Abstract

The wheat variety Kundan, along with the fast ruster variety Agra Local, were screened for seedling reaction and adult plant response for four years. Seedlings of Kundan were susceptible while adult plants showed lower susceptible response than Agra Local in the field. Kundan along with Agra Local were also screened for slow rusting components for two years in the glass house conditions. Kundan showed lower pustule number, pustule size and longer latency period under glasshouse conditions than the fast ruster. The consistent performance of AUDPC (four years) and components of slow rusting (two years) indicates the use of these parameters in identifying slow rusting genotypes / varieties. Test weight of Kundan never deviated significantly with the disease. The grain quality and test weight of this variety are highest and maintained consistently over the years. The long lasting resistance in the Kundan can be attributed to the slow rust resistance, which indicates its importance in the breeding programmes.

Introduction

Production and productivity of wheat are greatly influenced by the biotic and abiotic factors in wheat. Among the biotic factors, the rust pathogen assume particular significance as these can change genetically and multiply rapidly causing epidemic situation and thereby resulting into yield losses. Research into breeding for resistance to rust diseases in wheat is necessary to maintain productivity. There is strong consensus that growing genetically resistant cultivars is the most appropriate and cost effective means of managing rusts and is of one of the key components of crop improvement. The drawbacks of resistance breeding are few. One of the important concerns of resistance breeders is the specificity and durability of the disease resistance incorporated in their cultivars.

Leaf rust caused by Puccinia recondita Roberge ex Desmaz. f. sp. tritici Ericks. & E. Henn. is one of the most destructive and widely distributed disease in most wheat growing areas. Farrer (1898) in Australia recognized the general resistance in wheat to stem rust. With the discovery of physiological specialization in rusts by Stakman et al. (1962) and the classification of the genetic basis of resistance (Biffen, 1905; Flor, 1956) the hypersensitive type of resistance was incorporated in many cultivars. Van der Plank (1963) defined the theoretical basis of genetic resistance to leaf rust. For leaf rust alone 51 resistance genes have been identified from different germplasm of wheat and its wild relatives. Similarly for stem and stripe rusts, about 42 and 18 resistance genes have been identified, respectively.

Breeders commonly have relied on race specific leaf rust (Lr) resistance genes for hypersensitive resistance which are very effective in reducing the epidemic build up and easy to manage in breeding programmes because of their monogenic nature. But, the short-lived nature and improper use of race specific resistance genes on commercial scale led to their erosion within a short time. In fact, virulence is known for many of the named genes involved in resistance to leaf rust. A more recent example of the breakdown of the resistance afforded by the locus Lr26 in wheat against P. recondita Rob. ex Desm. f.sp. tritici in Pakistan (Aslam, 1996) and Yr9 against P. striiformis in Iran (Turabi, 1994) and in India the breakdown of leaf rust resistance genes Lr9 in 2000. This has resulted in the narrowing of the useful genetic diversity for resistance genes and has created the necessity of continuous search for more durable type of resistance.

Wheat cultivars grown in India are either direct selections, or are derived from germplasm developed at CIMMYT, Mexico. Following the wheat leaf rust epidemic in the 1970s affecting wheat crop in India, various genes for resistance to the diseases have been used in the breeding programmes. Genetic analysis of the germplasm in commercial production indicated that genes Lr1, Lr3, Lr10, Lr13, Lr23, Lr26 and Lr34 are the most frequently found in wheat grown in India.

There is a great interest in improving the durability of the intrinsically "non-durable" type of resistance by gene pyramiding or by use of multilines. Another possibility would be to identify and introduce resistant types that can be intrinsically durable like partial resistance. Partial resistance is characterized by a slow epidemic buildup despite a high infection type indicating a compatible host-pathogen interaction (Parlevliet, 1975). Partial resistance may be such an alternative and in several plant-pathosystems, partial resistance has been demonstrated to be inherited quantitatively.

The important components of slow rusting in leaf rust are longer latency period, smaller pustule size, fewer pustules and lower disease severity values (area under disease progress values). There are reports that this type of resistance is durable and race non-specific (Kuhn et al., 1978; Dinessen, 1993; Aslam, 1996; Singh et al., 1993). Reports on the association and genetics of components of slow rusting to leaf rust are limited and in most cases, slow rusting is inherited polygenically and hence selection for higher levels of resistance is less straight forward than for monogenically inherited traits.

Plant breeders require more information regarding the association and genetics of the components of slow rusting in order to breed more effectively for this type of resistance. Furthermore, efficient selection for such resistance requires methods for both glasshouse and field evaluations at the seedling and adult plant stages.

Materials and Methods

The experiments were conducted during 1998-2003 at Indian Agricultural Research Institute (IARI), New Delhi, India. Kundan, the dwarf wheat (DL 153-2) bred at the Indian Agricultural Research Institute, New Delhi involving parents Tonari 71 and NP 890, embodies a good degree of genetic homeostasis enabling it to register its presence among the elite cultivars over a wide range of growing conditions from rainfed to limited water and nutrient inputs as well as from normal to late planting. Its superior grains found mostly lacking in dwarf wheats are an attractive feature (Sawhney, 1993). Kundan is resistant to important rust diseases and constantly maintained the level of leaf rust resistance for over two decades, during which many rust pathotypes evolved.

Kundan was selected on the basis of historical observations of slow rusting to wheat leaf rust. It was evaluated for components of partial resistance to leaf rust both in the glasshouse and field conditions. The most widely virulent pathotype 77-5 (121R63-1) of Puccinia recondita f. sp. tritici was used for these experiments. The avirulence/ virulence formula for this pathotype is as follows: Lr9, Lr18, Lr19, Lr22a, Lr24, Lr 25, Lr28, Lr29, Lr30, Lr32, Lr 34, Lr48, Lr49 / Lr1, Lr2a, Lr3, Lr10, Lr11, Lr12, Lr13, Lr14a, Lr15, Lr16, Lr17, Lr20, Lr23, Lr26, Lr27+31, Lr33.

Glasshouse studies
Kundan along with fast ruster Agra Local, was tested in glasshouse at seedling stage following the procedure proposed by Joshi et al. (1986). Seedlings of the parents were raised in 10 cm pots and 10 seeds were sown in each pot. Infection types were recorded 12 days after inoculation following the scale proposed by Stakman et al. (1962).

Kundan and Agra Local were also sown in 30 cm pots with 5 plants per pot. Thinning was carried out to achieve one tiller per plant. Plants at flag leaf stage were sprayed with a suspension of freshly harvested urediospores where Tween-20 was used as surfactant. The suspension had 20 urediospores per microscopic field (10x X 10x) on an average. Each flag leaf was sprayed uniformly with urediospores. The inoculated plants were then transferred to a moist chamber for 48 hours after which the pots were transferred to open glasshouse benches. Experiment was repeated for a second year.

The infected flag leaves were individually evaluated for latency period, pustule size and pustule number. Latency period (days) was calculated by using the formula given by Das et al. (1993). Pustule size was calculated by using the formula, pi/4 x length x width (mm²) (Kochman & Brown, 1975). The length and breadth of the uredia were measured by the micrometer. From each flag leaf 10 uredinia were recorded and where less than 10 uredinia were present, the size of all the uredinia were measured. Pustule number was counted as the number of uredinia per unit area.

Field studies
Kundan along with Agra Local were also planted in the field for two consecutive crop seasons to compare the glasshouse observations with the field reaction where the disease increase is multicyclical. Plots consisted of pairs of 3 m rows seeded 10 cm apart with a distance of 18 cm between rows and 36 cm between the plots. Spreader rows composed of Agra Local, Kharchia Local and Lal Bahadur were planted all around the experimental block and between the beds. The spreader rows were inoculated using a hypodermic syringe with pathotype 77-5 after 55-60 days of sowing, so that the proper disease spread could occur. When the spreader plants were 50% infected, the genotypes were scored three times for rust severity and response using the "Modified Cobb's" scale (Peterson et al., 1948) at weekly intervals. The area under the disease progress curve (AUDPC) was calculated from these disease scores using the computer programme developed at CIMMYT.

Results and Discussion

The seedlings of Kundan showed high infection type of "3+" whereas the fast ruster, Agra Local showed "4" based on the "0-4" scale described by Stakman et al. (1962). In the field, Kundan showed the susceptibility as in the case of the fast ruster but the susceptibility is not as high as the Agra Local, which indicates the existence of the slow rusting in Kundan.

Under the glasshouse conditions, Kundan showed long latency period (20.89, 21.70), smaller pustule size (0.166, 0.134) and less number of pustules (10.75, 10.25) whereas the fast ruster, Agra Local had the short latency period (9.85, 11.60), larger pustule size (0.274, 0.396) and higher uredinial number (36.40, 34.10) in both the seasons (1998- 99, 1999- 2000), respectively. In the field conditions, Agra Local showed the susceptible response with AUDPC values (1455.0, 1300.0, 1550.00, 1380.00) whereas Kundan showed AUDPC values (218.5, 217.0, 245.00, 208.00) for four years. The components of slow rusting and AUDPC are consistent in Kundan over the years and they could be used as the most reliable direct parameters for measuring slow rusting resistance. Ohm and Shaner (1976), Kulkarni and Chopra (1980), Andres and Wilcoxson (1986), Singh et al. (1991) and Prabhu et al. (1993) also reported the similar trend in the case of AUDPC and for slow rusting components to leaf rust in wheat.

It can be observed that selection for the longer latency period, lower pustule size and pustule number would result in genotypes with low AUDPC. As there are many components involved in the slow rusting, each perhaps with its own system of genetic control, the evolution of aggressive races would expected to be slow and therefore slow rusting offers a durable form of host resistance.

The data indicated that over a period of 4 years the level of resistance to leaf rust in variety Kundan remained almost the same. The highest intensity of disease severity observed in Kundan was 30S. Similarly, the test weight (1000 grains) of Kundan (48-50g) remained more or less constant over years. When, Kundan is compared with the partial resistant cultivars for the components of slow rusting viz., pustule number, pustule size and latency period over the International confirmed slow rusting varieties, Galvez-87 and Trap to leaf rust, it performed better for all the components and the terminal disease severity. This may be the reason for the resistance of Kundan to leaf rust even it has been released in the mid eighties and indicates the importance of Kundan as a source for partial resistance to leaf rust in the breeding programmes.

Genetic analysis of the Kundan with Agra Local indicated that epistatic interactions are more predominant in the expression of the components of slow rusting and AUDPC (Lal Ahamed & Singh, 2003). There are reports that Kundan is having the leaf rust resistance gene, Lr34 (McIntosh et al., 1995). Lr 34 is known to be partial resistance gene for leaf rust (Rubiales & Niks, 1995) but the nature of this gene in controlling all the components of slow rusting and AUDPC has not yet been revealed.

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