Urediniospore

Abstract: SUMMARY 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 µ m 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

Abstract: SUMMARY Stripe (yellow) rust,caused by Puccinia striiformis f.sp.tritici (Pst), is a serious disease of wheat occurring in most wheat areas with cool and moist weather conditions during the growing season.The basidiomycete fungus is an obligate biotrophic parasite that is difficult to culture on artificial media. Pst is a macrocyclic, heteroecious fungus that requires both primary (wheat or grasses) and alternate (Berberis or Mahonia spp.) host plants to complete its life cycle. Urediniospores have the capacity for wind dispersal over long distances, which may, under high inoculum pressure, extend to thousands of kilometres from the initial infection sites. Stripe rust, which is considered to be the current major rust disease affecting winter cereal production across the world, has been studied intensively for over a century. This review summarizes the current knowledge of the Pst‐wheat pathosystem, with emphasis on the life cycle, uredinial infection process, population biology of the pathogen, genes for stripe rust resistance in wheat and molecular perspectives of wheat‐Pst interactions. Taxonomy: The stripe rust pathogen, Puccinia striiformis Westend. (Ps), is classified in kingdom Fungi, phylum Basidiomycota, class Urediniomycetes, order Uredinales, family Pucciniaceae, genus Puccinia. Ps is separated below the species level by host specialization on various grass genera,comprising up to nine formae speciales, of which P. striiformis f. sp. tritici Erikss. (Pst) causes stripe (or yellow) rust on wheat. Host range: Uredinial/telial hosts: Pst mainly infects common wheat (Triticum aestivum L.), durum wheat (T. turgidum var. durum L.), cultivated emmer wheat (T. dicoccum Schrank), wild emmer wheat (T. dicoccoidesKorn) and triticale (Triticosecale).Pst can infect certain cultivated barleys (Hordeum vulgare L.) and rye (Secale cereale L.),but generally does not cause severe epidemics. In addition,Pst may infect naturalized and improved pasture grass species, such as Elymus canadensis L., Leymus secalinus Hochst, Agropyron spp. Garetn, Hordeum spp. L., Phalaris spp. L and Bromus unioloides Kunth. Pycnial/aecial (alternative) hosts: Barberry (Berberis chinensis, B. koreana, B. holstii, B. vulgaris, B. shensiana, B. potaninii, B. dolichobotrys, B. heteropoda, etc.) and Oregon grape (Mahonia aquifolium). Disease symptoms: Stripe rust appears as a mass of yellow to orange urediniospores erupting from pustules arranged in long, narrow stripes on leaves (usually between veins), leaf sheaths, glumes and awns on susceptible plants. Resistant wheat cultivars are characterized by various infection types from no visual symptoms to small hypersensitive flecks to uredinia surrounded by chlorosis or necrosis with restricted urediniospore production. On seedlings, uredinia produced by the infection of a single urediniospore are not confined by leaf veins, but progressively emerge from the infection site in all directions, potentially covering the entire leaf surface. Individual uredinial pustules are oblong, 0.4‐0.7 mm in length and 0.1 mm in width. Urediniospores are broadly ellipsoidal to broadly obovoid, (16‐ )18‐30(‐32) × (15‐)17‐27(‐28) μm, with a mean of 24.5 × 21.6 μm, yellow to orange in colour, echinulate, and with 6‐18 scattered germ pores. Urediniospores can germinate rapidly when free moisture (rain or dew) occurs on leaf surfaces and when the temperatures range is between 7 and 12 °C. At higher temperatures or during the later growing stages of the host,black telia are often produced, which are pulvinate to oblong, 0.2‐0.7 mm in length and 0.1 mm in width. The teliospores are predominantly two-celled, dark brown with thick walls, mostly oblong-clavate, (24‐)31‐56(‐65) × (11‐)14‐25(‐29) μm in length and width,and rounded or flattened at the apex.

Abstract: SUMMARY Stem rust has been a serious disease of wheat, barley, oat and rye, as well as various important grasses including timothy, tall fescue and perennial ryegrass. The stem rust fungus, Puccinia graminis , is functionally an obligate biotroph. Although the fungus can be cultured with difficulty on artificial media, cultures grow slowly and upon subculturing they develop abnormal ploidy levels and lose their ability to infect host plants [Bushnell and Bosacker (1982) Can. J. Bot . 60 , 1827‐1836]. P. graminis is a typical heteroecious rust fungus with the full complement of five distinct spore stages that occur during asexual reproduction on its gramineous hosts and sexual reproduction that begins in the resting spore stage and culminates on the alternate host, barberry ( Berberis spp.). There appears to be little polymorphism for resistance/susceptibility in Berberis species, but complex polymorphisms of resistance/susceptibility and matching virulence/avirulence exist in gene-for-gene relationships between small grain species and the forms of P. graminis that infect them. Taxonomy: Puccinia graminis is a rust fungus in the phylum Basidiomycota, class Urediniomycetes, order Uredinales, and family Pucciniaceae, which contains 17 genera and approximately 4121 species, of which the majority are in the genus Puccinia [Kirk et al . (2001) Ainsworth and Bisby’s Dictionary of the Fungi . Wallingford, UK: CAB International]. Various subdivisions of P. graminis into subspecies, varieties and formae speciales have been proposed based on spore size and host range. Crossing studies and DNA sequence comparisons support the separation of at least two subspecies, but not the proposed separation based on spore size. Host range: The host range of P. graminis is very broad compared with that of most Puccinia spp.; it includes at least 365 species of cereals and grasses in 54 genera [Anikster (1984) The Cereal Rusts . Orlando, FL: Academic Press, pp. 115 ‐130]. Wheat stem rust, P. graminis f. sp. tritici , was shown to infect 74 species in 34 genera in artificial inoculations of seedlings, but only 28 of those species belonging to eight genera were known to be natural hosts of the fungus. Other formae speciales of P. graminis have narrower host ranges than P. graminis f. sp. tritici . Disease symptoms: Infections in cereals or grasses occur mainly on stems and leaf sheaths, but occasionally they may be found on leaf blades and glumes as well. The first macroscopic symptom is usually a small chlorotic fleck, which appears a few days after infection. About 8 ‐10 days after infection, a pustule several millimetres long and a few millimetres wide is formed by rupture of the host epidermis from pressure of a mass of brick-red urediniospores produced in the infection. These uredinial pustules are generally linear or diamond shaped and may enlarge up to 10 mm long. The powdery masses of urediniospores appear similar to rust spots on a weathered iron surface. With age, the infection ceases production of brick-red urediniospores and produces a layer of black teliospores in their place, causing the stems of heavily infected plants to appear blackened late in the season.