Common Rust Disease Research Articles

Quantitative trait loci (QTLs) for resistance to gray leaf spot and common rust diseases of maize

Gray leaf spot and common rust diseases can greatly reduce grain yield of maize in susceptible genotypes by between 10 and 70% on average. Control of these diseases through conventional measures has been quite ineffective and difficult to sustain. The most feasible way to control them is by breeding and deploying resistant maize genotypes. This study was carried out to evaluate germplasm for QTLs associated with resistance to GLS and common rust diseases by use of microsatellite markers and artificial inoculation with the two pathogens. A total of 41 genotypes comprising 23 recombinant inbred lines, 14 parental inbred lines and 4 hybrid checks were screened using 28 SSR markers and disease pressure by artificial inoculation (Tables 1 and 2). Out of the 14 parental inbred lines, only 4 were found to carry the QTL associating positively for the two diseases, and 10 out of the 23 recombinant inbred lines with possible lineage from any of the 14 parents, were positively associated with the traits and seven of the markers used (Table 3). GLS QTLs were significant for two markers; bnlg1258 with a LOD score of 16.0 and umc2019 with a LOD score of 17.9 from regions 2.06 and 2.08 of chromosome 2, respectively. Significant QTLs for common rust resistance were identified in three regions of chromosome 10, corresponding to markers phi054 with a LOD score of 14.0 at bin 10.00, umc1319 with a LOD score of 4.0 at bin 10.02 and bnlg1451 with a LOD score of 14.3 at bin 10.03. The effects of these QTLs were different from genotype to genotype. The disease severity scores (scale of 1-5) of artificial inoculation ranged from 1.5 to 2.5 for gray leaf spot with a mean of 1.88 and a range of 1.5 to 3.0 with a mean of 1.74 for common rust. All the inbred lines scored better for the two diseases than the four hybrid checks which scored 3.5. The most resistant genotypes showed a score of 1.5 for gray leaf spot and common rust. All parents showed a score of less than 2.5 for GLS and common rust. Parental genotypes MAL40, MAL9, MAL13, MAL41, MAL11, MAL19, MAL23-2, MAL24, and MAL19-1, carried QTLs associated with resistance to grey leaf spot and common rust and thus were identified as sources of resistance conferred to the inbred lines. The selected lines are being used to make single hybrids, double crosses, three way hybrids and synthetics resistant to diseases. The marker data was also used to analyze the diversity of the genotypes studied, with relevance to immune/resistant, tolerant or susceptible to the two diseases. Using the GLS genotypic data, 13 genotypes clustered into 11 groups, and using the common rust data, the 13 genotypes clustered into 12 clusters. This indicates that almost each of these genotypes was grouped in a cluster that contained lines that did not have positive association of marker and trait data. These results indicated that the putative QTLs for GLS are associated with the 13 genotypes and two markers in chromosome 2 (bnlg1258 and umc2019), whereas those of common rust are associated with the 13 genotypes and three markers on chromosome 10 (phi054, umc1319, and bnlg1451).

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. Puccinia triticina Eriks.: kingdom Fungi, phylum Basidiomycota, class Urediniomycetes, order Uredinales, family Pucciniaceae, genus Puccinia. 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. 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. 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. USDA Cereal Disease Laboratory: http://www.ars.usda.gov/mwa/cdl.

Unequal exchange and meiotic instability of disease-resistance genes in the Rp1 region of maize.

The Rp1 region of maize was originally characterized as a complex locus which conditions resistance to the fungus Puccinia sorghi, the causal organism in the common rust disease. Some alleles of Rp1 are meiotically unstable, but the mechanism of instability is not known. We have studied the role of recombination in meiotic instability in maize lines homozygous for either Rp1-J or Rp1-G. Test cross progenies derived from a line that was homozygous for Rp1-J, but heterozygous at flanking markers, were screened for susceptible individuals. Five susceptible individuals were derived from 9772 progeny. All five had nonparental combinations of flanking markers; three had one combination of recombinant flanking markers while the other two had the opposite pair. In an identical study with Rp1-G, 20 susceptible seedlings were detected out of 5874 test cross progeny. Nineteen of these were associated with flanking marker exchange, 11 and 8 of each recombinant marker combination. Our results indicate that unequal exchange is the primary mechanism of meiotic instability of Rp1-J and Rp1-G.

Evidence for the existence of a single race of common rust caused by Puccinia melanocephala, in Australian sugar cane cultivars

A detached-leaf method for testing for physiological races of common rust disease, caused by Puccinia melanocephala, in Australian sugar cane cultivars is described. Single-uredium isolates collected from various locations in Queensland, Australia, between 1978 and 1986 were screened. Host reaction type was specific to individual cultivars and did not vary when these cultivars were inoculated with different rust isolates. For each cultivar there was no difference in generation time for each isolate. These results suggest that there is only one physiological race present in the pathogen population in Queensland. Reported changes in the severity of common rust disease in sugarcane cultivars were caused by changes in environmental conditions rather than the development of new races of the pathogen. Rust infection fluctuated during the growth of sugar cane, with a major peak occurring at tillering and stem elongation, following moderate rainfall and temperatures between 12 and 25�C. Rust was more severe on older leaves than on younger leaves.