Teliospore Germination Research Articles

Observations on the gall rust fungus Prospodium transformans, a potential biocontrol agent of Tecoma stans var. stans (Bignoniaceae) in South Africa

Tecoma stans var. stans, a tree originating from the Americas, is emerging as an invasive weed in South Africa. A microcyclic gall inducing rust fungus, Prospodium transformans, has been investigated as a biological control agent against this plant. Development of germinating teliospores and symptoms on host plants are described, and the optimum temperature for teliospore germination was found to be 18-22ºC. Inoculations of plants grown from seed originating from South Africa and various localities in Mexico and Texas (USA) indicate that there are likely races specific to morphological variants of this widespread and highly variable plant species. Despite readily inducing galls on plants grown under quarantine glasshouse conditions in South Africa, this rust fungus failed to establish in the field upon release. It is suggested that the origin of the form of this plant, which has become invasive in South Africa, needs to be identified to source the correct race of P. transformans for release in South Africa.

The Effect of Exogenous Phytohormones on Resistance of Wheat Calluses to <i>Tilletia caries</i> (D.C.) Tul. & C. Tul.

The influence of exogenic hormones (indole-3-acetic acid (IAA), abscisic acid (ABA) and kinetin) on defense reaction of wheat (Triticum aestivum L.) calli to the bunt agent Tilletia caries (D.C.) Tul. & C. Tul. was studied. ABA and kinetin induced the oxalate oxidase activity, increased the Н2О2 level, decreased germination of fungi teliospores and induced on calli the occurrence of dense sites non-infected by pathogen. On the contrary, IAA led to the decrease of oxalate oxidase activity, loosening of calli and increase germination of bunt agent teliospores and growth of fungi mycelium, besides stimulated rhizoids formation of wheat calli. Probably, the accumulation of Н2О2 in wheat calli under the influence of kinetin and ABA connected with activity of oxalate oxidase is one of the factors increasing defense reaction of wheat to bunt agent.

Ustilago trichophora (H.F. Link) F. Körnicke, a fungus newly found in Poland

<em>Ustilago trichophora</em>, a smut fungus found for the first time in Poland, is characterized and illustrated. <em>Ustilago trichophora</em> affected <em>Echinochloa crus-galli</em> growing in the Lower Silesia voivodeship. In laboratory investigations, the germinability of teliospores, the morphological properties of promycelium, sporidia and colonies produced on potato dextrose agar, Sabouraud-glucose agar, and in water were determined. Attempts to infect seeds and seedling of <em>E. crus-galli</em> in a greenhouse pot experiment failed.

Initial Infection of Falcataria moluccana Leaves and Acacia mangium Phyllodes by Uromycladium tepperianum Fungi in a Laboratory Trial

Sengon is a fast growing species that is cultivated widely in Indonesia. Lately, sengon is severely attacked by fungus Uromycladium tepperianum that causing gall rust disease. It is also known to attack various types of acacia. This study aims to determine the fungal infection process U. Tepperianum on sengon leaves and the possibility of infection on Acacia mangium in the laboratory trial. Leave samples and fungal pathogen teliospores obtained from Cangkringan, Sleman, Yogyakarta. Several approach procedures conducted to achieve these objectives were: (1) identification of diseased trees, (2) collecting samples of diseased leaves, branches, twigs, and stems, and (3) artificial inoculation and investigating the infection process of U. tepperianum teliospores in the laboratory. The results showed that the process of infection in sengon started by teliospores germination and germ tube formation. Successive germ tube forming penetration pegs. In the plant tissue, the penetration peg formed hypha and further developed into intracellular and intercellular hyphae. The artificial inoculation on A. mangium leaf surface showed few spores can germinate. However, none of them managed to penetrate.

Successful establishment of epiphytotics of Puccinia punctiformis for biological control of Cirsium arvense

Canada thistle (Cirsium arvense, CT) is one of the worst weeds in temperate areas of the world. The rust fungus Puccinia punctiformis was first proposed as a biological control agent for CT in 1893. The rust causes systemic disease, is specific to CT, and is in all countries where CT is found. Despite a 120-year lapse since biological control with the rust was proposed, establishment of epiphytotics of the rust have previously been unsuccessful due to incomplete understanding of the disease cycle. In this study, newly-emerging rosettes in the fall are proposed as the physical and temporal infection courts for basidiospores, from germinating teliospores, to systemically infect CT and give rise to systemically diseased shoots the following spring. To test this hypothesis, rosettes of CT were inoculated in the fall with either telia-bearing leaves collected in mid-summer or with greenhouse-produced teliospores. Field sites were located near Kozani, Greece, Moscow, Russia, Christchurch, New Zealand, and Ft. Detrick, Maryland, USA. Telia-bearing leaves, which were used as inoculum in 12 of 13 field sites, were collected near each field site from CT shoots in close proximity to systemically diseased CT shoots producing aeciospores in the spring. Aeciospore infections of the leaves of these nearby shoots gave rise to uredinia which turned to telia in mid- to late-summer. Temperature and dew conditions at inoculation in the fall at each site were very favorable for teliospore germination. Rosettes inoculated in the fall were marked with flags, and systemically diseased shoots emerging near these flags the following spring were recorded. In 11 of the sites in these countries, individual rosettes were inoculated 2, 4, 6, or 8 times with telia-bearing leaves. Proportions of rosettes giving rise to systemically diseased shoots, out of the number of rosettes inoculated, were analyzed. Inoculations in all 13 sites produced systemically diseased shoots. A separate study on the phenology of CT showed that the maximum rate of leaf abscission occurred at the time of maximum emergence of new CT rosettes in the fall. This period coincided with an annually occurring period of sustained dew and favorable temperatures for teliospore germination. In nature, abscising telia-bearing leaves likely come into contact with a receptive rosette during favorable conditions for teliospore germination in the fall. This study demonstrates that epiphytotics of systemic rust disease of CT can be routinely established, by mimicking the natural disease cycle.

Transcriptome analysis of poplar rust telia reveals overwintering adaptation and tightly coordinated karyogamy and meiosis processes

Most rust fungi have a complex life cycle involving up to five different spore-producing stages. The telial stage that produces melanized overwintering teliospores is one of these and plays a fundamental role for generating genetic diversity as karyogamy and meiosis occur at that stage. Despite the importance of telia for the rust life cycle, almost nothing is known about the fungal genetic programs that are activated in this overwintering structure. In the present study, the transcriptome of telia produced by the poplar rust fungus Melampsora larici-populina has been investigated using whole genome exon oligoarrays and RT-qPCR. Comparative expression profiling at the telial and uredinial stages identifies genes specifically expressed or up-regulated in telia including osmotins/thaumatin-like proteins (TLPs) and aquaporins that may reflect specific adaptation to overwintering as well numerous lytic enzymes acting on plant cell wall, reflecting extensive cell wall remodeling at that stage. The temporal dynamics of karyogamy was followed using combined RT-qPCR and DAPI-staining approaches. This reveals that fusion of nuclei and induction of karyogamy-related genes occur simultaneously between the 25 and 39 days post inoculation time frame. Transcript profiling of conserved meiosis genes indicates a preferential induction right after karyogamy and corroborates that meiosis begins prior to overwintering and is interrupted in Meiosis I (prophase I, diplonema stage) until teliospore germination in early spring.

Sporulation Capacity and Longevity of Puccinia horiana Teliospores in Infected Chrysanthemum Leaves

Puccinia horiana Henn., a quarantine-significant fungal pathogen and causal agent of chrysanthemum white rust (CWR), was first discovered in the United States in 1977 and later believed to have been eradicated. Recently, however, the disease has sporadically reappeared in the northeastern US. Possible explanations for the reappearance include survival of the pathogen in the local environment, and reintroduction from other locations. To determine the possibility that the pathogen might be overwintering in the field, we undertook the study described here. Results from the study showed that P. horiana teliospores, imbedded in infected leaves, were capable of sporulating 2 weeks after inoculation, and this capacity continued until the leaf became necrotic and desiccated. This is the first report of the extreme susceptibility of P. horiana teliospores to leaf necrosis and desiccation and suggests that field infections following winter are unlikely to originate from teliospores. Teliospore germination on excised leaves was shown to be inhibited by light. Accepted for publication 3 April 2013. Published 23 August 2013.

Assessment of Risk of Establishment of Wheat Dwarf Bunt (Tilletia controversa) in China

Wheat dwarf bunt, caused by Tilletia controversa Kühn (TCK), is an important quarantine wheat disease throughout the world. Based on published research results of the biology and the epidemiology of the disease, the main factors including temperature, humidity, snow cover, and their parameters relating to teliospore germination, infection and epidemics of TCK were determined. The geophytopathological models for the risk analysis of wheat dwarf bunt establishment were modified. Fifty-year meteorologic data from about 500 weather stations in China were used to calculate the probabilities of TCK establishment in different geographic sites with the models. The map that displays the establishment risk of TCK in winter wheat growing regions in China was generated by using geographical information system (GIS). The zones showing high, moderate, low, and very low, including no risk, of TCK establishment accounted for 27.33, 27.69, 38.12, and 6.86% of total winter wheat growing areas in China, respectively. These results will provide useful information to formulate quarantine regulations and wheat importation policy in China.

Laboratory and field evaluation of fungicides for the management of sugarcane smut caused by Sporisorium scitamineum in seedcane

Sugarcane smut caused by Sporisorium scitamineum is one of the most devastating diseases of sugarcane. Two trialzole fungicides, propiconazole and triadimefon, have been used to protect seedcane from infection with sugarcane smut elsewhere, particularly after hot water treatment. These fungicides were registered under an emergency permit in Australia when smut was found for the first time in Queensland in 2006. A research program was initiated to screen and evaluate a range of fungicides against sugarcane smut. Nine fungicides were tested in vitro at various concentrations for their efficacy on smut spore germination. Azoxystrobin (Amistar®), quintozene (Quintozene® 750) and didecyl dimethyl ammonium chloride (Steri-max®) completely stopped germination of teliospores at 2.5 mg a.i./L. Propiconazole (Tilt®), triadimefon (Bayleton®), cyproconazole (Alto®) and acibenzolar-s-methyl (Bion®) significantly (P < 0.05) reduced spore germination at 50, 100 and 200 mg a.i./L. Two field trials were planted in spring 2008 and autumn 2009, where cane setts that had been inoculated with smut were dipped in a range of fungicide suspensions for five or 10 min, prior to planting. Cyproconazole, propiconazole, triadimefon and azoxystrobin significantly (P < 0.05) suppressed disease expression for up to 6 months in a summer experiment and 9 months in an autumn experiment. These results have important implication for selecting new fungicides for the control of sugarcane smut in seedcane that is provided to Australian sugarcane growers as a disease-free propagation source.

The infection process of Puccinia xanthii f. sp. ambrosiae-trifidae on Ambrosia trifida

Puccinia xanthii Schwein. f. sp. ambrosiae-trifidae S.W.T. Batra is an obligate parasitic rust fungus of Ambrosia trifida Linn. Field investigations in Liaoning Province, China, showed that it is an effective biocontrol agent of this alien invasive weed. Its infection of the plant was observed by light microscopy combined with Coomassie Brilliant Blue R-250 staining. We report the infection process, including teliospore germination and basidiospore formation on the host leaf surface, penetration of host tissue, and development of fungal hyphae within the host tissue. Fresh teliospores began to germinate from the germ pore within 1 h under suitable conditions and soon produced basidiospores or secondary basidiospores. Basidiospores falling on host leaves germinated from the end of the basidiospore opposite to the apiculus. Appressoria of germ tubes tended to orient along leaf epidermis cell ridges or at junctions near stomata rather than fixing randomly on the leaf surface. These germ tubes grew for short or longer distances before forming appressoria. The rust fungus directly penetrated the host epidermis by infectious pegs rather than through stomata. Within host tissues, the rust fungus formed intraepidermal vesicles, primary hyphae, intracellular hyphae, and M-haustoria. The intricate infectious structures formed by P. xanthii f. sp. ambrosiae-trifidae on or in host tissues suggest that the rust fungus is a suitable organism for researching the interaction between the pathogen and host plant.

In Vitro Influence of Selected Fungicides on Sphacelotheca Reiliana and Ustilago Maydis

The study evaluates the effects of selected fungicides on radial mycelial growth and the germination of teliospores and sporidia in two species. The influence of fungicides on the growth of the same developmental forms of both fungi varied at times. Germination of teliospores Ustilago maydis in all used concentrations completely inhibited the fungicides: benomyl, carboxin and thiram, flusilazole. Complete inhibition the germination of teliospores and sporidia Sphacelotheca reiliana caused grapefruit extract, benomyl, carboxin and thiram at concentrations of 10,000 to 100 mg/ml and azoxystrobin, flusilazole at concentrations of 10,000 to 1,000 mg/ml. Inhibition of mycelial growth of both species by the fungicides used was weaker than the inhibition the germination of teliospores and sporidia.

The modes of teliospore germination of Ustilago trichophora (Link) Kunze ex Körnicke occurring in Poland

Ustilago trichophora affects &lt;em&gt;Echinochloa crus-galli &lt;/em&gt;and other species of &lt;em&gt;Echinochloa &lt;/em&gt;genus. &lt;em&gt;U. trichophora &lt;/em&gt;is a member of the order Ustilaginales, of which the germinating teliospores form a promycelium with sporidia. Investigations by Ingold (1996) proved that spores coming from various parts of the world germinate in a different way. The aim of the investigation was to determine the mode of germination of &lt;em&gt;U. trichophora &lt;/em&gt;spores found in Poland.

The illustrated life cycle ofMicrobotryumon the host plantSilene latifolia

The plant-parasitic genus Microbotryum (Pucciniomycotina) has been used as a model for various biological studies, but fundamental aspects of its life history have not been documented in detail. The smut fungus is characterized by a dimorphic life cycle with a haploid saprophytic yeast-like stage and a dikaryotic plant-parasitic stage, which bears the teliospores as dispersal agents. In this study, seedlings and flowers of Silene latifolia Poir. (Caryophyllaceae) were inoculated with teliospores or sporidial cells of Microbotryum lychnidis-dioicae (DC. ex Liro) G. Deml &amp; Oberw. and the germination of teliospores, the infection process, and the proliferation in the host tissue were documented in vivo using light and electron microscopy. Although germination of the teliospore is crucial for the establishment of Microbotryum, basidium development is variable under natural conditions. In flowers, where the amount of nutrients is thought to be high, the fungus propagates as sporidia, and mating of compatible cells takes place only when flowers are withering and nutrients are decreasing. On cotyledons (i.e., nutrient-depleted conditions), conjugation occurs shortly after teliospore germination, often via intrapromycelial mating. After formation of an infectious hypha with an appressorium, the invasion of the host occurs by direct penetration of the epidermis. While the growth in the plant is typically intercellular, long distance proliferation seems mediated through xylem tracheary elements. At the beginning of the vegetation period, fungal cells were found between meristematic shoot host cells, indicating a dormant phase inside the plant. By using different microscopy techniques, many life stages of Microbotryum are illustrated for the first time, thereby allowing new interpretations of laboratory data.

In vitro effects of caffeic acid upon growth of the fungi Sporisorium scitamineum

Caffeic acid (CA) sometimes behaves as a potent phytotoxin affecting plant and fungi growth and physiology. The aim of the present study was to investigate whether CA at a concentration range similar to that found in sugarcane leaves, had any effect against different phases of Sporisorium scitamineum growth cycle. Leaf CA concentration from two different sugarcane cultivars, Mayari (My) 55-14, resistant, and Barbados (B) 42231, susceptible to smut, was chromatographically quantified by HPLC. A smut elicitor promoted an increase of CA concentration in the resistant cv. while no effect was produced in the susceptible one. The effect of CA upon S. scitamineum growth cycle showed to be dependent of both concentration and time. At 5.0 µg ml−1, CA produced and inhibition of teliospore germination, haploid sporidia production and dikaryotic mycelium appearance. At 30 µg ml−1, CA produced similar effects to these just described. Inhibition was more evident after 24 h or 28 h incubation of teliospores in CA solution than after 48 h. CA at 20 µg ml−1 reduced both germination of teliospores and production of haploid sporidia but it had no significant effect on dikaryotic mycelium appearance after 24-h incubation.

Effect of different meteorological parameters on germination of inoculum of teliospores of Tilletia indica in rice-wheat rotation system in Punjab, India

Dynamics of inoculum in relation to meteorological parameters affecting occurrence of Karnal bunt epidemic in wheat in Punjab was studied during the years 2001-02 and 2002-03. There existed an intricate relationship between germination of teliospores of Tilletia indica, the causal organism of karnal bunt of wheat and meteorological parameters. The ambient and soil temperature showed negative relationship with the germination of teliospores. Increase in temperature beyond 25 oC resulted decrease in germination of teliospores. Relative humidity had significant positive relationship with germination of teliospores and maximum teliospore germination was recorded when relative humidity was more than 75 per cent. Different models were developed for prediction of disease possibility based on viability of inoculums depending on meteorological conditions. Based on humid thermal ratio, the best fit models for forecasting viability and germination of teliospore of T. indica throughout the year and during crop season were developed which explained 65 to 89 per cent and 60 to 52 percent of the variations, respectively.

Factors Influencing Epidemiology and Management of Blackberry Rust in Cultivated Rubus laciniatus.

The blackberry rust pathogen Phragmidium violaceum was first observed in Oregon in spring 2005 on both commercially cultivated Rubus laciniatus (Evergreen blackberry) and naturalized R. armeniacus (Himalayan blackberry). Several commercial plantings suffered severe economic losses. In 2006 to 2008, all five spore stages of this autoecious, macrocyclic rust pathogen were observed annually, and asexual perennation of the pathogen on old leaves or in leaf buds was not evident in the disease cycle. In field experiments, teliospore germination and infection by basidiospores occurred mostly during April. On potted "trap" plants exposed for periods of 1 week under dense collections of dead leaves bearing teliospores, basidiospore infection was associated with wetness durations of >16 h with mean temperatures >8°C. Trap plants placed under the bundles of collected leaves frequently developed spermagonia, whereas only 1 of 630 trap plants placed in a production field of R. laciniatus became diseased, an indication that the effective dispersal distance of basidiospores may be limited. In growth chambers programmed for constant temperatures of 5, 10, 15, 20, 25, and 30°C, a minimum of six continuous hours of leaf wetness was required for infection by urediniospores, with >9 h required for moderate infection (>4 pustules/cm2) at 15 and 20°C. With diurnal temperature regimes averaging 5, 10, 15, 20, or 25°C, urediniospore germination and infection was highest in the range of 5 to 15°C; similarly, in the diurnal environment, >9 h of leaf wetness was required to attain moderate infection. In the field, lime sulfur applied as a delayed dormant treatment significantly suppressed teliospore germination and basidiospore infection. Over two seasons, one application of myclobutanil, a demethylation-inhibitor fungicide, applied in early May near the time of spermagonial appearance provided effective suppression of the summer epidemic.

&lt;i&gt;Puccinia melampodii&lt;/i&gt; Diet. and Holow. as a Biological Control Agent of &lt;i&gt;Parthenium hysterophorus&lt;/i&gt;

Parthenium hysterophorus is an annual, extremely prolific weed of Asteraceae that causes severe economic losses, health problems and habitat destruction in many tropical and sub tropical countries. In Sri Lanka, it was first observed in Vavuniya district in 1999 and subsequently it has spread to many areas of the country.The fungus Puccinia melampodii, introduced from Australia, was evaluated for its efficacy as bio control agent against P . hysterophorus under local conditions to use as a component of integrated pest management programme. Life cycle studies revealed that P . melampodii completes its life cycle within 8-11 days at 25°C night and 28°C day temperature. Continuous fungal infection resulted complete destruction of Parthenium plants within 30 days. In vitro studies to determine the effect of temperature on teliospore germination of P . melampodii indicated that it occurs over a wide range of temperatures from 5 to 30°C. The optimum temperature was 25°C and the fungus did not survive at 35°C after an inoculation period of 24 h and 48 h.Host specific studies of P . melampodii showed that, of 30 different plant taxa in 14 plant families tested, only P. hysterophorus were susceptible to the infection. Fungal infection at different growth stages of the Parthenium; rosette, preflowering and flowering stages, suppressed the plant growth, especially the leaf and flower production. Results revealed that the fungus P . melampodii can be effectively used as a potential bio-control agent against the Parthenium weed. Key words : Parthenium weed; Puccinia melampodii ; teliospore germination; mycelium DOI: 10.4038/jfa.v1i1.1835 Journal of Food and Agriculture Vol.1(1) 2008 pp.13-19

Biological control of sugarcane smut (Sporisorium scitamineum) through botanicals and Trichoderma viride

Culmicolus smut (Sporisorium scitamineum Meike), the sett borne pathogen causes considerable loss to sugarcane productivity especially to ratoon crop. Use of fungicides may cause environmental hazards besides residual problem. Therefore, twenty five plant species and Trichoderma viride was evaluated in vitro against smut pathogen (Sporisorium scitamineum). Out of twenty five plant species, leaf extracts (10%) of Calendula officinalis, Solanum nigrum and culture filtrate (5%) T. viride (bioagent) inhibited mycelial growth and teliospore germination of S. scitemineum. In plant crop, smut incidence was 2.07% in dipping of infected setts in leaf extract of S. nigrum before planting, while in remaining treatments it ranged between 3.38 to 7.28% as against 11.3% in control. In ratoon crop, the smut incidence was 5.38% in the same treatment, while in rest of the treatments it ranged between 6.85 to 9.94 per cent as against 15.3% in control. However, sett treatment with T. viride culture filtrate (5.0%) was superior among all the treatments as it improved germination (6.2%), millable canes (27.33%) and cane cane yield (38.18%) in plant crop and sprouting of clumps (12.57%), millable canes (51.46%) and yield (48.75%) in ratoon crop, respectively.

Puccinia jaceae var. solstitialis Teliospore Priming on Yellow Starthistle

Following the introduction of Puccinia jaceae var. solstitialis to California for biological control of yellow starthistle (Centaurea solstitialis, Asteraceae), teliospores, pycnia, and multiple urediniospore generations have been observed in the field. Because urediniospores have a relatively short life span in the field, functioning teliospores are expected to be necessary for the permanent establishment of P. jaceae var. solstitialis in California. To determine if conditions in California were conducive to this, teliospore emergence and priming were evaluated in the field. A factorial experiment in the laboratory with five incubation times and three incubation temperatures was used to determine teliospore priming requirements. Teliospore production coincided with plant senescence in August and September at two sites in 2 years; fewer teliospores were produced in 2006, suggesting inconsistent teliospore production may limit population growth and contribute to local extinctions in some areas. When teliospores were primed in the field, germination was low through the fall and abruptly peaked in January during both years. In the laboratory, teliospore germination increased as incubation time increased from 2 to 6 weeks and temperatures decreased from 12 to 4 degrees C. A degree-hour model derived from laboratory data accurately predicts when teliospores are primed for germination in the field. Based on the results obtained in this study, it is apparent that teliospore germination can occur over a range of priming conditions. However, lower temperatures and longer incubation periods are superior in breaking teliospore dormancy.

Carboxin tolerant strains of Ustilago nuda and Ustilago tritici in Canada

Seed treatment with systemic fungicides is commonly used to control loose smut of barley, caused by Ustilago nuda, and loose smut of wheat, caused by Ustilago tritici. Historically, the most widely used active ingredient in these fungicides has been carboxin. Isolates of U. nuda that are resistant to carboxin have been reported in France and Italy. Three isolates of U. tritici collected in 1997 and two isolates of U. nuda collected in 1999 in Canada appeared to be insensitive to carboxin when assessed using a teliospore germination assay on carboxin-amended media. In planta, studies were conducted by inoculation of these isolates onto susceptible lines of wheat or barley with treatment or no treatment of the inoculated seed with carboxin at the recommended rate in Canada of 55 g ai carboxin / 100 kg seed or twice the rate (110 g ai carboxin/100 kg seed). When the inoculated seed was subsequently grown to heading, assessment of the percent of smutted spikes on the plants indicated that the three isolates of U. tritici and the two isolates of U. nuda were unaffected by the carboxin seed treatment at 55 g ai carboxin/100 kg seed. However, the incidence of loose smut on their respective hosts by three of these isolates was significantly reduced when the seed was treated at a rate of 110 g ai carboxin/100 kg seed. These results would suggest that these isolates of U. tritici and U. nuda are tolerant to carboxin rather than resistant, because, in general, higher doses of the fungicide in seed treatments reduced host disease incidence caused by these pathogenic isolates. Key words: fungicide resistance, loose smut, barley, wheat, Hordeum vulgare, Triticum aestivum, T. turgidum. Les fongicides systémiques sont couramment utilisés pour protéger les semences contre le charbon nu de l’orge causé par Ustilago nuda et celui du blé causé par Ustilago tritici. Traditionnellement, l’ingrédient actif le plus couramment utilisé dans ces fongicides a été la carboxine. Des isolats d’ U. nuda tolérants à la carboxine ont été signalés en France et en Italie. Trois isolats d’ U. tritici collectés en 1997 et deux d’ U. nuda collectés en 1999, au Canada, ont semblé être insensibles à la carboxine lorsqu’ils ont été évalués au cours d’un biotest de germination de téleutospores dans un milieu gélosé modifié contenant de la carboxine. Des études in planta ont été menées en inoculant avec ces isolats des lignées de blé ou d’orge qui leur étaient réceptives et dont les semences inoculées avaient été traitées ou pas avec de la carboxine à raison de 55 g matière active de carboxine/100 kg de semence, dose recommandée au Canada, ou à deux fois la dose (110 g matière active de carboxine/100 kg de semence). Par la suite, lorsque les semences inoculées atteignirent le stade de l’épi, l’évaluation du taux d’infection des épis par le charbon nu a montré que ni les trois isolats d’ U. tritici ni les deux d’ U. nuda étaient affectés par la carboxine lorsque les semences étaient traitées selon la dose recommandée. Toutefois, l’incidence du charbon nu causé par trois de ces isolats sur leurs hôtes respectifs était de beaucoup réduite lorsque la dose du traitement était doublée. Ces résultats suggéreraient que les isolats d’ U. tritici et d’ U. nuda tolèrent davantage la carboxine qu’ils y sont sensibles parce que, en général, les traitements qui contiennent des doses plus élevées de fongicide contribuent à diminuer l’incidence de la maladie causée par ces isolats pathogènes. Mots-clés : tolérance aux fongicides, charbon nu, orge, blé, Hordeum vulgare, Triticum aestivum, T. turgidum.