Inheritance of resistance to spot blotch caused by Bipolaris sorokiniana in spring wheat A. K. Joshi 1 , S. Kumar 1 , R. Chand 2 and G. Ortiz-Ferrara 3 1 Department of Genetics and Plant Breeding, Institute of Agricultural Sciences, Banaras Hindu University, Varanasi 221 005, India, E-mail: joshi_vns@yahoo.co.in; 2 Department of Mycology and Plant Pathology, Institute of Agricultural Sciences, Banaras Hindu University, Varanasi 221 005, India; 3 CIMMYT South Asia, PO Box 5186, Kathmandu, Nepal With 3 figures and 6 tables Received October 10, 2002/Accepted October 20, 2003 Communicated by G. Fischbeck Abstract Three F 1 progenies and their families in the segregating generations (F 3 ,F 4 ,F 5 and F 6 ), obtained after crossing resistant · susceptible wheat genotypes were studied in the field to determine the genetics of resistance to spot blotch caused by Bipolaris sorokiniana. Spot blotch scores in the F 1 generation showed absence of dominance. Individu- ally threshed F 2 plants were used to advance the generations. Progenies (200–250) of resistant genotypes Acc. No. 8226, Mon/Ald, Suzhoe#8 crossed with susceptible ÔSonalikaÕ were evaluated in the F 3 , F 4 ,F 5 and F 6 generations under induced epiphytotic conditions. Based on disease score distribution in individual progeny rows, F 3 progenies were grouped into four classes: homozygous resistant, homozygous susceptible, segregating resistant and segregating sus- ceptible. Resistance appeared to be under the control of three additive genes. The presence of three genes was also noted in the distribution ofF 4 and F 5 lines. In the case of F 6 progeny rows, both quantitative and qualitative models were used to estimate the number of segregating genes based on a 2-year trial. It appeared that resistance to spot blotch was controlled by the additive interaction of more than two genes, possibly only three. Key words: Triticum aestivum — Bipolaris sorokiniana —foliar blight—resistancetospotblotch—genetics—springwheat Spot blotch, caused by Bipolaris sorokiniana (Sacc.) Shoe- maker,isanimportantwheatdiseaseinthewarmerandhumid growing regions of the world (Dubin and van Ginkel 1991, Joshietal.2002).Thediseasewasreportedasearlyas1914by Mohy (Joshi et al. 1986). However, its importance has increased recently (Saari 1998, Chaurasia et al. 1999) as the production of wheat has expanded into newer growing areas. Surveys indicate that spot blotch has become a serious disease constraint for wheat cropping in several parts of the world, includingeasternIndia,Bangladesh,TaraiofNepalandBrazil (van Ginkel and Rajaram 1998). These areas are characterized by high temperature and humidity at the late Ôgrowth stageÕ. The average yield loss due to spot blotch in South Asia and India has been estimated to be 19.6 and 15.5%, respectively (Dubin and van Ginkel 1991). Yield losses between 20 and 80% have been reported by Duveiller and Gilchrist (1994) and lossesupto100%mayoccurunderthemostsevereconditions of infection (Srivastava et al. 1971, Mehta 1993). Several attempts have been made to control spot blotch but no single control measure has been successful. Hence an integrated approach with host resistance as a major compo- nent is considered necessary (Joshi et al. 2002). Even if it is incomplete, any degree of resistance will complement chemical and other control measures (Joshi and Chand 2002). It is generally accepted that spot blotch resistance of high-yielding genotypes is not yet satisfactory (Duveiller et al. 1998a). The slow progress in breeding for resistance to spot blotch (Rajaram 1988) is suggested to be due to a variety of reasons, including the quantitative nature of inheritance (Duveiller et al. 1998a), the variability of the pathogen and the aggres- siveness of B. sorokiniana which seems to increase over time (Hetzler et al. 1991). Inheritance studies on resistance to spot blotch are limited and the nature of inheritance is still debatable. Reports indicate both monogenic (Srivastava et al. 1971, Srivastava 1982, Adlakha et al. 1984) and polygenic (Velazquez Cruz 1994) types of resistance. However, the experience of wheat workers to achieve partial resistance in breeding populations (Dubin and van Ginkel 1991, Duveiller and Gilchrist 1994, Dubin and Rajaram 1996) has suggested a polygenic type of resistance.Therefore,thepresentinvestigationwasundertaken to further enhance knowledge on the genetics of resistance to spot blotch and to help breeders find an efficient breeding approach for improving resistance to this disease in wheat. Materials and Methods Genotypes and experimental design: Three genotypes of Triticum aestivum, bread wheat with high resistance to B. sorokiniana and a susceptible genotype, ÔSonalikaÕ, were used (Table 1). Single plant selections of each genotype were multiplied and used in the crosses. Parents and F 1 progenies were evaluated for resistance to spot blotch under induced epiphytotic conditions in the field at Varanasi (North Eastern Plains Zone, India, 25.2°N and 83.0°E) in the year 1996–97. Plots (ca. 25–30 plants) consisted of two 1-m rows seeded 25 cm apart with 30 cm between plots. Genotype A-9-30-1, a susceptible durum wheat(T. turgidum),wasplantedinalleysandborders,2weeksbefore sowing the experiment to enhance the spread of inoculum. For each cross, the susceptible parent and resistant parents were planted for comparison. The progenies of the three crosses were advanced to the F 3 generation after threshing individual F 2 plants. These crosses were chosen because they had been consistently the most resistant during the previous 5 years. The F 2 generation was raised in the off-season nursery, Wellington, Tamil Nadu, India, during the summer of 1997. F 3 families obtained from around 200 F 2 plants were sown as separate rows in the crop season of 1997–98. The corresponding parents were used as checks. The progenies of the three crosses were advanced to F 4 and F 5 generations following the pedigree method, where a random plant in Plant Breeding 123, 213—219 (2004) Ó 2004 Blackwell Verlag, Berlin ISSN 0179-9541 U. S. Copyright Clearance Center Code Statement: 0179–9541/2004/2303–0213 $ 15.00/0 www.blackwell-synergy.com