Quest to elucidate the life cycle of Puccinia psidii sensu lato

There is controversy surrounding the described life cycle of the rust fungus Puccinia psidii sensu lato, which causes disease on several plant species in the family Myrtaceae. The objective of this study was to determine whether P. psidii s.l. is autoecious by performing basidiospore inoculations, and microscopically examining the fate of basidiospores on the leaf surface and nuclear condition at different stages of rust development. No spermogonia developed on leaves of Agonis flexuosa inoculated either with a teliospore suspension or basidiospores naturally discharged from telia. Uredinial sori that developed in all three inoculations with teliospore suspensions and in one of the five inoculations with naturally-discharged basidiospores from telia were most likely the result of urediniospore infections. Microsatellite analysis revealed that isolates made from these uredinial sori had the same multilocus genotype as that of the original isolate. No signs of penetration of plant cells by basidiospores were observed on A. flexuosa and Syzygium jambos. The nuclear condition of mycelia of uredinial sori, urediniospores, teliospores, and four-celled metabasidia was typical of that in many rust fungi. Our study could not provide unequivocal proof that P. psidii s.l. is autoecious. While it is possible that it could be heteroecious, with an unknown alternate aecial host, it is also possible that basidiospores have lost the ability to infect Myrtaceae or are infrequently operational.     

Life cycle of <Emphasis Type="Italic">Uromyces appendiculatus</Emphasis> var. <Emphasis Type="Italic">azukicola</Emphasis> on <Emphasis Type="Italic">Vigna angularis</Emphasis>

Field observations and inoculation experiments revealed that Uromyces appendiculatus var. azukicola has an autoecious and macrocyclic life cycle and produces spermogonia, aecia, uredinia, and telia on Vigna angularis var. angularis and V. angularis var. nipponensis. From inoculation experiments, it was suggested that this rust fungus has different host relationships from other varieties. Morphological examinations revealed that the characteristics of urediniospores and teliospores are different among varieties, although aeciospores are morphologically similar to each other.Contribution no. 182, Laboratory of Plant Parasitic Mycology, Institute of Agriculture and Forestry, University of Tsukuba, Japan     

Aphanocladiutn album, a fungus inducing teliospore production in rusts

The hyphomycetous fungus Aphanocladiutn album can grow over and around uredia of the rusts Puccinia coronata, P. hordei, P. graminis f.sp. avenae and P. recondita f.sp. triticina when host plants are kept under very humid conditions, but not on such plants not infected with rusts; uredia are adversely affected and telia develop in their vicinity. Plants inoculated with these rusts and with five isolates of A. album (one from a dead insect) showed: (1) much earlier development of telia on detached and non-detached rusted leaves inoculated with A. album than on corresponding leaves not thus inoculated; (2) telial induction by A. album in some isolates of rust species which hitherto had rarely or never produced telia; (3) precocious telial formation, in comparison with controls, when A. album spores were sprayed on leaves as much as 3 days before and 9 days after rust inoculation, and occasionally after uredia had already matured. As affected leaves remained green until the whole leaf became moribund, senescence is apparently not the factor inducing telia formation. The normal-appearing teliospores of some isolates were induced to germinate, whereas others did not. Rhamnus palaestina inoculated with basidiospores of one isolate of A. album-treated P. coronata f.sp. avenae produced pycnia and fertile aecia. The importance of A. album as a working tool in rust research and as a possible means for biological control of rust epiphytotics is discussed.     

Puccinia galiiuniversa,a new caricicolous rust fungus systemically inhabiting Galium aparine in its spermogonial–aecial stage

Field observations and inoculation experiments revealed that spermogonia and aecia produced on systemically infected plants of Galium aparine and uredinia and telia on Carex maackii are different stages of the life cycle of a Puccinia fungus. By comparative morphology with allied species, the fungus was concluded to be a new species and named as Puccinia galiiuniversa.     

Microscopic observations of a compatible host/pathogen interaction between a potential biocontrol agent (Uromyces pencanus) and its target weed (Nassella neesiana)

Uromyces pencanus is a prospective biocontrol agent for Nassella neesiana in Australia and New Zealand. The infection process of U. pencanus urediniospores in leaves of its susceptible host N. neesiana was found to be similar to that of most other rust species. A pattern against which to compare nonhost reactions in host range experiments was achieved using a modification of Bruzzese and Hasan’s whole leaf clearing and staining technique. The staining of microtome leaf sections provided supplemental information about this pathosystem.     

Self-fertility and other distinguishing characteristics of a new morphotype of Puccinia coronata pathogenic on smooth brome grass

A new morphotype of Puccinia coronata, pathogenic to Bromus inermis, a common roadside and pasture grass in the northern United States, was discovered in the 1990s and described as P. coronata f. sp. bromi by Delgado et al in 2001. Puccinia coronata f. sp. bromi does not require fertilization of pycnia to produce aecia on its alternate host, whereas fertilization is required in all other varieties or formae speciales of P. coronata with aecial hosts in the family Rhamnaceae and for which life cycles have been described. Promycelia of P. coronata f. sp. bromi produce only two basidiospores, and each receives a pair of nuclei from the promycelium. The nuclei divide again so that mature basidiospores each contain four nuclei. Puccinia coronata f. sp. bromi has smaller teliospores than P. coronata var. avenae, and its substomatal vesicles are non-septate and distinctly shaped compared to those of P. coronata var. avenae. In addition, nuclei of P. coronata f. sp. bromi contain less DNA than those of P. coronata var. avenae. Puccinia coronata f. sp. bromi is further distinguished from P. coronata var. avenae and P. coronata var. hordei in being avirulent on both oat and barley, whereas neither P. coronata var. avenae nor P. coronata var. hordei are virulent on Bromus inermis.     

Initiation of Meiotic Recombination in Ustilago maydis

A central feature of meiosis is the pairing and recombination of homologous chromosomes. Ustilago maydis, a biotrophic fungus that parasitizes maize, has long been utilized as an experimental system for studying recombination, but it has not been clear when in the life cycle meiotic recombination initiates. U. maydis forms dormant diploid teliospores as the end product of the infection process. Upon germination, teliospores complete meiosis to produce four haploid basidiospores. Here we asked whether the meiotic process begins when teliospores germinate or at an earlier stage in development. When teliospores homozygous for a cdc45 mutation temperature sensitive for DNA synthesis were germinated at the restrictive temperature, four nuclei became visible. This implies that teliospores have already undergone premeiotic DNA synthesis and suggests that meiotic recombination initiates at a stage of infection before teliospores mature. Determination of homologous recombination in plant tissue infected with U. maydis strains heteroallelic for the nar1 gene revealed that Nar⁺ recombinants were produced at a stage before teliospore maturation. Teliospores obtained from a spo11Δ cross were still able to germinate but the process was highly disturbed and the meiotic products were imbalanced in chromosomal complement. These results show that in U. maydis, homologous recombination initiates during the infection process and that meiosis can proceed even in the absence of Spo11, but with loss of genomic integrity.     

New geographic distribution records and assumed microcyclic life cycle of Triphragmiopsis jeffersoniae (Pucciniales)

Triphragmiopsis jeffersoniae was found on Jeffersonia dubia in Sapporo, Japan, in 2003. This was the first geographic distribution record of the fungus out of its native distribution range in continental Far East Asia. This fungus was found also in Nikko, Japan, in 2011. Triphragmiopsis jeffersoniae formed Aecidium-type sori and tripartite teliospores subtended by a pedicel. In the early spring, the fungus formed telia with or without Aecidium-type sori on petioles of emerging host leaves. Subsequently clusters of Aecidium-type sori were formed on the abaxial leaf surface. These sori did not repeat; but they were soon densely surrounded by telia. Both the Aecidium-type sori and telia were densely crowded and no small isolated telia were formed. The assumption that the spores from Aecidium-type sori do not have infective ability was confirmed by inoculation experiments and microscopic observations for the Japanese materials. The Aecidium-type spore germlings failed to invade the Jeffersonia leaf either directly or through a stoma under the experimental conditions. Thus, T. jeffersoniae was assumed to have microcyclic life cycle, comprising the functional teliospores and the non-functional Aecidium-type aeciospores.     

Life cycle of Melampsora coleosporioides,a leaf rust of Salix babylonica in Japan

Melampsora coleosporioides produces uredinia and telia on the leaves of Salix babylonica. Since the life cycle of this fungus is largely unknown, inoculation experiments were conducted to find alternate host plants. Results showed that M. coleosporioides can use Corydalis incisa as a spermogonial and aecial host. The morphological characteristics of the spermogonial and aecial states of the fungus are first described. Furthermore, field observations and histological studies demonstrated that the fungus was able to overwinter in twigs of S. babylonica and produced urediniospores in early spring. Thus, the leaf rust occurs on S. babylonica every year and alternation with the spermogonial and aecial host is not necessary.     

Melampsora salicis-sinicae (Melampsoraceae,Pucciniales), a new rust fungus found on willows in China

Based on morphological characteristics and molecular phylogeny, a rust fungus on Salix hsinganica, S. sinica, S. starkeana, S. wallichiana and S. xerophila in China is described as a new species, Melampsora salicis-sinicae. This new species morphologically resembles M. capraearum and M. epiphylla in having subcuticular teliospores with a thickened apical wall, but it differs from these two species in its amphigenous telia. In addition, this new species differs from M. capraearum in its slender teliospores and thinner apical thickness of teliospores, and it also differs from M. epiphylla in the dimension of urediniospores, the position of teliospores and the apical thickness of teliospores. Phylogenetic analyses using partial sequences of the D1/D2 region of nuclear large subunit and the internal transcribed spacer (ITS1 and ITS2) regions including 5.8S of ribosomal RNA gene and the translation elongation factor 1 alpha (EF-1α) gene revealed that this rust fungus was distinct from its morphologically similar species, M. capraearum and M. epiphylla.     

Species composition and distribution of Coleosporium species on the needles of Pinus densiflora at a semi-natural vegetation succession site in central Japan

Coleosporium species cause pine needle rust. Most species have heteromacrocyclic life cycles, and 12 species use Pinus densiflora as aecial hosts. To understand the biology of rust fungi and develop better methods for controlling rust diseases, it is necessary to clarify that which Coleosporium species affect pine trees. However, Coleosporium on pine trees have rarely been identified at the species level because of their morphological similarities. We used polymerase chain reaction - restriction fragment length polymorphism (PCR-RFLP) to clarify the species composition, abundance, and distribution of Coleosporium in a P. densiflora forest. We surveyed a site where several Coleosporium species might complete their life cycles. PCR-RFLP revealed four species on the pines: C. asterum, C. clematidis-apiifoliae, C. lycopodis, and C. phellodendri. Coleosporium phellodendri was distributed throughout the forest and was the most abundant. Aecia of C. phellodendri formed mainly on 2-y-old needles. The abundance and distribution of C. phellodendri appeared to be affected by the longer effective dispersal range of basidiospores and the existence of abundant inoculum sources. The age of leaves where C. phellodendri form aecia mainly was thought to be influenced by the characteristic life cycle, with aecial formation requiring 2?y after basidiospore infection.     

牛紏吴茱萸柄锈菌新种

本文描述了采自海南省琼中县牛紏吴茱萸Evodia trichotoma (Lour.) Pierre(芸香科)上的柄锈菌一新种——Puccinia evodiae-trichotomae J. -y. Zhuang。本种与巴布亚一新几内亚产的吴茱萸Evodia sp.上的另一种柄锈菌Puccinia evodiae Cumm.略近似,但冬孢子堆裸露呈垫状,而后者则深埋于病瘿中。冬孢子形状与大小和后者相仿,然而少数孢子可长达60μm,顶壁有时略增厚,颜色很浅或近几色。镜下可见一些孢子已萌发,推测可能属无眠冬孢型。芸香科植物上的柄锈菌目前仅知10种,本种特征与它们均不相同,故鉴定为新种。本新种模式标本保藏在中国科学院微生物研究所真菌标本室。     

Spore dimensions of Puccinia species of cereal hosts as determined by image analysis

Digital image analysis was used to measure dimensions of spores produced by Puccinia coronata, P. graminis, P. hordei, P. recondita, P. striiformis and P. triticina. Included were teliospores, basidiospores, urediniospores and, except for P. striiformis, pycniospores and aeciospores. Length, width and projection area of spores were measured with NIH Image or Scion software. By using limits on size, spores were automatically selected and measured, except for teliospores, which required manual elimination of the pedicel and separation of images of adhering spores. Length and width were determined as the major and minor axes of the best fitting ellipse for each spore. This procedure gave values for length and width close to results obtained with an ocular micrometer. Projection area was determined as the number of pixels within spore boundaries multiplied by the area represented by each pixel, giving values that are not feasible to obtain accurately with an ocular micrometer. Of the species studied, spores of P. recondita had the largest dimensions, P. triticina had the smallest. The rank of the six species based on increasing width, length or projection area was almost the same, using each spore type except pycniospores. Generally, differences of 5% in a given spore dimension between two species were significant. Differences between species were greater with basidiospores and aeciospores than with other spore types. Teliospores were unique in that length and width were negatively correlated, resulting in less variation in area than in length or width. The results indicate that image analysis is useful for measuring spore dimensions, that projection area of spores is a useful added parameter for characterizing rust species and that dimensions of teliospores, basidiospores, aeciospores and urediniospores each are potentially useful for differentiating species.     

On the hunt for the alternate host of Hemileia vastatrix

Coffee leaf rust (CLR), caused by the fungal pathogen Hemileia vastatrix, has plagued coffee production worldwide for over 150 years. Hemileia vastatrix produces urediniospores, teliospores, and the sexual basidiospores. Infection of coffee by basidiospores of H. vastatrix has never been reported and thus far, no alternate host, capable of supporting an aecial stage in the disease cycle, has been found. Due to this, some argue that an alternate host of H. vastatrix does not exist. Yet, to date, the plant pathology community has been puzzled by the ability of H. vastatrix to overcome resistance in coffee cultivars despite the apparent lack of sexual reproduction and an aecidial stage. The purpose of this study was to introduce a new method to search for the alternate host(s) of H. vastatrix. To do this, we present the novel hypothetical alternate host ranking (HAHR) method and an automated text mining (ATM) procedure, utilizing comprehensive biogeographical botanical data from the designated sites of interests (Ethiopia, Kenya and Sri Lanka) and plant pathology insights. With the HAHR/ATM methods, we produced prioritized lists of potential alternate hosts plant of coffee leaf rust. This is a first attempt to seek out an alternate plant host of a pathogenic fungus in this manner. The HAHR method showed the highest‐ranking probable alternate host as Psychotria mahonii, Rubus apetalus, and Rhamnus prinoides. The cross‐referenced results by the two methods suggest that plant genera of interest are Croton, Euphorbia, and Rubus. The HAHR and ATM methods may also be applied to other plant–rust interactions that include an unknown alternate host or any other biological system, which rely on data mining of published data.     

Uromycladium tepperianum, the gall rust fungus from Falcataria moluccana in Malaysia and Indonesia

Batai (Falcataria moluccana) is a valuable tree species for forest plantations in Malaysia and Indonesia. Since 1993, a gall rust disease has caused severe damage to all growth stages, from seedlings in the nursery to mature trees in the field. To identify the fungus causing gall rust disease on F. moluccana in Malaysia and Indonesia, study of the mode of infection and changes in the anatomy of infected cells were carried out in the anatomy laboratory. The disease in Malaysia and Indonesia is caused by Uromycladium tepperianum. The fungus produces three longitudinally ridged teliospores on each head, with spores measuring 13–20 μm wide and 17–28 μm long. The fungus is microcyclic, completing its entire life cycle on F. moluccana. This study confirmed that the teliospores themselves cannot infect the host. Under favorable conditions, about 10 h after inoculation, teliospores germinate to produce basidiospores that form penetration pegs about 6 h later, and it is this peg which penetrates the host cells directly through the epidermis. Pycnia, recognized as small brown pustules, break through the epidermis about 7 days after inoculation.     

Battling the biotypes of balsam: the biological control of Impatiens glandulifera using the rust fungus Puccinia komarovii var. glanduliferae in GB

Impatiens glandulifera, or Himalayan balsam, is a prolific invader of riverine habitats. Introduced from the Himalayas for ornamental purposes in 1839, this annual species has naturalised across Great Britain (GB) forming dense monocultures with negative affects across whole ecosystems. In 2006 a programme exploring biocontrol as an alternative control method was initiated and to date, two strains of the rust fungus Puccinia komarovii var. glanduliferae have been released. To better understand the observed differences in susceptibility of GB Himalayan balsam stands to the two rust strains, inoculation studies were conducted using urediniospores and basidiospores. Experiments revealed large variation in the susceptibility of stands to urediniospores of the two rust strains, with some resistant to both. Furthermore, the infectivity of basidiospores was found to differ, with some stands fully susceptible to the urediniospore stage, being immune to basidiospore infection. Therefore, before further rust releases at new sites, it is necessary to ensure complete compatibility of the invasive stands with both urediniospores and basidiospores. However, for successful control across GB it is essential that plant biotypes are matched to the most virulent rust strains. This will involve additional strains from the native range to tackle those biotypes resistant to the strains currently released.     

Studies on the rustMaravalia cryptostegiae,a potential biological control agent of rubber-vine weed (Cryptostegia grandiflora,Asclepiadaceae: Periplocoideae) in Australia,I: Life-cycle

Three spore types are described forMaravalia cryptostegiae: hemileioid urediniospores; thin-walled, hyaline, ellipsoidal, non-resting teliospores and ovoid to lacrimoid basidiospores. Field surveys in the Madagascan native range of rubber-vine failed to confirm the existence of spermogonia and morphologically distinct aecia within the life-cycle. Greenhouse inoculations with basidiospores were unsuccessful. Cytological studies revealed that rubber-vine rust has a similar nuclear cycle to that reported for coffee leaf rust,Hemileia vastatrix. The working hypothesis is proposed thatMaravalia cryptostegiae is a primitive, autoecious tropical forest rust with only a short or partially expanded life-cycle represented by two teliospore forms. The predominant, functional form is uredinioid with a novel nuclear cycle, in which there is a delayed meiotic division (the Kamat phenomenon). The non-dispersed form appears to be vestigial or non-functional since it germinates to produce a metabasidium with genetically variable and unstable basidiospores. The relationships and evolutionary significance of the generaMaravalia andHemileia are discussed.     

Metamorphosis of the Basidiomycota Ustilago maydis: Transformation of yeast-like cells into basidiocarps

Ustilago maydis (DC) Cda., a phytopathogenic Basidiomycota, is the causal agent of corn smut. During its life cycle U. maydis alternates between a yeast-like, haploid nonpathogenic stage, and a filamentous, dikaryotic pathogenic form that invades the plant and induces tumor formation. As all the members of the Subphylum Ustilaginomycotina, U. maydis is unable to form basidiocarps, instead it produces teliospores within the tumors that germinate forming a septate basidium (phragmobasidium). We have now established conditions allowing a completely different developmental program of U. maydis when grown on solid medium containing auxins in dual cultures with maize embryogenic calli. Under these conditions U. maydis forms large hemi-spheroidal structures with all the morphological and structural characteristics of gastroid-type basidiocarps. These basidiocarps are made of three distinct hyphal layers, the most internal of which (hymenium) contains non-septate basidia (holobasidia) from which four basidiospores develop. In basidiocarps meiosis and genetic recombination occur, and meiotic products (basidiospores) segregate in a Mendelian fashion. These results are evidence of sexual cycle completion of an Ustilaginomycotina in vitro, and the demonstration that, besides its quasi-obligate biotrophic pathogenic mode of life, U. maydis possesses the genetic program to form basidiocarps as occurs in saprophytic Basidiomycota species.     

Are the microcyclic rust species Puccinia melampodii and Puccinia xanthii conspecific?

The taxonomic relationship between two microcyclic rust species, Puccinia melampodii and Puccinia xanthii, recorded from a number of Asteraceae hosts, was explored by comparing teliospore morphology, host specificity and sequence data. Teliospore morphology varied between and within individual rust accessions but, in general, teliospores of P. xanthii were larger than those of P. melampodii. Field observations and cross-inoculation greenhouse studies showed that P. melampodii accessions were highly specific to their original host species and unable to establish compatible interactions with related Asteraceae species. Though small amounts of genetic variation were detected between P. melampodii and P. xanthii accessions, the ITS and TEF phylogenetic analyses comprising several other rust species grouped these accessions within a well supported clade. Our data indicate that despite differences between P. xanthii and P. melampodii accessions these rusts cannot readily be separated at the species level, supporting earlier taxonomic conclusions and the recent proposal to group them into a morphospecies. Based on host-specificity data, the name Puccinia xanthii var. parthenii-hysterophorae is proposed to accommodate all Mesoamerican records of P. melampodii associated with the host Parthenium hysterophorus.     

Putative natural hybrid between Puccinia lagenophorae and an unknown rust fungus on Senecio madagascariensis in KwaZulu-Natal,South Africa

Several specimens of an aecial rust fungus were collected on Senecio madagascariensis during a field survey carried out in KwaZulu-Natal, South Africa. As telia were not present in the specimens collected, DNA sequence analyses were undertaken to determine the identity of the rust species. ITS and β-tub1 sequencing confirmed that one of the isolates recovered is Puccinia lagenophorae sensu lato. On the other hand, sequencing and RFLP analysis revealed the presence of two divergent copies of ITS and β-tub1 in all the other six isolates investigated. In both phylogenetic trees, one copy of the gene region grouped within a well supported clade with sequences of P. lagenophorae accessions from different geographical origins and hosts, and the Australian rusts Puccinia saccardoi and Puccinia stylidii. The other copy of these gene regions grouped within a separate clade comprising European accessions of Puccinia dioicae (ITS) and Uromyces sommerfeltii (β-tub1) that occur on Asteraceae hosts. Multiple copies of these gene regions were not observed in Australian isolates of P. lagenophorae. Our study provides some evidence that an interspecific hybrid rust fungus, with P. lagenophorae as one of its parents, may occur on S. madagascariensis in South Africa. The identity of the other parent remains unknown.