The Infection of Festuca arundinacea by Puccinia graminis subsp. graminicola

Puccinia graminis subsp. graminicola has caused economically important losses of tall fescue (Festuca arundinacea Schreb.) in North America. This rust infects leaves, culms, and spikelets of the host. Initital symptoms consist of very small, chlorotic flecks, followed by long, narrow lesions. Invasive hyphae were diffuse and filamentous, changing to dense and blocky with the formation of hymenia. Tissues were extensively colonized, including development caryopses. Seedborne infection consisted of mycelium within the embryo and urediniospores carried on both surfaces of the glumes. It is not known whether caryopses that are internally infected can germinate to produce infected individuals, but this could be an important quarantine consideration. Although, at the ultrastructural level, urediniospore development in this fungus is similar to that of other rusts, we detected an additional layer at the interface of the urediniospore and its pedicel. This layer may play a role in the, release of urediniospores from their pedicels.     

Competition Alters Temporal Dynamics of Sporulation in the Wheat Stem Rust Fungus

To investigate the effects of competition on the timing of pathogen reproduction, urediniospores of two strains of Puccinia graminis f.sp. tritici (SR22 and SR41) were inoculated onto leaves of wheat seedlings singly and in 1 : 1 mixture at three inoculum densities. On randomly sampled leaves, uredinia were counted 9 days after inoculation and urediniospores were collected and quantified every other day from the seventh to the 29th day after inoculation. Increases in inoculum density resulted in progressively smaller increases in uredinial numbers. However, total urediniospore production per leaf was not significantly affected by inoculum, and hence uredinial, density over a range of approximately 10-300 uredinia on the leaf. Total urediniospore production per uredinium generally decreased with increasing inoculum or uredinial density. At high densities, sporulation per uredinium peaked earlier in the sporulation period, had a less distinct peak, and dropped off earlier than for the lower densities. Logistic model fits to cumulative sporulation curves over time revealed that strain SR41 had a greater epidemic rate parameter (r) than SR22 at low and intermediate inoculum or uredinial densities, while SR22 had a higher r-value than SR41 at high density. Both strains also exhibited greater r-values in the presence of the other strain than when alone. Results suggest that strains may have different ecological strategies in their timing of reproduction, and that both intra- and interstrain competition can have complex effects on the temporal dynamics of sporulation in pathogen strains.     

Detection of Puccinia pelargonii‐zonalis‐infected Geranium Tissues and Urediniospores

The occurrence of geranium rust (caused by Puccinia pelargonii‐zonalis) in commercial greenhouses can result in unmarketable plants and significant economic losses. Currently, detection of geranium rust relies solely on scouting for symptoms and signs of the disease. The purpose of this research was to develop a rapid detection assay for P. pelargonii‐zonalis‐infected tissues or urediniospores on greenhouse‐grown geraniums. Two oligonucleotide primers were designed based on internal transcribed spacer sequence data from three isolates of P. pelargonii‐zonalis. The primers amplified a 131‐bp product from genomic DNA from each isolate of P. pelargonii‐zonalis but did not amplify a product from genomic DNA from twelve other rust fungi or four other plant pathogenic fungi. A PCR product was amplified consistently from solutions that contained 1 ng or 100 pg/ml of purified P. pelargonii‐zonalis DNA in conventional PCR and at 1 pg/ml using real‐time PCR. The detection threshold was 10² urediniospores/ml for real‐time PCR and 10⁴ urediniospores/ml for conventional PCR using urediniospores collected by vacuum from sporulating lesions. Puccinia pelargonii‐zonalis DNA was amplified by real‐time PCR from urediniospores washed from a single inoculated leaf, but recovered urediniospores were below detection threshold from one inoculated leaf with 5, 10, 25 and 50 non‐inoculated leaves. Conventional and real‐time PCR did not detect P. pelargonii‐zonalis in infected leaf tissues, presumably due to PCR inhibitors in the geranium leaf tissue. The inhibition of both conventional and real‐time PCR by geranium tissues suggests that a detection assay focusing on urediniospore recovery and microscopic examination with subsequent species verification by PCR may be the most efficient method for assessing the presence of geranium rust in greenhouses.     

Exogenous double-stranded RNA inhibits the infection physiology of rust fungi to reduce symptoms in planta

Rust fungi (Pucciniales) are a diverse group of plant pathogens in natural and agricultural systems. They pose ongoing threats to the diversity of native flora and cause annual crop yield losses. Agricultural rusts are predominantly managed with fungicides and breeding for resistance, but new control strategies are needed on non-agricultural plants and in fragile ecosystems. RNA interference (RNAi) induced by exogenous double-stranded RNA (dsRNA) has promise as a sustainable approach for managing plant-pathogenic fungi, including rust fungi. We investigated the mechanisms and impact of exogenous dsRNA on rust fungi through in vitro and whole-plant assays using two species as models, Austropuccinia psidii (the cause of myrtle rust) and Coleosporium plumeriae (the cause of frangipani rust). In vitro, dsRNA either associates externally or is internalized by urediniospores during the early stages of germination. The impact of dsRNA on rust infection architecture was examined on artificial leaf surfaces. dsRNA targeting predicted essential genes significantly reduced germination and inhibited development of infection structures, namely appressoria and penetration pegs. Exogenous dsRNA sprayed onto 1-year-old trees significantly reduced myrtle rust symptoms. Furthermore, we used comparative genomics to assess the wide-scale amenability of dsRNA to control rust fungi. We sequenced genomes of six species of rust fungi, including three new families (Araucariomyceaceae, Phragmidiaceae, and Skierkaceae) and identified key genes of the RNAi pathway across 15 species in eight families of Pucciniales. Together, these findings indicate that dsRNA targeting essential genes has potential for broad-use management of rust fungi across natural and agricultural systems.     

SCANNING ELECTRON MICROSCOPY OF THE UREDINIAL STAGE OF PUCCINIA CANALICULATA ON YELLOW NUTSEDGE,CYPERUS ESCULENTUS (CYPERACEAE)

The ultrastructure of the uredinial stage of the rust fungus, Puccinia canaliculata (Schw.) Lagerh., on yellow nutsedge (Cyperus esculentus L.) is described using scanning electron microscopy. This rust has been proposed as a potential biological control agent against this serious weed species. Urediniospores are ellipsoid to ovoid, and echinulate with the spines distributed over the surface except at basal portions. Spines are in circular depressions, encircled by annuli. Germination is through one of two equatorial germ pores. Germ tube growth is primarily perpendicular to the longitudinal orientation of epidermal cells. Appressoria completely cover stomata. Uredinia develop within linearly aligned intercellular cavities in substomatal regions. Uredinia become erumpent, exposing pedicellate urediniospores and smooth paraphyses scattered throughout the sori. Uredinia are nonperidiate.     

A new species of Melampsora on Populus yunnanensis from China

Melampsora nujiangensis, a rust fungus found on Populus yunnanensis in China, is described as a new species. Light and scanning electron microscopy with herbarium specimens of the rust fungus show that the shape of its urediniospores differs from that of other known species of Melampsora, and its urediniospore walls are thinner than the other species. Furthermore, in phylogenetic trees based on the DNA sequences (28S and ITS) the rust fungus is phylogenetically separated by high bootstrap values. These results indicate that the fungus is an isolated species among the genus Melampsora. Contribution no. 202 from the Laboratory of Plant Parasitic Mycology, Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan     

Effect of light exposure on in vitro germination and germ tube growth of eight species of rust fungi

The effects of light on urediniospore germination and germ tube elongation was studied with eight species of rust fungi that infect ornamental plants or row crops. Exposure of six species of fungi to cool white fluorescent light at 400 or 600 micromol s(-1) m(-2) for 24 h significantly reduced germination with largest decreases typically observed at 600 micromol s(-1) m(-2). Germination and germ tube elongation did not recover during 24 h dark incubation after 18 h exposure to fluorescent light at 600 micromol s(-1) m(-2), indicating the effects were not reversible. Germ tube elongation of all fungi was negatively affected by increased length of exposure to fluorescent light. Increased exposure to fluorescent light differentially affected germination of the fungi with Puccinia hemerocallidis, Phakopsora pachyrhizi, Pucciniastrum vaccinii and Puccinia menthae negatively affected and Puccinia sorghi, Puccinia triticina, Puccinia pelargonii-zonalis and Puccinia iridis relatively unaffected in 10 h incubation. Exposure of Ph. pachyrhizi and P. triticina urediniospores to sunlight rapidly reduced germination and germ tube elongation with no germination observed for Ph. pachyrhizi after 2.5 h. Germ tube elongation but not germination of hydrated urediniospores of Ph. pachyrhizi and P. triticina was significantly reduced compared to dry urediniospores exposed to 10 h fluorescent light followed by 24 h dark incubation. Exposure to fluorescent light (all fungi) or sunlight (two fungi) negatively affected urediniospore germ tube elongation. Differences observed in urediniospore germination between fungi suggest some species have co-evolved with their host for differing light conditions. Our data suggests exposure of urediniospores to strong light could inactivate rust fungi on plant surfaces or in the atmosphere.     

A new species of <Emphasis Type="Italic">Puccinia</Emphasis> (Pucciniales,Basidiomycota) and new records of rust fungi from Panama

A new species of Puccinia with diorchidioid teliospores is described from Urochloa decumbens (Poaceae) in Panama. It differs from the other known species of Puccinia with diorchidioid teliospores by uredinia with paraphyses and urediniospores with 3–4 scattered germ pores. Twenty-one known species of rust fungi are cited for Panama for the first time. Alveolaria cordiae is a poorly known species which is reported, described and illustrated in detail for the first time on Cordia spinescens in Panama.     

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.     

Observations on the ultrastructure of the uredinial stage ofPuccinia polypogonis onPolypogon monspeliensis

The ultrastructure of the uredinial stage of the rust fungus,Puccinia polypogonis onPolypogon monspeliensis is described, using scanning and transmission electron microscopy. This study examined the urediniospores, intercellular hyphae, and haustoria of the fungus. The formation and structure of urediniospores is similar to those of otherPuccinia species. The ultrastructure of intercellular hyphae and haustoria is similar to those of other rust fungi, but with some differences. No modifications are observed in the wall of the haustorial mother cells during penetration. A collar is found only around old haustoria. In most cases, one nucleus is detected inside the haustorial body and no nucleoli are seen in the nuclei of intercellular hyphae and haustoria. The host-parasite interface, including extrahaustorial matrix and extrahaustorial membrane, is also discussed and compared with those of other rust fungi.     

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     

A new species, Dicheirinia panamensis, and new records of rust fungi from Panama

Based on a recent fieldwork in Panama, 25 species of rust fungi and several new hosts are reported for the first time from this country. Among the new records is one new species, Dicheirinia panamensis on Cojoba rufescens (Fabaceae). It differs from known species in the genus Dicheirinia by the presence of uredinia and telia without paraphyses, irregularly tuberculate urediniospores with two germ pores on the flattened sides, and tuberculate teliospores formed by three probasidial cells, subtended by a pedicel with three hyaline, apical cells. Taxonomical novelty: Dicheirinia panamensis J.R. Hern.,M. Piepenbr. and Vega Rios.     

Host and environmental effects on the infection of maize by Puccinia sorghi. II. Post-penetration development

The numbers of pustules which subsequently developed on maize seedlings incubated at 5, 10, 15, and 20 °C and 100% r.h. were directly related to the temperature and length of incubation. The generation time was 16 days at 10 °C, 10 days at 15 °C, 7 days at 20 °C and 5 days at 25 °C. Increase in urediniospore production was observed with increase in temperature. Increase in the inoculum concentration up to 5 × 10⁴ urediniospores/ml increased the number of resultant uredinia. However, further increases did not increase uredinia-formation.     

Wheat leaf rust caused by Puccinia triticina

Leaf rust, caused by Puccinia triticina, is the most common rust disease of wheat. The fungus is an obligate parasite capable of producing infectious urediniospores as long as infected leaf tissue remains alive. Urediniospores can be wind‐disseminated and infect host plants hundreds of kilometres from their source plant, which can result in wheat leaf rust epidemics on a continental scale. This review summarizes current knowledge of the P. triticina/wheat interaction with emphasis on the infection process, molecular aspects of pathogenicity, rust resistance genes in wheat, genetics of the host parasite interaction, and the population biology of P. triticina. Taxonomy: Puccinia triticina Eriks.: kingdom Fungi, phylum Basidiomycota, class Urediniomycetes, order Uredinales, family Pucciniaceae, genus Puccinia. Host range: Telial/uredinial (primary) hosts: common wheat (Triticum aestivum L.), durum wheat (T. turgidum L. var. durum), cultivated emmer wheat (T. dicoccon) and wild emmer wheat (T. dicoccoides), Aegilops speltoides, goatgrass (Ae. cylindrica), and triticale (X Triticosecale). Pycnial/aecial (alternative) hosts: Thalictrum speciosissimum (= T. flavum glaucum) and Isopyrum fumaroides. Identification: Leaf rust is characterized by the uredinial stage. Uredinia are up to 1.5 mm in diameter, erumpent, round to ovoid, with orange to brown uredinia that are scattered on both the upper and the lower leaf surfaces of the primary host. Uredinia produce urediniospores that are sub‐globoid, average 20 µm in diameter and are orange–brown, with up to eight germ pores scattered in thick, echinulate walls. Disease symptoms: Wheat varieties that are fully susceptible have large uredinia without causing chlorosis or necrosis in the host tissues. Resistant wheat varieties are characterized by various responses from small hypersensitive flecks to small to moderate size uredinia that may be surrounded by chlorotic and/or necrotic zones. Useful website: USDA Cereal Disease Laboratory: http://www.ars.usda.gov/mwa/cdl     

Inheritance of resistance to yellow rust (Phragmidium rubi-idaei) in red raspberry

The inheritance of resistance in red raspberry (Rubus idaeus) to yellow rust (Phragmidium rubi-idaei) was studied in a diallel cross among the cultivars Boyne, Meeker, Mailing Jewel, Glen Prosen and Glen Clova. The progenies and clonally propagated parents were exposed initially in a rust-infected plantation where the incidence and severity of infection was assessed at the telial stage. The following spring leaves on new canes of the same plants were inoculated with urediniospores in a glasshouse at 18 ± 2 °C. The latent period and number of uredinia per cm² of abaxial leaf surface were determined 13 and 18 days after inoculation. Cv. Boyne developed only chlorotic flecking on inoculated leaves in the glasshouse and no sporulation was observed. It is postulated that this reaction was determined by a major gene which is designated Yr. Some of the other parents showed incomplete resistance of the slow rusting type, and segregants of Boyne selfed that were susceptible also showed low levels of rust, which suggests that Boyne carries both major- and minor-gene resistance. Diallel analyses of both the field and glasshouse data from the susceptible segregates indicated that the degree of incomplete resistance present was determined predominantly by additive gene action, though small but significant non-additive effects also occurred: cv. Meeker was the most resistant parent and contributed the most resistance to the progenies. The Meeker progenies also segregated for gene H, which determines cane hairiness and is known to be associated with resistance to other diseases. Segregants without hairs (genotypes hh) were found to be the more resistant to rust.     

Microscopy and proteomic analysis of the non-host resistance of <Emphasis Type="Italic">Oryza sativa</Emphasis> to the wheat leaf rust fungus, <Emphasis Type="Italic">Puccinia triticina</Emphasis> f. sp. <Emphasis Type="Italic">tritici</Emphasis>

Rice (Oryza sativa) cv. Nipponbare expresses non-host resistance (NHR) to the wheat leaf rust fungus, Puccinia triticina f. sp. tritici (Ptt). When the leaves of cv. Nipponbare were inoculated with Ptt, approx 93% of the urediniospores germinated on the leaf surface, but only 10% of the germinated spores formed appressoria over the stomata at one day post inoculation (1 dpi). Hydrogen peroxide (H₂O₂) accumulated in host cells around the appressoria at 3 dpi. Approx. 3% of the appressoria produced short hyphae inside the leaf, and fluorescence was observed in tissue invaded by the hyphae by 7 dpi. At 22 dpi, 0.2% of the sites with appressoria formed branching infection hypha in mesophyll cells, but no substomatal vesicles, haustorial mother cells or haustoria were observed. Proteins were extracted from leaves 3 dpi and analyzed by two-dimensional gel electrophoresis (2-DE). A total 33 spots were reproducibly up-regulated and 9 were down-regulated by infection compared to the water inoculated control. Of these, 30 were identified by MALDI-TOF Mass Spectrometry. The identified proteins participate in defense/stress responses, energy/carbohydrate metabolism, oxidation–reduction processes, protein folding/turnover/cleavage/degradation, signal transduction and cell death regulation. The results indicates that NHR of rice to Ptt is consistent with a shift in protein and energy metabolism, increased antimicrobial activities, possibly including phytoalexin accumulation and cell wall reinforcement, increased cell repair, antioxidive and detoxification reactions, and enhanced prevention of plant cell death. Nearly half of the up-regulated identified proteins were associated with chloroplast and mitochondrial physiology suggesting important roles for these organelles during NHR.     

Does Rust Infection of Willow Affect Feeding and Oviposition Behavior of Willow Leaf Beetles?

Willows are often attacked by both herbivorous insects and rust fungi. Little is known about interactions between these two willow enemies. We studied whether feeding and oviposition behavior of the willow leaf beetle Plagiodera versicolora upon the willow hybrid Salix x cuspidata is affected when the rust fungus Melampsora allii-fragilis has attacked the plant. Laboratory bioassays revealed that adult willow leaf beetles significantly avoided feeding and oviposition on rust-infected leaves when compared to healthy leaves. Further bioassays aimed to elucidate the temporal and spatial scale of effects of rust infection on feeding behavior of adults. While infected parts of leaves were avoided at all times past infection tested (8, 12, and 16 days), symptom-free parts of infected leaves were only avoided 16 days past infection. Systemic effects extended only one leaf position up and two leaf positions down from the infection site.