Abstract

A total of 844 durum wheat genotypes and 4429 bread wheat genotypes were field evaluated under heavy inoculum pressure for resistance to stem and leaf rusts during 1996-2000. Based on the means from four crop seasons, 86% of the bread wheat lines showed resistance to stem rust, compared to only 8% of the durum wheat lines. However, 73% of durum wheat lines were resistant to leaf rust, compared to 57% of the bread wheat lines. Only 6% of the bread wheat genotypes showed intermediate to high levels of susceptibility to stem rust, compared to 73% of the durum wheat stocks. In contrast, only 9% of the durum wheat lines exhibited intermediate to high levels of susceptibility to leaf rust, compared to 25% of the bread wheat lines. Thus, the "complementary" nature of resistance manifested by the two wheat species, bread wheat to stem rust and durum wheat to leaf rust, coupled with their "differential" response observed in other studies to individual pathotypes of the two rusts, is potentially useful for the management of both the rust diseases.

Introduction

While recording observations on the field responses of wheat lines to stem and leaf rusts, it was noticed that bread wheats were generally more resistant to stem rust, and durum wheats to leaf rust. Hence, the field observations recorded during 1996-2000 at IARI-Regional Station, Indore of a large number of durum wheat and bread wheat genotypes, were analysed to quantify the relative levels of resistance currently existing in durum wheat and bread wheat gene pools to stem rust and leaf rust diseases.

Materials and Methods

A total of 211, 232, 216 and 185 durum wheat lines; and 1151, 1138, 1001 and 1139 bread wheat lines were evaluated during 1999-2000, 1998-99, 1997-98 and 1996-97 crop seasons, respectively. Seed was hand-drilled in 1.0 m rows, with 23 cm row-to-row spacing. Check-cum-disease spreader rows consisting of mixtures of highly susceptible wheats were planted after every 20 test rows. The screening block was surrounded on all sides by two lines of disease spreader rows. Local standard recommended agronomic practices were adopted for raising the crop. Heavy inoculum pressure was created with the help of hypodermic syringe inoculations and spore suspension sprays, using 11-13 pathotypes of stem rust and 17-20 pathotypes of leaf rust during each season, including all the important Indian pathotypes of the two rusts. Field responses of the test lines to the two rusts were recorded, combining disease severity as per the modified Cobb's scale (Peterson et al., 1948) with the reaction (infection type). The wheat lines studied were grouped into various 'reaction classes' and sub-classes, based on the infection types and the disease severity, as per the details given in the foot notes under Table 1.

Results and Discussion

During all the four crop seasons, a far greater percentage of bread wheat lines were resistant to stem rust, compared to durum wheat lines (Table 1). In contrast, durum wheats consistently showed better resistance to leaf rust than the bread wheats (Table 2). Stem rust resistance of bread wheats can be attributed mainly to the two resistance genes, Sr2 and Sr31, frequently present in the Indian bread wheats, either singly or generally in combinations with other resistance genes (Directorate of Wheat Research 1999, Nayar et al., 1994). While Sr2 is an adult-plant resistance gene (effect of the gene best expressed in post-seedling stages) that has contributed to the durability of stem rust resistance worldwide (McIntosh et al., 1995), Sr31 provides over-all resistance (resistance functioning throughout the plant life including the seedlings) which is effective against stem rust populations in most parts of the world including India (Bhardwaj et al., 1999). The stem rust resistance base of Indian durum wheats is narrow, comprising mostly of Sr9e and few other resistance genes like Sr11 or Sr7b (Directorate of Wheat Research 1999). None of these genes is effective against all the Indian stem rust pathotypes.

Durum wheats have shown wide spectrum of effectiveness against Indian leaf rust populations, particularly to the widely prevalent and virulent race 77-pathotypes (Mishra et al., 2001a, Mishra et al., 2001b, Honrao and Rao 1996, Nayar et al., 1996, Sharma et al., 1996, Pandey and Rao 1984). In contrast, Indian bread wheats are generally susceptible to leaf rust race 77-pathotypes, particularly to 77-5 (Mishra et al., 2001a, Mishra et al., 2001b, Sharma et al., 1996). Leaf rust resistance genes Lr23 and Lr26 are common in Indian bread wheat lines (Directorate of Wheat Research 1999, Sawhney et al., 1998, Nayar et al., 1994). Extensive use of these genes led to the evolution of several pathotypes that carry virulence for these genes. At present, there are eight leaf rust pathotypes that possess virulence for Lr26 and nine that possess virulence for Lr23, with different avirulence/virulence gene combinations (Sawhney et al., 1998). Combined virulence for Lr23+Lr26 increased from 8.3% in 1992 to 93.6% in 1996 (Sawhney et al., 1998).

Thus, it is evident that the levels of resistance to stem rust and to leaf rust are relatively low in the Indian durum wheat and bread wheat gene pools, respectively. Long-term evaluation of world collections at the Vavilov All-Russian Institute of Plant Industry, Uzbekistan, also showed that 76.3% of the durum wheat accessions were resistant to leaf rust, compared to 53.3% of the bread wheat stocks (Udachin, 1998).

Moreover, almost 50% of the durum lines exhibited `MR' reaction to leaf rust, compared to about 24% of the bread wheat lines. However, 14% of the durum lines remained free from leaf rust infection, while 24% of the bread wheat lines did not show any signs of leaf rust infection (Table 2). These observations suggest differences between the genetic architecture of leaf rust resistance in durum and bread wheats. A number of studies in different parts of the world have strongly indicated that the leaf rust resistance functioning in durum wheat is unique, and is different from its counterpart in bread wheat. Generally, durums show resistance to the bread wheat-virulent leaf rust pathotypes, while bread wheats to the leaf rust cultures which are relatively more virulent on durums (Mishra et al., 2001a, Mishra et al., 2001b, Mishra 1996, Singh et al., 1992, Zhang and Knott 1990, Mishra et al., 1989, Sharma et al., 1986, Casulli et al., 1983). Furthermore, differences in response of the two kinds of wheat to the Indian pathotypes of stem rust have also recently been observed (Mishra et al., 2001b). Hence, the "complementary" nature of resistance manifested by the two wheat species, durum wheat to leaf rust, and bread wheat to stem rust, and their "differential" response to individual pathotypes of the two rusts, are potentially useful for the management of both the rust diseases. Breeding strategies can be planned to transfer leaf rust resistance from durum wheats to bread wheats, and stem rust resistance from bread wheats to durum wheats. Another practical approach that can immediately be adopted is to encourage the simultaneous cultivation of durum and bread wheats, which can help in keeping the inoculum levels of both stem and leaf rusts under check, thus, minimising the chances of the two rusts' assuming any epidemic proportions. Cultivation of durum wheat varieties in central India contributed in arresting the build-up of leaf rust epidemic during 1997-98, despite an early outbreak of leaf rust in the region and prolonged favourable weather conditions for disease development (Mishra et al., 2001c).

Acknowledgements

Receipt of the nucleus inoculum of stem and leaf rust pathotypes used in the study from the Head, Directorate of Wheat Research, Regional Station, Flowerdale, Simla, India, is gratefully acknowledged. We thank Mr Jagdish for helping with the rust inoculations.

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