Abstract

Knowledge of leaf rust resistance in wheat cultivars is useful in breeding new cultivars and anticipating changes in leaf rust races. The objective of this study was to test for the presence of known Lr genes conferring seedling resistance in important Egyptian bread wheat and durum wheat cultivars. Twenty five bread wheat and six durum cultivars were inoculated with 22 isolates of leaf rust previously characterized for avirulence/virulence to 21 Lr genes. Resulting leaf rust reaction types were consistent with the presence of Lr1 in cultivars Sids 3-9, Giza 157, Giza 162, Giza 165, Sakha 61, Sakha 69, Gemmeiza 3 and Gemmeiza 5; Lr3 in Sids 4-9, Giza 167, Sakha 8, and Sakha 69; Lr10 in Sids 1-9, and Giza 162; Lr17a in Sids 4-9, and Gemmeiza 5; Lr18 in Sids 4-8, and Gemmeiza 5; Lr23 in Sids 1-3, Gemmeiza 1, Giza 164, and Sahel 1; and Lr26 in Sids 4-8, Giza 164, Sahel 1, Gemmeiza 1, Gemmeiza 3, and Gemmeiza 5. Four bread wheat cultivars had none of the 21 known Lr genes tested, five cultivars were postulated to have a single Lr gene, eight cultivars had two Lr genes, one had three Lr genes, and two cultivars had four Lr genes. All of these 20 cultivars also had at least one unidentified resistance gene. Five other cultivars were postulated to have six Lr genes each. Evidence indicated that Lr1 was present in 14 of the 25 bread wheat cultivars, and Lr10 and Lr26 were each present in 10 bread wheat cultivars. Resistance genes in the six durum cultivars mostly conferred intermediate resistance and appeared to differ from those in bread wheat, although Lr23 may be present in durum cv. Sahog 3.

Introduction

Leaf rust caused by Puccinia triticina is one of the most common diseases of wheat, occurring nearly everywhere that wheat is grown (Wiese, 1987). Leaf rust can cause yield losses of 10% or more in susceptible bread wheat if the infection becomes severe by the mid-dough stage of development, but losses can be much greater if the leaves are destroyed by rust at earlier stages of development (Chester, 1946). In Egypt, losses of 5-10% from leaf rust were common in years when bread wheat cultivars lacked adequate resistance (Abdel Hak et al., 1966; Nazim et al., 1982; Nazim et al.,1983). Generally wheat leaf rust is controlled by resistant cultivars. High levels of resistance to leaf rust are race-specific. The durability of race-specific resistance may depend on the number of effective resistance genes that are combined in wheat cultivars (Schafer and Roelfs, 1985). When pyramiding resistance genes in cultivars, it is important to know the resistance genotypes of the cultivars and germplasm used as parents in the breeding programs.

The gene-for-gene relationship (Flor,1956) between race-specific resistance in host plants and virulence in pathogens makes it possible to postulate resistance genotypes of host cultivars based on their reactions to a selected set of pathogen races. Loegering (1978, 1985) developed the concept of interorganismal genetics of host:pathogen associations in gene-for-gene interactions and used it to detect the presence of specific resistance genes in host cultivars based on their reactions to pathogen isolates with suitable combinations of avirulence and virulence alleles. Data for infection-type (IT) of the host:pathogen interaction have been used to postulate or verify the genes present in wheat cultivars for leaf rust and stem rust resistance (McVey, 1989, 1992; McVey and Long, 1993; Modawi et al., 1985; Singh, 1991, 1993; Singh et al., 1999; Statler, 1984). The objective of this study was to postulate the leaf rust resistance genes in commonly grown Egyptian wheat cultivars. We limited this study to resistance genes that are expressed in the seedling as well as adult plant stage.

Materials and Methods

Pathogen isolates - A set of 22 isolates of P. triticina with different combinations of avirulence and virulence genes were selected for the test (Table 1). The isolates included five from Egypt, 11 from various states of the United States, and six from other countries. Each isolate was tested on a set of 21 differential wheat lines, each with a different single Lr gene for resistance to leaf rust. The first 19 Lr gene lines, which are in a 'Thatcher' spring wheat background, were developed at the Agriculture Canada Wheat Research Center, Winnipeg, Canada. The near-isogeneic line with Lr36 in a 'Manitou' spring wheat background was developed at University of Saskatchewan, Saskatoon, Canada. The Lr42 line in a 'Century' winter wheat background was developed at the Wheat Genetics Resource Center, Kansas State University. The 21 differential lines and their low infection types (LIT) are shown in Table 2. The LIT characteristic of each differential line is indicated according to the Stakman et al. (1962) scale of 0 (no visible reaction), ; (hypersensitive necrosis or chlorotic flecks), 1 (small uredinia often surrounded by necrosis), 2 (small to medium uredinia often surrounded by chlorosis), or various combinations of these reactions. In a few cases IT3 (medium-sized uredinia without chlorosis or necrosis), which is regarded as a high infection type (HIT), was seen in combination with a low IT. Each isolate was increased on a selected Lr line susceptible to that isolate but not to other isolates to avoid potential contamination of cultures used in subsequent tests.

Inoculation procedures - The near-isogenic Lr lines and the cultivars to be tested were planted in vermiculite in 5.3 cm square plastic pots. Four entries were planted per pot with 10 to 15 seeds per entry planted in each corner of the pot. Pots were grouped in plastic trays designed to hold six pots per tray. Plants were grown in a rust-free greenhouse until inoculation. Plants were fertilized at 5 and 8 d after planting with a water-soluble fertilizer (23:19:17 NPK) at a rate of 2.5 g per tray of six pots.

At 7 d after planting, when first leaves were fully extended, the seedlings were inoculated by spraying them with a suspension of urediniospores in a light mineral oil carrier. The oil was allowed to evaporate from the leaves for 30-60 min, and the seedlings were placed overnight in a dew chamber at 17°C. They were then transferred to a greenhouse with mean temperature approximately 20 to 21°C and light supplemented with 100 to 200 mol m^-2s^-1 photon flux fluorescent light. At 14 d after inoculation, plants were scored for IT according to the Stakman et al. (1962) scale. The same procedure was used to test cultivars against each P. triticina isolate, but the inoculations with different isolates were done at different times and the inoculators were sterilized between inoculations to avoid cross contamination.

Postulation of resistance genotypes - Reactions of the 22 isolates to the 21 near-isogenic lines are shown in Table 3. The 25 Egyptian bread wheat (Triticum aestivum) cultivars and six durum wheat (T. turgidum) cultivars and their pedigrees are shown in Table 4. Presence or absence of specific genes for leaf rust resistance were deduced for each cultivar:isolate combination. For example, a cultivar with a HIT to a leaf rust isolate was positive evidence that the cultivar did not possess Lr gene(s) that correspond to gene(s) for avirulence identified for that pathogen isolate when tested on the set of single Lr gene differentials. A LIT indicated that the host possessed either a known gene for which the isolate was avirulent or an unidentified gene or genes conferring a similar LIT to the isolate. The likely identity of an Lr gene for resistance could be hypothesized when the cultivar of unknown resistance genotype expressed a LIT identical to that of the near-isogenic line when inoculated with the same pathogen isolates.

Results

Infection types of the 25 Egyptian bread wheat and six durum wheat cultivars to the 22 leaf rust isolates are shown in Table 5 with the postulated leaf rust resistance genotype for each cultivar. The cultivars Sids 1 and Sids 2 illustrate the process by which the presence of Lr genes was postulated. 'Sids 1' and 'Sids 2' were susceptible to isolates 9, 10, 11, 15, and 21 (Table 5), which indicates that 'Sids 1' and 'Sids 2' do not have Lr1, 2a, 2c, 3ka, 9, 11, 16, 17a, 18, 21, 26, or 30, all of which condition a LIT to one or more of these isolates. The LITs of 'Sids 1' and 'Sids 2' to isolates 6, 7, 12, 14, 18, and 19 are consistent with the LIT conferred by Lr1 against avirulent races of leaf rust. The LITs with isolates 4, 5, 7, 12, 13, 16, and 19 can be attributed to Lr23. Four of these, isolates 4, 5, 13, and 16, produced a HIT on the Lr10 differential. Isolates 4 and 16 produced a 23 LIT on 'Sids 1' and 'Sids 2', which is typical of the LIT for these isolates on the Lr23 differential. The LITs of 'Sids 1' and 'Sids 2' to isolates 1, 8, 17, and 20 indicates the additional presence of an unidentified gene(s) in these cultivars. Also, isolates 2 and 3 were avirulent to 'Sids 1' but virulent to 'Sids 2'. This indicates that 'Sids 2' has a least one other unidentified leaf rust resistance gene.

By a similar process, the leaf rust resistance genotypes of the other Egyptian wheat cultivars were postulated. The data indicated the probable presence of Lr1, 3, 10, 17a, 18, and 26 in 'Sids 4', 'Sids 5', 'Sids 6', 'Sids 7', and 'Sids 8' (Table 5). Other cultivars have fewer Lr genes. Three of the Giza cvs. and 'Sakha 92 have none of the 21 Lr genes tested, but each of these cultivars appears to have an unidentified gene(s) for leaf rust resistance. Among the bread wheat cultivars, five were postulated to have a single Lr gene, eight to have two Lr genes, one to have three Lr genes, two to have four Lr genes, and as described above, five Sids cultivars were postulated to have six Lr genes.

Lr1 was the most common leaf rust resistance gene, being postulated in 16 of the 25 Egyptian bread wheat cultivars. Other common Lr genes include Lr10 and Lr26 (10 cultivars), Lr3 (nine cultivars), Lr17a (seven cultivars), Lr18 (eight cultivars), and Lr23 (six cultivars). The LIT combinations produced by isolates in this study were not consistent with the existence of Lr2a, Lr2b, Lr2c, Lr3ka, Lr9, Lr11, Lr21, Lr24, Lr30, Lr36, or Lr42 in any of the wheat cultivars tested. In addition, Lr16 can be excluded because the necrosis typical of its LIT with avirulent isolates was not seen with any of the Egyptian wheat cultivars. Patterns of LIT among several of the cultivars tested do not necessarily exclude Lr14b or Lr15 from being present, but neither gene is necessary to explain any of the LIT patterns, and there is no reason to assume that either gene is present in the pedigrees of the Egyptian wheat cultivars.

The Egyptian durum wheat cultivars showed intermediate reactions to most of the leaf rust isolates tested. Only isolates 1, 17, 18, and 22 gave HITs to most of the six durum cultivars. The patterns of LITs in the durum cultivars were not consistent with those of any of the Lr genes, except that resistance of 'Sahog 10' might be attributed to the presence of Lr23 plus an unidentified resistance gene.

Discussion

The pool of resistance genes postulated to be present in the common Egyptian bread wheat cultivars is small and would not be effective in many areas of the world. Nine of the 25 cultivars appeared to have either none or only one known Lr gene. While these and other Egyptian cultivars appear to have one or more unidentified leaf rust resistance genes, the resistance provided by most of these unidentified genes seems to be of slight value against wheat leaf rust races known to be present in Egypt. Among the Lr genes considered in this study, only Lr18 is effective against all five Egyptian isolates used in the study. 'Sids 4-9' and 'Gemmeiza 5' were postulated to have Lr18 and are resistant to all five Egyptian isolates. "Sids 1', 'Giza 167', and 'Sahel 1', which do not have Lr18, also were resistant to all five Egyptian isolates, but their postulated Lr genes do not account for their resistance to at least some of the Egyptian isolates. Therefore, these cultivars appear to have unidentified gene(s) for leaf rust resistance that may be useful in Egypt. For example, the unidentified resistance gene(s) present in combination with Lr3 in 'Giza 167' accounts for resistance to all five Egyptian isolates, because Lr3 does not confer resistance to any of the five. The unidentified resistance gene(s) present in combination with Lr10 and Lr23 in 'Sids 1' accounts for resistance to two of the Egyptian isolates.for which neither Lr10 nor Lr23 confers resistance. Similarly, the unidentified resistance gene(s) in 'Sahel 1' confers resistance to the two Egyptian isolates with virulence to overcome the resistance of Lr23 and Lr26.

Huerta-Espino (1992) found that in many parts of the world, the populations of P. triticina on durum cultivars are quite distinct from P. triticina populations on bread wheat cultivars. Most durum leaf rust isolates tested by Huerta-Espino were avirulent on nearly all bread wheat cultivars. Within durum cultivars, however, there was a wide range of specificity of resistance to isolates of leaf rust from durum wheat. This indicates that durum wheats contain a diversity of leaf rust resistance genes that differ from those of bread wheat. Thus, it is not surprising that none of the Lr genes in near-isogenic lines of bread wheat, with the possible exception of Lr23, could account for the resistance of the six Egyptian durum cultivars tested.

Acknowledgements

This research was funded by the Agricultural Technology Utilization and Transfer (ATUT), Agency for International Development (AID). We thank Mark Hughes for assistance with data analysis and Lucy Wanschura for technical assistance in testing resistance of the cultivars and virulence of the isolates.

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