Tolerance of Selected Onion Genotypes and Their Polycrosses to Onion Thrips (Thrips tabaci Lind.) in Isfahan

Onion thrips (Thrips tabaci Lind.) is one of the important factors which indirectly restrict the proper yield of onion (Allium cepa L.). Therefore, finding tolerant cultivars to onion thrips is important. Because of many small florets, cross pollination in onion which is a biennial plant by using polycross method for breeding is suitable and easy. To produce polycross seed, bulbs of 9 self pollinated onion genotypes were planted in the shape of equilibrium lattice design for 9 treatments in May 2002. To study tolerance to onion thrips and assess other agronomical characteristics, the polycross seeds produced from free pollination and self pollinated plant seeds (totally 18 genotypes) were planted in a randomized block design with four replications. The main plots were sprayed and not sprayed, and sub-plots were onion genotypes. From August to October 2003, samples were taken from each plot bi-weekly, and number of thrips, and percentage of injury on plants were recorded. As a result of this study, Azarshahr self pollinated genotype showed maximum population density of thrips (22.24) and injury, but Ghom and Abarkooh genotypes showed the lowest amount (2.65 and 5.68 respectively) among all genotypes. Among pollycross genotypes, Ghom and Kashan genotypes showed low population densities (4.3 and 4.24 respectively) and injury, but Azarshahr and Tarom showed high population densities (20.66 and 19.84 respectively) and percentage of injury. Generally, polycross reduced population density of thrips and percentage of injury.