Host and environmental effects on post-penetration development of Puccinia graminis avenae and P. coronata avenae

After inoculation of Avena sterilis and the oat cultivars Algerian and Garry with Puccinia graminis avenae the time required for the eruption of pustules of the rust was markedly less at 30–35 than at 20–25 ^oC. In addition, the area of pustules and number of uredospores produced were significantly greater at 30–35 than at 20 ^oC. Peaks of uredospore production occurred between 12 and 22 days after inoculation. In comparable experiments, the time required for pustules of P. coronata avenae to erupt, and the size of pustules, were relatively insensitive to change of temperature, although weight of uredospores produced was greater at 20 than at 30 ^oC. Peaks of uredospore production occurred between 14 and 18 days after inoculation. Both rusts showed straight-line relationships between pustule area and number of uredospores produced. The percentage of infection foci that developed into pustules was similar with both rusts and on all the oat cultivars examined. Both rusts produced susceptible reaction types on all the hosts tested. Pustules of P. graminis avenae were smaller and fewer and generation time longer on cv. Garry than on cv. Algerian or Avena sterilis and the numbers of pustules per unit of inoculum of both rusts were greatest on Algerian, least on Garry. It is suggested that these quantitative differences in phases of the infection process contribute towards the ‘slow-rusting’ reaction of cv. Garry.     

Host and environmental effects on the penetration of oats by Puccinia graminis avenae and Puccinia coronata avenae

The frequency of penetration from appressoria of Puccinia graminis avenae and P. coronata avenae varied among Avena species and between oat cultivars, although both rusts produced susceptible infection type pustules on the cultivars tested. Penetration on cv. Garry was significantly less than that on the Avena species (A. barbata, A.fatua and A. sterilis) studied and penetration of these Avena species was significantly less than on the cvs Algerian and Fulmark. When the rusts were allowed to develop into pustules on seedlings which had been inoculated with fixed amounts of inoculum, there was a direct relationship between number of pustules produced and penetration frequency. The effects of temperature, light and dew period on penetration from appressoria of ‘single race’ and ‘mixed race’ inocula was also studied on these cultivars and species. Penetration by P. graminis avenae was greatest at 30–35 °C and at light intensities of 5625 lux and above, whereas that by P. coronata avenae was greatest at 20 °C and was unaffected by artificial light intensities up to n 250 lux. Maximal penetration by P. graminis avenae and P. coronata avenae was observed after inoculated plants had been exposed to dew periods of 16 and 12 h respectively. Some penetration was observed after a dew period of 8 h. The time taken for each rust to attain maximum penetration varied from 36 to 52 h after inoculation, depending on the environment, and was usually less for P. coronata avenae than for P. graminis avenae.     

Puccinia coronata f. sp. avenae: a threat to global oat production

Puccinia coronata f. sp. avenae (Pca) causes crown rust disease in cultivated and wild oat (Avena spp.). The significant yield losses inflicted by this pathogen make crown rust the most devastating disease in the oat industry. Pca is a basidiomycete fungus with an obligate biotrophic lifestyle, and is classified as a typical macrocyclic and heteroecious fungus. The asexual phase in the life cycle of Pca occurs in oat, whereas the sexual phase takes place primarily in Rhamnus species as the alternative host. Epidemics of crown rust happens in areas with warm temperatures (20–25 °C) and high humidity. Infection by the pathogen leads to plant lodging and shrivelled grain of poor quality. Disease symptoms : Infection of susceptible oat varieties gives rise to orange–yellow round to oblong uredinia (pustules) containing newly formed urediniospores. Pustules vary in size and can be larger than 5 mm in length. Infection occurs primarily on the surfaces of leaves, although occasional symptoms develop in the oat leaf sheaths and/or floral structures, such as awns. Symptoms in resistant oat varieties vary from flecks to small pustules, typically accompanied by chlorotic halos and/or necrosis. The pycnial and aecial stages are mostly present in the leaves of Rhamnus species, but occasionally symptoms can also be observed in petioles, young stems and floral structures. Aecial structures display a characteristic hypertrophy and can differ in size, occasionally reaching more than 5 mm in diameter. Taxonomy : Pca belongs to the kingdom Fungi, phylum Basidiomycota, class Pucciniomycetes, order Pucciniales and family Pucciniaceae. Host range : Puccinia coronata sensu lato can infect 290 species of grass hosts. Pca is prevalent in all oat‐growing regions and, compared with other cereal rusts, displays a broad telial host range. The most common grass hosts of Pca include cultivated hexaploid oat (Avena sativa) and wild relatives, such as bluejoint grass, perennial ryegrass and fescue. Alternative hosts include several species of Rhamnus, with R. cathartica (common buckthorn) as the most important alternative host in Europe and North America. Control : Most crown rust management strategies involve the use of rust‐resistant crop varieties and the application of fungicides. The attainment of the durability of resistance against Pca is difficult as it is a highly variable pathogen with a great propensity to overcome the genetic resistance of varieties. Thus, adult plant resistance is often exploited in oat breeding programmes to develop new crown rust‐resistant varieties. Useful website : https://www.ars.usda.gov/midwest-area/st-paul-mn/cereal-disease-lab/docs/cereal-rusts/race-surveys/ .     

Development and characterization of novel, polymorphic microsatellite markers for oat crown rust, Puccinia coronata

We report the development of 37 novel and polymorphic microsatellite markers for oat crown rust, Puccinia coronata f.sp. avenae. The allelic diversity ranged from two to 16 alleles per locus. Observed heterozygosity ranged from 0.000 to 0.971, and expected heterozygosity from 0.057 to 0.848. Thirteen of the loci were not in Hardy-Weinberg equilibrium, due to either the presence of null alleles, small sample size, or the effects of population subdivision (Wahlund's effect). All 37 primer pairs were tested with P. graminis and P. triticina showing that they are specific to P. coronata.     

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.     

Incidence of crown rust and virulence of puccinia coronata cda. f.sp. avenae eriks. and the effectiveness of pc genes for resistance in europe,middle east and north africa: Inzidenz von kronenrost und virulenz von puccinia coronata cda. f.sp. avenae eriks. und die wirksamkeit von pc-genen für resistenz in europa,nahost und nordafrika

Crown rust on oat incited by Puccinia coronata Cda f.sp. avenae is a wide-spread disease in Europe, Middle East (Israel) and North Africa (Morocco). High natural levels of the disease were recorded in Austria, Bulgaria, Czech Republic, Estonia, Israel, Italy, Poland, Russia and the former Yugoslavia. The severity of the disease in the European and Mediterranean Oat Disease Nursery (EMODN) between 1995 and 1999 showed that it occurred at damaging levels in a number of countries. Considerable differences in a disease resistance index (DRI) to crown rust among 67 oat lines in the period 1995?–?2000 were found. The values of the DRI ranged from18 (KR 8122) to over 290 (Pc 59, Pc 68). Detailed studies of the virulence combinations of P.coronata f.sp. avenae in Europe, Middle East and North Africa were carried out between 1995?–?2001. There were considerable differences in the average number of virulence among countries. Virulence to Pc 39 and 68 was found for the first time and is a significant finding. Nevertheless the major genes Pc 68, Pc 39, Pc 50-2, Pc Pc 59, Pc 60, Pc 61 proved to be highly effective.     

Development of simple sequence repeat markers for the plant pathogenic rust fungus Puccinia triticina

Eighteen polymorphic di‐ and trinucleotide simple sequence repeat markers were developed for the phytopathogenic rust fungus Puccinia triticina. The allelic diversity varied from two to nine alleles per locus. Levels of observed heterozygosity ranged from 0.095 to 0.952. Seven of the loci deviated significantly from Hardy–Weinberg equilibrium (P < 0.002) with 70% having levels of observed heterozygosity higher than expected heterozygosity. Null allele(s) were observed for locus PtSSR76 with a frequency of 9%. A preliminary screen of other cereal rust fungi (P. coronata, P. graminis, P. recondita and P. striiformis) indicated that these primer pairs are specific to P. triticina.     

Preliminary Investigations on the Rusts of Perennial Ryegrass in Central Italy

The results of a four-year investigation carried out in Central Italy on the occurrence of perennial ryegrass rusts from 1989—92 are reported. The crown rust (Puccinia coronata) was the most frequently recorded in the surveyed are while P. graminis was only occasionally detected. The causal agents were identified by the morphological criteria proposed by CUMMINS (1971).
A single-pustule isolate of P. coronata was used to test the resistance of seven ryegrass cultivars under greenhouse conditions. Two cultivars, 'Artal' and 'Score', proved resistant to the fungus isolate while the remaining ('Elka', 'Patora', 'Pleno', 'Sisu' and 'Vejo') were high susceptible.     

The Association of Brome Mosaic Virus and Wheat Rusts.

Evidence is presented in support of transmission of brome mosaic virus by uredospores of wheat stem rust (Puccinia graminis f. sp. tritici). The presence of BMV associated with uredospores is shown by the use of sensitive serological assays such as, fluorescent antibody binding studies, enzyme-linked immunosorbent assay and immunosorbent electron microscopy. The virus is shown to be carried externally on the uredospores. The nature of the association of the virus with uredospores and its relation to the mode of carriage and transmission of the virus by the uredospores, as well as the importance of this association in the epidemiology of the virus is discussed.     

Transient transformation of the rust fungus Puccinia graminis f. sp. tritici

The biotrophic rust fungus Puccinia graminis f. sp. tritici (Pgt) was transformed by particle bombardment. The promoter from the Pgt translation elongation factor 1alpha (EF-1alpha) gene was fused to the bacterial marker genes hygromycin B phosphotransferase (hpt) and beta-glucuronidase (GUS). Transformation constructs were introduced into uredospores of Pgt, an obligate pathogen of wheat, by biolistic bombardment. Uredospores transformed with the construct containing the hpt gene germinated and initiated branching on selective medium, indicating that they had acquired resistance to hygromycin B. However, transformants stopped growing 5 days after bombardment. GUS activity in uredospores and germlings was histochemically detected 4-16 h after bombardment. GUS expression was also obtained using the INF24 promoter from the bean rust fungus Uromyces appendiculatus, demonstrating that heterologous genes can be expressed in P. graminis under the control of regulatory sequences from closely related organisms.     

Puccinia chunjii, a close relative of the cereal stem rusts revealed by molecular phylogeny and morphological study

A rust specimen with macroscopic similarities to the cereal stem rusts was collected on Elymus sp. from Gansu province, China. Phylogenetic analyses of ITS and COI DNA sequences indicated the fungus was closely related but distinct as a strongly supported sister taxon to the Puccinia graminis species complex. Microscopic examination revealed diagnostic teliospore characteristics, differentiating it from P. graminis and other morphologically similar rusts. Herein, we designate a name for this new lineage, Puccinia chunjii sp. nov.     

Deciphering species complexes: Puccinia andropogonis and Puccinia coronata, examples of differing modes of speciation

Species of macrocyclic, heteroecious grass rusts often have been defined with wide host ranges and variation in spore morphology. Consequently, some are species complexes and contain genetically distinct forms. Molecular analyses, together with morphological and biological methods, provide powerful means to dissect these complexes. Puccinia coronata is a complex species that has a broad telial host range including more than 45 genera of grasses and a narrow aecial host range. Phylogenetic analysis of nuclear ribosomal internal transcribed spacer (ITS) DNA sequences from 15 aecial and telial collections grouped P. coronata into six distinct clades supporting separation of this complex into four distinct species. Puccinia andropogonis, a common rust of tall prairie grasses in North America, is also a complex species. However, in contrast to P. coronata, P. andropogonis has a narrow telial host range and a broad aecial host range. DNA sequence analysis grouped 15 collections of P. andropogonis into six distinct clades representing at least four distinct species. Speciation of P. coronata appears to have occurred primarily by radiation onto new telial hosts, whereas in P. andropogonis speciation appears to have occurred primarily by radiation onto new aecial hosts.     

Short Note: Effect of Acremonium obclavatum glucan on groundnut leaf rust disease caused by Puccinia arachidis Speg.

A water soluble glucan purified from the culture filtrate of Acremonium obclavatum, an antagonist to the groundnut rust, Puccinia arachidis, inhibited germination of uredospores of P. arachidis. Prior treatment of groundnut leaves with glucan prolonged the incubation period between inoculation and development of rust disease, and caused a decrease in the number of pustules and uredospores/sorus. Treated groundnut leaves showed an increase in endogenous salicylic acid, intercellular chitinase and -1,3 glucanase activities.     

Regional variation of Puccinia coronata avenae in Australia and its implications for oat breeding

A virulence survey of Puccinia coronata avenae was conducted in Australia from 1977 to 1980 with a chosen series of resistant oat lines as a tester set, viz., Ascencao, Pc-38, Pc-39, Pc-45, Pc-48, Pc-50, Pc-55, Pc-56 and TAM 0–312. Isolates of the pathogen were classified by using these nine tester lines in combination with the International Differential Set. The survey area was divided into six zones (1) -Queensland; (2) - northern New South Wales; (3) - southern New South Wales; (4) - Victoria; (5) - South Australia and (6) - Western Australia. Virulence for Pc-38 was widespread throughout Australia and virulences or partial virulences for lines Pc-39, Pc-45, Pc-48 and Pc-55, although not as frequent, were common in Zones 1, 2 and 3. No isolates were found with virulences for oat lines Pc-56 and TAM 0–312, while virulences for Ascencao or Pc-50 were rare. The combination of virulences or partial virulences for lines Pc-39, Pc-45 and Pc-48 was relatively common only among samples from Zones 1, 2 and 3. The diversity of the pathogen population varied between zones. Isolates from Zones 1 to 6 comprised 16, 21, 9, 8, 5 and 4 different standard races and mean virulence values were 5-0, 3–8, 3-0, 2–7, 2–6, 3-1 and 3-0, respectively. The proportion of complex strains (virulence value 5 or higher) decreased from 48-5% (Zone 1) and 17-0% (Zone 2) to 1–7% (Zone 3), 6-3% (Zone 4), 0% (Zone 5) and 13-9% (Zone 6). Greater diversity of the pathogen population in Zones 1 and 2was indicated by a higher incidence of strains with virulence for one or more of the nine resistant oat tester lines.     

Reactive Oxygen Species are not Increased in Resistant Oat Genotypes Challenged by Crown Rust Isolates

Crown rust (Puccinia coronata Corda f.sp. avenae) can devastate oats (Avena sativa). Oxidative stress is part of the resistance mechanism in several pathosystems, but in the oat–crown rust system, it is unclear, especially regarding partial resistance. We evaluated the effects of P. coronata on oxidative stress in oat cultivars: URS 21 (partially resistant), Leggett (race‐specific resistant), URS22 and Clintland 64 (susceptibles). Seedlings and plants were inoculated with P. coronata uredospores. Cultivars were assessed for antioxidant enzyme activity and the reactive oxygen species (ROS) hydrogen peroxide and superoxide. Due to the importance of the partial resistance of URS21, this cultivar and URS 22 were also appraised for total phenolics and the relative expression of oxidative stress genes. Postinoculation, Leggett and URS 21 showed no increased peroxide levels. The susceptible cultivars increased ROS and ascorbate peroxidase activity. Clintland 64 increased also catalase activity, whereas URS 22 increased glutathione reductase and the expression of genes encoding antioxidant enzymes. URS 21 showed almost no antioxidant enzyme induction. Shortly after inoculation, URS 21 showed increased expression of genes encoding lipoxygenase and peroxidase. Cultivars URS 21 and Leggett accumulated cell wall fluorescent compounds, phenolics being detected in the former. Oxidative stress appears not to cause the hypersensitive response in this pathosystem, but late ROS accumulation did occur in the susceptible cultivars. Cultivar URS 21 may, differently from other known mechanism to date, reduce ROS accumulation by increasing the level of phenolics, resulting in later pathogen and cell death, showing non‐specific resistance to races of the pathogen also at seedling stage.     

Global risk levels for corn rusts (Puccinia sorghi and Puccinia polysora) under climate change projections

Common rust (Puccinia sorghi) and southern rust (Puccinia polysora) are two of the most important foliar corn diseases worldwide. These fungi have caused severe economic loss to corn yields worldwide. The current and future potential distribution of these diseases was modelled with CLIMEX using the known current geographic locations of the rusts, growth and stress indices. The models were run under the A2 scenario using CSIRO‐Mk3·0 and MIROC‐H for 2050 and 2100. The current projection shows areas with marginal to optimal suitability in all the continents. The models for future projections display a general reduction in the Southern hemisphere and increase in the Northern hemisphere, especially for the southern rust. The overlay of the General Circulation Models produce an estimation of the common areas under risk for future climate conditions for the simultaneous occurrence for both corn rusts, with a reduction of the medium‐ and high‐risk categories by 2100. This study highlights the possible effects of climate change at a global level for common and southern rust, as well as the risk of occurrence of both diseases in common areas for future climate that could be particularly harmful for crops.     

Dew and the growth of the uredospore germ tube of Puccinia graminis on the wheat leaf

Water droplets simulating dew, formed on wheat leaves held in a condensation chamber, appeared first on the leaf hairs and coalesced into lines along the ridges of the leaf surface in close proximity to the stomata. On leaves thus wetted, germ tubes from uredospores of Puccinia graminis showed a thigmotropic response, growing at right angles to the ridges: when they reached the stomatal region the penetration of the stoma may be further assisted by chemotropic or hydrotropic attraction. Studies on plastic surfaces showed that germ-tube growth took place only between and not within water droplets.     

Germination of Puccinia striiformis uredospores on leaves of adult winter wheat plants

In experiments with intact adult winter wheat plants and detached leaves, higher percentages of Puccinia striiformis uredospores germinated on the adaxial leaf surface, particularly the distal parts, than on the abaxial surface. This is consistent with the observation that the distal parts of the adaxial leaf surface are those which are most susceptible to yellow rust. Higher percentages of uredospores germinated on leaves of some of the winter wheat varieties examined than on those of others. These varietal differences depended on the part of leaf and the race of P. Striiformis used for the comparisons. This suggests that the differences between varieties are partly race specific.     

The development of Puccinia hordei on barley cv. Zephyr

Germination of uredospores of Puccinia hordei was similar on cover-slips and on the first leaves of barley seedlings (cv. Zephyr) at 100 % r.h. over the range 5–25 °C, being greatest at 20 °C. At 15, 20 and 25 °C maximum germination was attained in 6 h. No uredospores germinated on coverslips in humidities below saturation. The numbers of pustules which subsequently developed on plants incubated at 5, 10, 15 or 18 °C and 100 % r.h. for varying periods up to 24 h, were directly related to rise in temperature and length of incubation. The time from inoculation to eruption of pustules (generation time) was 6 days at 25 °C, 8 days at 20 °C, 10 days at 15 °C, 15 days at 10 °C and 60 days at 5 °C. Pustule production on inoculated plants which had been kept at 5 °C was rapidly accelerated when they were transferred to 20 °C. Data obtained at constant temperatures were used to predict generation times of the fungus in the field. The productivity of pustules, determined as weight of uredospores, was examined at 10, 15 and 20 °C. Significantly more spores were produced at 15 than at 10 °C and most were produced at 20 °C. The results are discussed in relation to those obtained by other workers and to the development of brown rust in the field.     

Wachstum und Sporenbildung einer schweizerischen Rasse von Puccinia graminis tritici in vitro

Growth and spore formation of a Swiss race of Puccinia graminis f. sp. tritici in vitro On a medium containing Evans Pepton and yeast Extract, the rust formed aecidiospores in great number. The aecidiospores were able to infect wheat plants. On media 4–9 (Table 1) uredospores and teliospores are formed. Highest numbers of teliospores were produced on substrates with ATP, nucleotide bases, and ribose.