Wheat leaf rust caused by Puccinia triticina

Abstract

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 microm 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.

Figure 1

Life cycle of Puccinia triticina. (A) Uredinia on leaf containing single cell dikaryotic urediniospores originating from aeciospores or urediniospores. This asexual uredinial stage may originate either from aeciospores or urediniospores and the uredinial stage is repeated on the wheat host as long as favourable conditions for infection occur. Top inset: surface view of a uredinium (×100); bottom inset: echinulate surface of a single urediniospore (×3000). (B) Telia typically form beneath the leaf epidermis near the end of the growing season, are the size of uredinia, and are black and erumpent at maturity. Top inset: teliospores which originate from telia and signal the beginning of the sexual stage of the life cycle; bottom inset: karyogamy and meiosis occur in the mature teliospore. Each teliospore can germinate producing a promycelium which gives rise to four haploid basidiospores, two of each mating type (+ and –). The nuclei undergo mitotic division so that mature basidiospores each have two haploid nuclei (not shown). (C) Pycnia, produced by basidiospores on Thalictrum, appear as yellow–orange pustules on upper leaf surfaces. (D) Diagrammatic cross‐section of Thalictrum with pycnia. Top inset: basidiospores infect Thalictrum in which the fungus produces haploid pycnia (×400); bottom inset: pycniospores and flexuous hyphae are immersed in a liquid ‘nectar’ exudate (×500). Fertilization occurs when pycniospores and receptive hyphae fuse in compatible pairs of opposite mating type combinations (+/–). (E) Diagrammatic cross‐section of Thalictrum with both pycnia and aecia. Top inset: following fertilization, a dikaryotic aecium develops (×200); bottom inset: the aecium produces chains of dikaryotic aeciospores (×1250). Inset pictures in A, C, and E courtesy of Brown and Brotzman (1979), by permission of the University of Missouri Extension Division. Top inset picture in B from Anikster et al. (2005a).

Figure 2

Infection structures of Puccinia triticina on wheat leaf surface. (A) Urediniospore (U) germ tube (G) has produced an appressorium (A) over stomatal aperture (bar = 40 µm). (B) Appressorium contents have moved into substomatal vesicle (SSV) (bar = 12 µm). (C) Inside of leaf showing SSV orientated perpendicular to leaf surface (bar = 10 µm). (D) SSV produces primary infection hyphae (PH) and haustorial mother cell (HMC). The HMC produces a haustorium (not shown) within a mesophyll cell. A secondary haustoria (SH) may emerge in the proximity of HMC (bar = 7 µm). Pictures courtesy of Hu and Rijkenberg (1998), by permission of Elsevier.

Figure 3

Leaf rust seedling infection types. The lower leaves have resistant infection types that range from hypersensitive flecks to small–moderate size uredinia surrounded by chlorosis and necrosis. The infection type of the top leaf is fully susceptible with large uredinia without surrounding chlorosis or necrosis in the leaf tissue.