VARIABILITY IN VIRULENCE OF WHEAT LEAF RUST (PUCCINIA TRITICINA) IN THE SINDH PROVINCE, PAKISTAN

Article History Received: November 11, 2022 Revised: May 20, 2023 Accepted: May 29, 2023 Leaf rust of wheat caused by an obligate biotrophic fungus (Puccinia triticina), is one of the widespread diseases of wheat. The emergence of new virulent races of fungal pathogens threatens wheat cultivars’ resistance, leading to outbreaks that can cause substantial damage to crops and result in economic losses to farmers. Developing wheat cultivars that have resistance to virulent races is an effective means of reducing the frequency and impact of these outbreaks. An experiment was conducted at 5 locations (a trap nursery consisting of 37 lines of wheat leaf rust differentials and 115 commercial wheat cultivars was established against leaf rust pathogen), for evaluating the virulence pattern of leaf rust under natural field environments across wheat-producing regions of Sindh, Pakistan. Results revealed that Lr9, Lr19 and Lr28 genes were found effective under field environments. The moderate resistance was recorded for Lr18, Lr23 and Lr34 genes at two locations while Lr36 & Lr37 genes had moderate resistance at most of the locations. Results also revealed that Rawal-87, Marvi-2000, Bhittai, Pirsabak-08, Faisalabad-08, Benazir-13 and Shalakot-13 were the commercial wheat varieties having resistance at all locations of Sindh during (year 1 and year 2) years while most of the cultivars showed susceptibility. The scenario clue to a dire necessity to widen the genetic base of Pakistani cultivars by incorporating genes for the resistance against disease. Furthermore, strong monitoring and regular surveys should be conducted for determining current virulence status and resistance genes.


INTRODUCTION
Bread wheat (Triticum aestivum L.) is the main source of protein and energy and the majority of our population depends on it as it is a major source of nourishment (FAOSTAT, 2016).Wheat yield is seriously influenced by biotic and abiotic constraints (Singh et al., 2008) and many biotic stresses are responsible for destabilizing the wheat production throughout the world.Among them, three rust species viz., yellow, brown and black rust are of major importance (Singh et al., 2004;Chen, 2005).
Stem rust, leaf rust, and stripe rust have had a significant impact on cultivated cereals, causing economic losses due to the reduction in the quantity and quality of grains in various regions across the world, especially in Asia.These deadly pathogens are obligate parasites belong to the genus Puccinia having narrow host range and are highly specialized (Mendgen and Hahn, 2002;Kolmer et al., 2009).Of these three notorious wheat diseases, brown rust is the most serious striking wheat disease (Bolton et al., 2008).Characteristics of P. triticina are wide adaptation of the pathogen to the distinct climatic situations and increased levels of virulence distinction where wheat is grown and contribute to regular reductions in the worldwide production of wheat (Roelfs, 1992;Kolmer et al., 2011).Mains and Jackson (1926) were the first to explain the physiologic specialization of leaf rust of wheat as a pathogen has a long history of studies of population.Mains and Jackson (1926) initially described the primary differentials set, including the eight lines of wheat having distinct genes for resistance i.e. ([Lr1 gene Mediterranean, [Lr11] Hussar, and [Lr3a] Democrat) and broadly utilized for investigation and monitoring structure of pathogen races globally.Dyck (1968) had initially developed lines of Thatcher differential and designated them as fundamental set for examining virulence diversity of pathogens globally.A particular differential set, comprising nine lines of wheat, was reported in the early 1980s to investigate pathogens.A constant record of knowledge regarding the evolution of diverse populations and epidemiology of the pathogen has been provided by virulence surveys of pathogen races.Until the late 1960s, single-uredinial isolates of pathogen were characterized for virulence on eight differential varieties (Kolmer et al., 1995).A sequence of near-isogenic wheat lines of Thatcher were utilized by (Dyck et al., 1974) that varied by single resistance genes of pathogen.The Thatcher lines have been exercised to differentiate virulence in populations of pathogen in Egypt (McVey et al., 2004b) and throughout the world (Singh, 1991).Use of nearisogenic differential lines has made possible the distinction of P. triticina isolates that vary by an individual virulence (Samborski and Dyck, 1976).Genetic resistance against wheat leaf rust is widely classified into adult plant resistance (APR) and seedling resistance.According to various reports (Leonova et al., 2020;McIntosh et al., 2017) more than 100 genes conferring resistance to pathogen to date, have been effectively categorized, of them the most of confer seedling resistance.According to (Flor, 1971) usually seedling resistance is regulated by a single gene with a main influence that communicates with the pathogen in a 'gene-for-gene' association.Whereas adult plant resistance (APR) is typically efficacious at the postseedling development phases, is either regulated by an individual gene with the main influence or several genes each with minor influence.Various researchers (Lagudah, 2011;McCallum et al., 2012;Burdon et al., 2014) reported that partial resistance is provided by some adult plant (APR) genes which is useful (i.e.racenonspecific) against all races of a particular species of pathogen.In contrast, most of genes possess adult plant resistance interact additively and increase resistance to certain immunity levels (Singh et al., 2014).However, pleiotropic resistance is shown by some genes of APR against numerous diseases viz., Lr67, Lr46 and Lr34 offer partial resistance to powdery mildew, yellow, stem and wheat leaf rust diseases (Lagudah, 2011;Risk et al., 2012;Ellis et al., 2014).The virulence of the pathogen has been reported in coastal zones where wheat develops and matures in warm humid environments viz., Central Asia (Morgounov et al., 2007), Uruguay (German and Kolmer, 1994;Germán et al., 2007), Turkey (Kolmer et al., 2013) and also in continental zones (the Great Plains of North America) which have warm to hot summer temperatures (Roelfs, 1989).In Egypt, high diversity of virulence and broad variations in populations of pathogens are recorded (McVey et al., 2004a).Since the 1930s, extensive studies of physiologic specialization in pathogens have been conducted in the Canada (Johnson, 1956) and the U.S. (Johnston, 1968).On an annual basis numerous virulence phenotypes of the fungus are detected due to highly variable pathogen populations.Similarly, pathogen is found annually all over wheat-producing areas in Pakistan resulting in frequent yield reductions (Saari and Prescott, 1985;Lawlor, 2013).According to reports of researchers (Nagarajan and Joshi, 1985) pathogens can survive on the crop during the summer in the mountainous western areas and then proliferates to wheat-producing areas of the provinces of Punjab and Sindh.The objective of the study was to investigate the virulence pattern of leaf rust in various wheatgrowing areas in Sindh, Pakistan.

Germplasm collection
A set of 37 near-isogenic (differential) lines and 105 cultivated wheat varieties were provided by International Wheat and Maize Improvement Centre (CIMMYT) Mexico and National Agricultural Research Center (NARC) Islamabad, Pakistan respectively, while additional 10 varieties were obtained from the Nuclear Institute of Agriculture (NIA), Tandojam for the current study.

Field experiments
A trap nursery consisting of 152 wheat genotypes, including 115 Pakistani wheat cultivars and 37 wheat near-isogenic lines containing single Lr gene was planted at five locations (Tandojam, Sakrand, Sanghar, Larkana and Badin) of Sindh Province.The nurseries were planted over 2 years during the 2014-15 and 2015-16 wheat cropping seasons.All locations represented distinct agroecological conditions conducive to infection and development of brown rust.Every genotype was sown in an unrepeated individual line of one-meter length where row to row distance was 30cm.Susceptible check (Morocco) two lines were sown for uniform infection.

CONCLUSION
Current work revealed that Lr9, Lr19 and Lr28 resistance genes demonstrated their effectiveness and Lr36 and Lr37 were found moderately resistant whereas most of the resistance genes were ineffective against the population of (P.triticina) pathogen across all tested locations during both cropping years.However, seven commercial wheat cultivars (Rawal-87, Marvi-2000, Bhittai, Pirsbak-08, Faisalabad-08, Benazir-13 and Shalakot-13) were found resistant out of 115 cultivars tested.Strong monitoring should be established and regular surveys should be conducted for determining genes for resistance and the current status of virulence.As virulence analysis provides a timely warning before the pathogen becomes a menace to crop production.

Table 1 .
Modified cobb's scale used for data recording during present study.
and Lr18, Lr23 and Lr34 had moderate resistance at two locations and Lr36, Lr37 had moderate reactions at all tested locations.The infection on susceptible Morocco was recorded up to 90S at all locations (Table2).