Abstract

For creation of wheat cultivars with durable protection to leaf rust in modern breeding, adult-plant resistance and partial resistance or "slow rusting" genes are widely used. To identify adult plant resistance genotypes we selected eight test - pathotypes of Puccinia triticina with matching virulence to adult plants of Thatcher near-isogenic lines with rust resistance gene Lr 12, Lr 13, Lr 34, Lr 37 and their combinations. Adult-plant resistance genes have been postulated for 20 wheat cultivars and lines produced from interspecific hybridization with Aegilops speltoides. The resistance gene Lr 13+ were postulated in winter wheat cv. Mironovskaya 29, Lr 37+ in cv. Mirleben, and Lr 12 in cv. Rodina. Resistance genes Lr 12, Lr 13, Lr 34, Lr 37 and their combinations were postulated in some lines with alien genes. The latent period has been determined for 13 wheat cultivars described as "slow rusting" resistance in field conditions. Specific interactions between resistance of cultivars and aggressiveness of pathotypes of P. triticina were revealed.

Introduction

Leaf rust caused by Puccinia triticina is one of the most important diseases of wheat. It occurs everywhere that wheat is grown in Russia. Yield losses reach 10-20% annually. Breeding of resistant cultivars is the most economically profitable and effective way to achieve disease control. Difficulty of breeding for durable resistance against leaf rust is due to continuous appearance of new virulent races and pathotypes in pathogen populations. To developing a strategy for breeding for disease resistance it is necessary to understand the processes of pathogen evolution in this host-parasite interaction and to understand the resistance mechanisms.

Cultivation of the wheat cultivars with durable resistance (adult-plant resistance and partial resistance) reduces yield losses, reduce the risk of epidemics, constrains evolution of the pathogen, and by definition increases the longevity of resistant cultivars.

The most durable leaf rust resistance in wheat (Triticum aestivum) is conditioned by adult-plant resistance genes (Liu & Kolmer, 1997; Singh et al., 1999a). Adult plant resistance genes have been widely deployed in wheat cultivars and are sources of highly effective leaf rust resistance. Some of them are important in North American spring wheat, in breeding programmes in South America, and in the CIMMYT programme (Barcellos et al., 2000). The gene Lr 34 has been widely used in North and South American spring wheat over the last 30 years (Singh et al., 1999b). Lr 13 was first used in Canada in the cultivar Manitou, which was released in 1966 when it was highly resistant. Lr 35 was derived from Aegilops speltoides (Kolmer, 1997) and has already been used in wheat breeding programmes (Liu & Kolmer, 1997; Singh et al., 1999a; Ionesco-Cojocary et al., 1979/80).

The main parameters of a "slow rusting" resistance are the dynamics of development of the disease as expressed by the area under the disease progress curve under field conditions, latent period, quantity of pustule and type of reaction in the greenhouse or an artificial climate chambers conditions (Lehman & Shaner, 1996; Line & Chen, 1995; Ahamed, 2003).

Materials and Methods

Pathogen isolates
A set of eight pathotypes P. triticina with different virulence and avirulence genes to adult plant resistance Lr-lines of wheat were selected for the test. The pathotypes were isolated from single pustules on wheat plants from Central (pathotype 261-5), Central-Chernozem (317-6) and West-Siberian (277-1, 277-4, 277-10), and Low-Volga regions (252-3) in 2000. The pathotypes were divided into different phenotypes according to Long and Kolmer (1989). Each isolate was increased on a susceptible cultivar.

Experiments were carried out in a controlled climate chamber ( temperature, light duration and humidity) and in a greenhouse. Inoculation of plants was carried out on seedlings and adult plants at leaf stage flag-1. Plants were sprayed with a suspension of spores with water and Tween20 and maintained in the moisture chamber for 24 hours. Plants were scored for infection 10-12 day after inoculation on the Mains and Jackson (1926) scale. Infection types 0, 0;, 1, 2, were classed as resistant and 3, 4, as susceptibe.

Postulation of resistance genotypes
The pathotypes 277-1, 277-4, 277-10, 252-3, 261-5, 265-5, 261-6, 317-6 with different virulence combinations were used to determine the adult-plant resistance genotype of the test cultivars and lines of wheat. The virulence of the pathotypes race type are shown in Table 1.

The presence of adult plant resistance genes were tested for in foreign cultivars from the Vavilov Institute of the Plant Industry, and from hybrids of wheat with alien genes. Presence or absence of specific genes for leaf rust resistance were deduced for each cultivar - isolate combination by comparison with the adult-plant resistance of Lr lines and the same isolates.

Latent period
The latent period was evaluated comparing tested cultivars with susceptible wheat control. The suscepti-ble check was cv. Khakasskaya. Mean latent period was determined in days from the time of inoculation until 50% uredinia have erupted.

Results and Discussion

For identification of adult plant resistance genotype it is necessary to have a set the test pathotypes of the pathogen with appropriate genes for virulence to corresponding isogenic lines cv. Thatcher. For this purpose the collection pathotypes P. triticina from various geographical populations was utilised and tested on isogenic wheat lines with adult plant resistance genes. As a result of these experiments effective adult plant resistance genes were identified in the test materia: Lr 22a, Lr 35, Lr 37. From a large number of P. triticina pathotypes collected, eight were selected and used in the tests and adult plant resistnce genes present were postulated in the wheat cultivars and lines with alien genes (Table 2).

Four adult plant resistance genes and their combinations were detected in the wheat genotypes tested. Gene Lr 12 was revealed in genotypes Rodina and 119/00i, Lr 34 - in 39/02i and 140/97i, and Lr 37 in 120/00i. Combination of two genes Lr 13+Lr 37 were found in Frontana and Lr 12+Lr 34 in 142/97i. Combination of three genes Lr 12+Lr 37+Lr 34 were revealed in the line 142/02i together with alien genes. Known and unknown genes were postulated in lines 142/01i (Lr13++), 422/02i (Lr 34++) and 425/02i (Lr 12++).

Cultivars General Urquiza and Timgalen were susceptibility to all pathotypes and therefore have no postulated adult plant resistance. Other hybrids showed signs of adult resistance probably from alien genes from Ae. speltoides. The cultivars with the effective gene Lr 37 in the combinations Lr 34+Lr 37, Lr 12+Lr 34+Lr 37 are the most interesting for further research.

Analysis of variance showed significant influence of many factors the on duration of the latent period. The pathotypes differed significantly by Fisher's F-criterion (P<0.01) *. The average latent period of pathotypes 261-5 was shorter than for pathotypes 277-10 or 317-6. There were no differences between the latent periods for pathotypes 277-10 and 317-6. There were significant differences in the latent period of some cultivars (P<0.01). Cultivar Khakasskaya had the shortest average latent period (10 day) and cv. Exchange had the longest (14 day). Interactions between cultivars and isolates were also was statistically significant (P<0.05).

Three groups of types of interaction of common wheat and leaf rust pathotypes were revealed:
1. Cultivars susceptible to all pathotypes and latent period of 12 days (General Urquiza and Frassinetto, Table 3).
2. Cultivars resistance to some pathotypes and having the longest latent period (Frontana and Kundan). These cultivars combine race-specific and partial resistance.
3. Cultivars susceptible to all pathotypes and having a longer latent periods to most part of them (Albatros odesskiy, Da-Lu-Us-Huan and Chanute).
Cultivars combining partial and race-specific resistance are more useful for breeding.

Thus, specific interaction between resistance of cultivars and aggressiveness of pathotypes P. triticina was revealed. Results of researches have shown that this type of resistance is race-specific. In this connection for the selection of the cultivars keeping durable resistance it is necessary to use a wide spectrum of the races described high virulence and aggressiveness.

Acknowledgemets

The authors are gratefully the financial support from International Science and Technology Center. The presented research was supported by project 1721 ISTC.

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