AbstractDue to the non‐availability of sources of resistance in Sri Lanka toPhytopbthorablight in sesame (Sesamum indicum) caused byPhytopbthora nicotianaevar.parasitica, a mutation breeding programme was initiated. Seeds of three genotypes were subjected to six doses of gamma rays from 100 Gy to 750 Gy from a60Co source. Seeds of M2bulks, sampled by variety and treatment from the first five capsules formed on M1plants, were grown in a field with a history of repeated incidence of the disease. The best 21 lines, having the highest survival and seed production, identified in the screening of M3and M4progeny rows of selected M2single plants were then tested in a replicated field trial with the recommended cultivar ‘MI 3’ as a control. The plant survival in selected lines averaged 43.3 percent as against 7.2 percent in the recommended variety. Eight selections recorded significantly higher seed yield than ‘MI 3’ at P<0.01 and another three at P<0.05. Gamma ray treatments of 450 Gy and 600 Gy produced more lines tolerant to the disease than the other d
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