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Varieties resistant to biotic stresses
At the time of the introduction of the exotic high yielding varieties, pest problem was not as serious as it assumed subsequent to their advent. As against three insect pests viz stem borer, gallmidge and earbug and two diseases viz blast and brownspot affecting rice, today the number has increased to 13 diseases and as many insect pests and their presence in varied pathotype/biotype forms with higher virulence and viruliference. Among the insect pests, stemborer, gallmidge and brown planthopper (BPH) and among diseases blast, bacterial leaf blight (BLB) and sheath blight still remain the major killer pests. Rice gallmidge and BPH existing in 6 and 3 biotypes while BLB in atleast 2-3 pathotypes and blast in many racial complexes make the pest problem all the more complex and not easily manageable in rice, warranting development of varieties of multiple resistance using pest and biotype/pathotype specific resistance gene sources, fortunately available in abundance in nature. The need for aggressive breeding research to insulate varieties under development with desired level of genetic resistance then, therefore, assumed importance and urgency during the second decade. Rice germplasm rich in resistance gene sources became handy to breeders to access them and use adopting different gene deployment strategies. Identification of resistance source in W 1263, a derivative of the landrace Eswarakora and its use in cross breeding resulted in the first ever gallmidge resistant variety Phalguna, which is a breakthrough in resistance breeding against insect pests. Incidentally, location specific expression of resistance to the pest observed for the first time led to the development of the biotype concept in insect pests. Over the years as many as six biotypes, distinctly differing in their viruliference have been found to exist in the country itself. Identification of biotype specific resistance gene sources and their deployment variedly had led to evolution of varieties having specific and broad spectrum resistance. The concept of biotype while giving a new direction to resistance breeding against gallmidge, proved a forerunner to extension of the concept to other rice insect pests, especially hoppers. As a result, over 55% of the high yielding varieties planted today are resistant to one or more pests In resistance breeding, emphasis has been largely for managing the pests that continue to appear in one form or the other causing serious crop losses over large areas. In particular, breeding efforts has therefore been, for development of varieties with broad spectrum resistance against multiform pests like BPH, BLB, gallmidge and blast adopting gene pyramiding strategy. As a result, compared to the varieties of the 1970s and 1980s, many of the present day varieties are combining resistance not only for more than one pest but also for more than one biotype/pathotype/race of pests. Multiple resistant varieties quite popular till date include India bred Vikramarya, Birupa, Krishna, Tara, MD 43, Khavveli, Surendra, Udaya and IRRI bred IR 60, IR 64, IR 70 etc. Inspite of such successful genetic interventions coupled with integrated pest management (IPM) strategies placing emphasis on no or need-based use of chemical pesticides, still one pest or the other appearing on epidemic scale causing high yield losses suggests evolution of more and more virulent/viruliferous strains of the major pests a continuous process. This is particularly true in the case of BPH and BLB, which have been reported in the recent years to have appeared in endemic scale in certain pockets. Recognizing that by stacking genes of resistance to pathotypes/biotypes critical to the country and regions within, broad spectrum resistance could be achieved against them, efforts made with the aid of molecular markers have enabled insulation of popular varieties like Samba Mahsuri and Pusa Basmati 1 selectively with two different resistance genes (Xa21, xa5/xa13). Thus, there is enormous scope of selective improvement of varieties otherwise popular and important with desired level of resistance.