alternaria spp. pathogenic on Citrus

European Journal of Plant Pathology - In this study, inhibitory effects of the aqueous and ethanolic leaf extracts obtained from marshmallow (A. officinalis) were evaluated against Alternaria spp.,...     

Etiology and Population Genetics of Colletotrichum spp. Causing Crown and Fruit Rot of Strawberry

ABSTRACT Isolates of Colletotrichum spp. from diseased strawberry fruit and crowns were evaluated to determine their genetic diversity and the etiology of the diseases. Isolates were identified to species using polymerase chain reaction primers for a ribosomal internal transcribed spacer region and their pathogenicity was evaluated in bioassays. Isolates were scored for variation at 40 putative genetic loci with random amplified polymorphic DNA and microsatellite markers. Only C. acutatum was recovered from diseased fruit. Nearly all isolates from crowns were C. gloeosporioides. In crown bioassays, only isolates of C. gloeosporioides from strawberry caused collapse and death of plants. A dendrogram generated from the genetic analysis identified several primary lineages. One lineage included isolates of C. acutatum from fruit and was characterized by low diversity. Another lineage included isolates of C. gloeosporioides from crowns and was highly polymorphic. The isolates from strawberry formed distinctive clusters separate from citrus isolates. Evaluation of linkage disequilibrium among polymorphic loci in isolates of C. gloeosporioides from crowns revealed a low level of disequilibrium as would be expected in sexually recombining populations. These results suggest that epidemics of crown rot are caused by Glomerella cingulata (anamorph C. gloeosporioides) and that epidemics of fruit rot are caused by C. acutatum.     

Diversity and population dynamics of Penicillium spp. on apples in pre- and postharvest environments: consequences for decay development

Isolates of Penicillium spp. were collected regularly from 2001 to 2003 from the surfaces of apple fruit pre- and postharvest, and from the atmosphere of orchards and storage rooms in France. Penicillium spp. were not detected from the atmosphere of conventional orchards, while their density did not exceed 50 spores m⁻³ in the atmosphere of organically managed orchards. Penicillium spp. were seldom detected on apple surfaces in the orchard. The density of Penicillium on apples increased from 10 to 50 spores cm⁻² after 1 month in storage to 300–400 spores cm⁻² after 6 months. The level of airborne Penicillium increased by up to 2 × 10⁴ and 2·5 × 10³ spores m⁻³ within nondisinfected and previously disinfected warehouses, respectively. Penicillium expansum (30–62%) and P. solitum (6–45%) were the most prevalent species on apple or in storage rooms. Other species of Penicillium isolated included P. commune, P. verrucosum, P. chrysogenum, P. rugulosum and P. digitatum. Apple fruit were also surveyed for wounds and the number of open lenticels using the sulphur dioxide test. The incidence of wounding at harvest varied from 12 to 36%, depending on cultivar and locality. When apples were inoculated at harvest by either aqueous or aerial inoculum of P. expansum, the decay incidence was constantly higher than the incidence of wounding. The number of open lenticels per cm² of apple surface varied from 0·5 on cv. Boskoop to 4·4 on cv. Golden Delicious. An average of 13 and 2·1% of lenticels, respectively, were infected when they were inoculated by P. expansum and P. verrucosum. Cultivars of apple fruit that showed a greater number of open lenticels, combined with a large diameter varying from 100 to 200 µm, were more susceptible to P. expansum.