Histological analysis of spindle and spheroid root galls caused by Plasmodiophora brassicae

Clubroot of crucifers, caused by the obligate parasite Plasmodiophora brassicae, is characterized by the formation of conspicuous root galls. These galls usually have a club- or spindle-shaped morphology, and interfere with water and nutrient uptake by infected plants. Smaller galls, historically regarded as resistance structures and distinct from the typical spindle-shaped galls, have also been identified and termed ‘spheroid galls’ because of their spherical or nearly spherical form. An assessment of various Brassica species and varieties revealed that spheroid galling could be observed in all genotypes investigated, but occurred regularly only in a few particular host/P. brassicae combinations. While spindle gall formation was coincident with the expansion of the stele and infection of secondary tissues by P. brassicae, spheroid galls typically had a region of proliferating tissue that corresponded to the secondary cortex and periderm of the healthy plants, with the outer proliferating tissue less infected than the inner portions. The underlying host tissue showed limited secondary tissue development, was largely uninfected, and, where infection occurred, a continuous stele was maintained. An active host defensive reaction, in the form of cell lignification or the hypersensitive response, was not observed, while pathogen resting spores were visible in one longitudinal section of a spheroid gall. These findings suggest that while the proliferation of P. brassicae is restricted in spheroid galls, these structures are not indicative of complete resistance to clubroot.     

Gall production on hawthorns caused by <Emphasis Type="Italic">Gymnosporangium</Emphasis> spp. in Hatay province,Turkey

Three hawthorn and related rust diseases caused by Gymnosporangium confusum on Crataegus monogyna, Gymnosporangium clavariiforme on Crataegus orientalis and Gymnosporangium sabinae on Pyrus communis were detected in Hatay province, Turkey. G. confusum was also found causing telial galls on Juniperus communis. G. confusum and G. clavariiforme produced aecial horns on overwintered galls on hawthorn twigs from May to June. The production of galls caused by Gymnosporangium on the secondary host is unusual. Portions of the nuclear ITS and LSU rDNA were sequenced for all three species of Gymnosporangium for use as a species barcode; sequences were deposited in GenBank. Sequence data from G. clavariiforme and G. sabinae matched those in GenBank; however, this is the first study to deposit sequence data from G. confusum to GenBank. The life cycles of G. confusum and G. clavariiforme are discussed along with implications for disease control.     

柽柳柽瘿蚊的生物学特性

报道了在新疆柽柳上造成虫瘿的齿腿柽瘿蚊、毛尾柽瘿蚊、土库曼柽瘿蚊和害柽瘿蚊。对4种柽瘿蚊分布、寄主植物、生活史、行为及对寄主植物的选择性等生物学特性进行了观察。4种瘿蚊造成柽柳枝条形成性状各异的虫瘿，多一室一虫。齿腿柽瘿蚊在新疆乌鲁木齐市一年4代，毛尾柽瘿蚊在新疆阜康一年1代，均以老熟幼虫在虫瘿中越冬；在室内养虫盒和罩笼中，齿腿柽瘿蚊平均产卵42粒，害柽瘿蚊平均产卵56粒，卵多单产。寄生齿腿柽瘿蚊、毛尾柽瘿蚊的寄生蜂均分别有姬小蜂科和金小蜂科各1种。寄生蜂对齿腿柽瘿蚊越冬代的寄生率为38．5％，第1代蛹末期为58．0％，第2代蛹末期为91．3％，第3代蛹末期为3．6％；对毛尾柽瘿蚊蛹期寄生率为58．8％。接种卵试验表明，齿腿柽瘿蚊可以在采自美国的盆栽多枝柽柳上成功地取食、造成虫瘿，但不能在盆栽杨树和柳树上取食、存活。     

Basal stem rot of oil palm (Elaeis guineensis); mode of root infection and lower stem invasion by Ganoderma boninense

Reproducible infection of intact roots of oil palm ( Elaeis guineensis ) with Ganoderma boninense , the cause of basal stem rot, showed penetration followed by rapid longitudinal progression of hyphae and colonization of the lower stem (bole). Light and transmission electron microscopy showed invasion of the root cortex, with no evidence of selective progression through the vascular system or lacunae. In newly colonized tissue the fungus behaved as a hemibiotroph, with numerous, wide, intracellular hyphae occupying entire host cells that possessed intact cell walls and contained discernible cytoplasm and organelles. In the bole this phase coincided with a complete depletion of previously abundant starch grains in advance of invasion. Subsequently, in the roots and colonized stem base, widespread necrotrophic, enzymatic attack of all layers of the host cell walls occurred. Hyphae were intra- and intercellular and intramural and associated host cell wall degradation was often at a distance from hyphae, resulting in cavities within cell walls. A third developmental stage was the formation of an extensive, melanized, tough mycelium or pseudo-sclerotium which surrounded roots and comprised many very thick-walled cells encasing more typical thin-walled hyphae. Macroscopic observation of and isolation from the bole of randomly felled, commercial palms provided confirmatory evidence that multiple infections originated in the roots before spreading into the base of long-established palms.     

枸杞红瘿蚊和枸杞白瘿蚊所致虫瘿形态结构差异

枸杞红瘿蚊是宁夏枸杞上重要成灾害虫之一。枸杞白瘿蚊是继枸杞红瘿蚊后在宁夏枸杞上新发现的另一种瘿蚊科害虫。两种瘿蚊均以幼虫为害宁夏枸杞幼嫩花蕾使其不能正常发育,但形成的虫瘿结构差异明显。本文通过外部形态观察、石蜡切片等方法对两种虫瘿发育过程中的外部形态特征、内部组织结构进行了对比。结果表明,两种瘿蚊幼虫的取食不会引起花部结构的增加或缺失,但均影响花部结构发育,使其无法正常开花结果。枸杞红瘿蚊主要造成子房细胞数量增多形成虫瘿,枸杞白瘿蚊主要造成花冠细胞体积增大形成虫瘿。此外两种虫瘿在花柱、花丝、蜜腺、花药、花柄等处均存在一定差异。     

Anatomy of cranberry stem gall and localization of bacteria in galls

ABSTRACT Cranberry stem gall is characterized by tumors that girdle stems, thereby killing all distal leaves, flowers, and fruit. Bacteria that produce high levels of the plant growth hormone indole-3-acetic acid (IAA) are associated with and believed to cause cranberry stem gall. The anatomy of naturally occurring galls on woody cranberry plants and galls caused by inoculation of micropropagated cranberry plants with Pantoea agglomerans strain 4/99 was consistent with elevated levels of IAA in plants. Field galls exhibited hypertrophy and hyperplasia of tissue external to the vascular cambium, resulting in extensive stem swelling and splitting of the periderm. Similarly, galls on micropropagated plants contained enlarged cortical parenchyma cells. The current year's xylem vessels in field galls were narrow and dense compared with xylem vessels of healthy stems. Curved xylem elements apparently developed de novo within field galls and galls on inoculated plants. Cavities and fissures in both types of galls contained dense aggregates of bacteria. Treatment of micropropagated plants with synthetic IAA caused hypertrophy of cortical parenchyma and formation of adventitious roots. The results support the hypothesis that IAA-producing bacteria cause cranberry stem gall.     

Suppression of clubroot (Plasmodiophora brassicae) development in Arabidopsis thaliana by the endophytic fungus Acremonium alternatum

This study investigated the ability of an endophytic fungus Acremonium alternatum to reduce clubroot formation in the model plant Arabidopsis thaliana, which is highly susceptible to Plasmodiophora brassicae . Quantitative PCR demonstrated that A. alternatum colonized the P. brassicae -infected roots and shoots of the host plant. When Arabidopsis plants were co-inoculated with P. brassicae and A. alternatum , gall formation was reduced as shown by the reduction of the disease index (DI) by up to 50% compared to plants only infected with P. brassicae, whereas the infection rate was lowered by about 20% only in several, but not all, experiments. Clubroot was similarly suppressed when plants were inoculated with autoclaved A. alternatum spores or spore extracts, showing that viable spores were not needed. However, A. alternatum spores did not inhibit P. brassicae resting spore germination. Compared to the normal root galls, the smaller root galls on A. alternatum -inoculated plants contained fewer resting spores of the clubroot pathogen. It was thus hypothesized that inoculation with A. alternatum delayed the development of P. brassicae . Using quantitative RT-PCR to monitor the expression of P. brassicae genes differentially expressed during the development of the disease, a delayed pathogen development was corroborated. Furthermore, greenhouse experiments identified a time window in which the endophyte had to be administered, where the latest effective time point was 5 days before inoculation with P. brassicae and the optimum treatment was to administer A. alternatum and P. brassicae at the same time. These results indicate that A. alternatum and perhaps similar endophytes could be useful for the management of clubroot disease.     

Histological investigation of stripe rust (Puccinia striiformis f.sp. tritici) development in resistant and susceptible wheat cultivars

The wheat cultivar Kariega expresses complete adult plant resistance against stripe rust, whereas cv. Avocet S is susceptible. Using confocal laser scanning microscopy, initial fungal penetration into flag leaves was identical in both cultivars, with directional germ-tube growth towards stomata that were penetrated without the formation of an appressorium, followed by differentiation of a substomatal vesicle, infection hyphae, haustorial mother cells and haustoria. During the following 4 days, further fungal development occurred more quickly in the resistant than in the susceptible cultivar. However, by 7 days postinoculation (dpi) the situation changed, with exponential growth of the pathogen occurring only in the susceptible line. Induced cellular lignification, a typical defence reaction of cereals, was observed at 4 dpi in the resistant cultivar, and 2 days later lignified tissue completely surrounded the fungal colonies. In the susceptible cultivar, isolated lignified host cells occurred at 6 dpi, and long, unbranched fungal hyphae outgrowing the resistance reaction were observed.     

Subcuticular-Intracellular Hemibiotrophic and Intercellular Necrotrophic Development of Colletotrichum acutatum on Almond

ABSTRACT The early infection and colonization processes of Colletotrichum acutatum on leaves and petals of two almond cultivars with different susceptibility to anthracnose (i.e., cvs. Carmel and Nonpareil) were examined using digital image analysis of light micrographs and histological techniques. Inoculated tissue surfaces were evaluated at selected times after inoculation and incubation at 20 degrees C. Depth maps and line profiles of the digital image analysis allowed rapid depth quantification of fungal colonization in numerous tissue samples. The results showed that the early development of C. acutatum on petals was different from that on leaf tissue. On petals, conidia germinated more rapidly, germ tubes were longer, and fewer appressoria developed than on leaves. On both tissues, penetration by the pathogen occurred from appressoria and host colonization was first subcuticular and then intracellular. On petals, colonizing hyphae were first observed 24 h after inoculation and incubation at 20 degrees C, whereas on leaves they were seen 48 to 72 h after inoculation. Intercellular hyphae were formed before host cells became necrotic and macroscopic lesions developed on petals >/=48 h and on leaves >/=96 h after inoculation. Histological studies complemented data obtained by digital image analysis and showed that the fungus produced infection vesicles and broad hyphae below the cuticle and in epidermal cells. In both tissues, during the first 24 to 48 h after penetration fungal colonization was biotrophic based on the presence of healthy host cells adjacent to fungal hyphae. Later, during intercellular growth, the host-pathogen interaction became necrotrophic with collapsed host cells. Quantitative differences in appressorium formation and host colonization were found between the two almond cultivars studied. Thus, on the less susceptible cv. Nonpareil fewer appressoria developed and host colonization was reduced compared with that on cv. Carmel.     

Interaction of Root-knot Nematodes (RKN) and the Bacterium Agrobacterium Tumefaciens in Roots of Prunus Cerasifera: Evidence of the Protective Effect of the Ma RKN Resistance Genes Against Expression of Crown Gall Symptoms

Agrobacterium tumefaciens (AT) is the causal agent of crown gall, a major problem in the family Rosaceae and particularly for Prunus spp. Crown gall symptoms result from the bacterial infection of the cells damaged mechanically at the collar or by root parasitic nematodes. Myrobalan plum (P. cerasifera) is susceptible to AT and is not a host for the root-knot nematode (RKN), M. hapla. Some clones of this plum carry single Ma resistance genes that control M. arenaria, M. incognita and M. javanica. The four above mentioned RKN and Myrobalan progenies segregating for Ma were used in experiments aimed at obtaining a better knowledge of the interaction between AT and RKN in relation to the RKN resistance genes. Prunus rooted cuttings, naturally infected with the bacterium were repotted, grown and inoculated individually with RKN. In a first experiment, Prunus plants were (i) either inoculated with 10,000 juveniles (J2s) of M. arenaria to provide a short inoculum pressure (SIP) or (ii) inoculated by association with one M. arenaria-galled tomato root system that produced a high and durable inoculum pressure of the same nematode species. Four months after RKN inoculation, plants were rated for nematode and bacterial root galling symptoms. RKN and AT galls were more numerous and more homogenous under DIP than under SIP. Nevertheless, for both inoculum regimes, AT galls were present in the RKN-susceptible clones (= carrying none of the Ma genes) and absent in the RKN-resistant clones. Subsequent experiments, conducted under DIP with M. arenaria, M. incognita, M. javanica and M. hapla, also showed, for the three first species, the presence of AT galls only in RKN-susceptible clones whereas Prunus plants inoculated with M. hapla and nematode-free controls were free of AT galls. Consequently RKN act as a wound agent in the AT infection process of Myrobalan plum only when the plant develops a compatible reaction (i.e. when it lacks the Ma resistance genes). Considering that J2s do penetrate the roots of resistant plants, the absence of crown gall symptoms on this material even under durable inoculum pressure strengthens the hypothesis that this nematode stage has a very weak effect on plant cells during the infection process. This is the first evidence of the protective effect of a RKN resistance gene against the expression of root crown gall consecutive to RKN infection. The protective effect of Ma and presumably of other RKN resistance genes against AT is a strong argument for their introgression into Prunus and other Rosaceae or bacterium-susceptible crops.     

Susceptibility of hairy root lines of Brassica species to Plasmodiophora brassicae and in an in vitro subculture system

To investigate the susceptibility of hairy root lines of Brassica species to Plasmodiophora brassicae, hairy roots were induced in a number of Brassica species with Agrobacterium rhizogenes. Turnip hairy root was highly susceptible to P. brassicae; infection rates were high and large galls formed. In contrast, the rates of root hair infection and gall formation on intact Brassica plants did not differ significantly from the control. To induce resting spore formation, turnip hairy roots were incubated at 15°, 20°, or 25°C after 3 weeks of incubation at 25°C. The number and fresh mass of the galls per hairy root were higher and formation of resting spores was greatest after a 7-week incubation at 20°C. To subculture P. brassicae using turnip hairy root, turnip hairy roots were reinoculated with resting spores and gall with resting spores then formed on the hairy roots. In this way, P. brassicae using hairy roots could be subcultured in vitro two or three times on three single-spore isolates of P. brassicae. This is the first report of in vitro subculture of P. brassicae using hairy root.     

禾谷镰刀菌在小麦穗部侵染过程的细胞学研究

 采用扫描和透射电镜技术系统地观察了禾谷镰刀菌(Fusarium graminearum)在小麦穗部的侵染过程。接种后6~12 h,分生孢子在小麦穗部的任何部位均可萌发,每个孢子可产生1至多个芽管,新产生的芽管并不立即入侵寄主组织,而是在寄主体表生长扩展;接种后36~48 h观察,小穗颖片、外稃、内稃的内侧和子房的表面形成了密集的菌丝网,然而在小麦穗轴表面、颖片和内稃的外表面,菌丝生长缓慢、分布稀疏,但颖片外表边缘的菌丝可跨越边缘扩展到颖片的内表皮上;接种后36 h,寄主体表菌丝产生入侵菌丝,以直接入侵方式由颖片、外稃、内稃的内侧及子房的顶部侵入寄主组织体内,随后,菌丝以胞间和胞内生长的方式向下扩展;接种后4~5 d,菌丝由上述组织扩展到达穗轴后,在穗轴内沿微管束组织和皮层组织向上和向下扩展,延伸到相邻小花,随菌丝在小麦穗部组织内不断地生长扩展,使得寄主细胞坏死、解体,并最终导致整个麦穗的枯死。     

Interaction of red raspberry cultivars with isolates of Phragmidium rubi-idaei

Seven cultivars of red raspberry ( Rubus idaeus ) showed different susceptibility to infection by yellow rust ( Phragmidium rubi-idaei ) at two commercial plantations in Scotland and North Wales, The cultivars Latham, Chief and Boyne were completely resistant. Mailing Delight and Glen Clova supported more telial more than Mailing Jewel. In contrast the last named was the most susceptible cultivar at the aecial stage in Scotland but developed similar numbers of aecia to Glen Clova and Mailing Delight in Wales.
Meeker had an extended latent period, fewer and smaller pustules and a very small yield of urediniospores compared to the other susceptible cultivars, thus showing the characteristics of slow-rusting genotypes. At the Scottish site in 1982, where rust was less than in Wales, Meeker and Mailing Jewel performed differently to the other cultivars at the telial stage. Inoculation studies under controlled environmental conditions with urediniospores from each site revealed an interaction between cultivars and isolates suggesting that variation in pathogenicity is present in populations of P. rubi-idaei in the UK.     

Genetic Diversity in Poplar Leaf Rust (Melampsora medusae f. sp. deltoidae) in the Zones of Host Sympatry and Allopatry

ABSTRACT Poplar leaf rust caused by Melampsora medusae f. sp. deltoidae is a widespread disease in North America, where epidemics occur within zones of sympatry and allopatry of telial hosts (Populus spp.) and aecial hosts (Larix spp.). To test the hypothesis that epidemics originate in the zone of sympatry where the rust can complete its life cycle, populations in sympatry and allopatry were analyzed with single-strand conformational polymorphism for codominant detection of alleles directly from uredinia. More alleles were detected in rust populations in the zone of host sympatry than in allopatry. Almost all alleles found in the zone of allopatry were a subset of the allelic diversity present in the zone of host sympatry. Distance analyses clustered populations according to geographic origin, but not sampling year or type of stand (plantation or natural stands). Large differences in allelic and genotypic frequency were observed between years in allopatry but not in sympatry, suggesting new colonizations in allopatric populations. Our results point to a dynamic and complex pattern of inoculum dissemination in polar leaf rust. The hypothesis most consistent with our results is that populations in sympatry represent a source of inoculum for epidemics, with some annual recolonization in allopatry, possibly via intermediate population jumps.     

Leaf Rust on Aegilops speltoides Caused by a New Forma Specialis of Puccinia triticina

ABSTRACT A leaf rust attacking Aegilops speltoides in its natural habitat is reported for the first time. It was found in two locations in northern and central Israel. The two collections from A. speltoides resemble wheat leaf rust, Puccinia triticina, in most spore dimensions, in the morphology of the substomatal vesicle of the urediniospore, and in DNA content in pycniospore nuclei. Similarly to P. triticina isolates from wheat, isolates taken from A. speltoides are compatible with Thalictrum speciosissimum as an aecial host and they are crossed easily with wheat leaf rust isolates. However, isolates from A. speltoides differ from wheat leaf rust in their telial host range. They are avirulent to cultivated wheat cultivars, but attack hundreds of A. speltoides accessions that were immune to wheat leaf rust. This distinct host preference justifies delineation of the newly found leaf rust as a forma specialis (f. sp. speltoides) within P. triticina.     

Waarnemingen betreffende Ditylenchus radicicola (Greeff, 1872) Filipjev, 1936

Summary On account of recent records ofDitylenchus radicicola (Greeff, 1872)Filipjev, 1936 in Dutch meadows, a study was made of its biology. Literature data were summarized.The eelworm attacks the roots of Gramineae and causes spiralshaped galls, which may contain up to 60 adults and some thousands of eggs and larvae each. Young larvae may actively hatch in water from whole, undecayed galls at temperatures around 17°C (Graph. 1). Root exudate of host plants did not stimulate the hatching process. The first larval stage is infective and may penetrate young root tips and also young galls (Fig. 1); it can withstand 2 month desiccation and also desinfection in a solution of an organic mercury compound. The host plant influences the number and size of the root galls (Graph. 2). Fig. 2 shows the anatomy of a young gall.The parasite is only found in grasses in this country and damage has not yet been found. Rye, wheat, barley and oats proved to be susceptible in inoculation trials; 17 plant species have been mentioned as natural hosts in different countries now and 3 are only found susceptible in inoculation trials.     

Cystiphora sonchi (Vallot, 1827) (Diptera: Cecidomyiidae), first record for Tunisia and Africa

During 2017 and 2018, from June to October, leaf galls were observed for the first time on Sonchus oleraceus L. (Asteraceae), sow thistle, in Akouda‐Sousse, Tunisia. Leaves with galls were collected and brought to the laboratory. After a few days, adults emerged from the leaf galls. Morphological identification showed the emerged gall midges to be Cystiphora sonchi (Vallot, 1827) (Diptera: Cecidomyiidae). This is the first record of this species in Tunisia and Africa.     

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.     

葡萄根癌土壤杆菌的不同生化型对葡萄及其它植物致病性的初步研究

 从北京、内蒙古、吉林、辽宁、山东等地采到的葡萄根癌病标本中,分离到根癌土壤杆菌[Agrobacterium fumefaciens(Smith&Town.) Conn]3种生化型菌株。它们对玫瑰香葡萄的致病性明显不同。生化Ⅲ型菌株致病性强,形成大、中型瘤。生化Ⅰ型、Ⅱ型菌株致病性弱,只形成小瘤。生化Ⅲ型的不同菌株对葡萄、向日葵、烟草、番茄等8种植物的致病性也很不同。一类只能侵染葡萄,是窄寄主型;另一类除葡萄外,还能侵染向日葵、烟草、番茄,是宽寄主型。玫瑰香、龙眼等葡萄对3种生化型菌株比较敏感;莎巴珍珠、贝达等比较抗病。生化Ⅲ型和Ⅰ型的一些菌株也能侵染梨、李、杏、樱桃、榆叶梅等几种果树。从山东白雅葡萄上分离的BS33-10菌株(胭脂碱质粒类型)接种在烟草、番茄上能形成畸嵌瘤。     

本土寄生蜂对移殖天敌昆虫泽兰实蝇的寄生

泽兰实蝇（Procecidochares utilis Stone）是我国入侵杂草紫茎泽兰的专食性天敌昆虫。为摸清本土寄生蜂对泽兰实蝇的寄生情况，在云南昆明市选取林下和开阔地等两种生境，分别采用对角线五点取样法与邻接格子取样法，调查了紫茎泽兰的生长及感染虫瘿情况；通过饲养与解剖虫瘿，观察了寄生泽兰实蝇的本土寄生蜂种类及其寄生率。结果表明，共有6种寄生蜂从虫瘿中羽化，对虫瘿的寄生率达70％以上；其中大部分虫瘿被3种寄生蜂寄生。虽然在开阔地生境中紫茎泽兰株高和每株枝条数显著大于林下生境，植株和枝条感虫（瘿）率也显著高于林下，但寄生蜂对虫瘿的总寄生率、各种寄生蜂的寄生率在两种生境之间差异不显著。解剖表明，每虫瘿羽化出的寄生蜂数随虫瘿横径的增大而显著增多。