Leaf miner(Liriomyza spp.) is a major insect pest of many important vegetable crops, including spinach (Spinacia oleracea). Chemical control is not long lasting, and it is well documented that leafminers can develop a high degree of resistance to insecticides. Resistant varieties remain the most economical means of insect control. The purposes of the present experiments were to evaluate differences in spinach genotypes to leafminer damage, to compare results obtained from insect cages and from the field, and to study the association among different resistant traits. We screened 345 spinach genotypes from the USDA germplasm collection and 441 genotypes from CGN (Holland) and IPK (Germany) spinach collections for leafminer resistance in an outdoor insect cage and in the field. Significant genotypic differences were found for leafminer stings per unit leaf area, mines per plant, and mines per 100 g of plant weight. The sting result from the field was highly correlated (r = 0.770) with the result from the insect cage, demonstrating that a cage test could be used to screen for leafminer resistance in the field. Mines per plant were not correlated with plant weight, suggesting that leafminer flies did not lay their eggs randomly and oviposition-nonpreference occurred in these plants. Stings per unit leaf area was not correlated with mines per plant or per 100 g plant weight, which suggests that feeding-nonpreference does not necessarily mean oviposition-nonpreference for a spinach genotype and these two traits can be improved independently. These findings suggest that genetic improvement of spinach for leafminer resistance is feasible. A phenotypic recurrent selection method is used to increase the level of leafminer resistance in spinach.
The entire USDA spinach (Spinacia oleracea) germplasm collection (338 accessions) and 11 commercial cultivars were screened for oxalate concentration. There were significant differences in oxalate concentration among the genotypes evaluated, ranging from 5.3% to 11.6% on a dry weight basis. The low-oxalate genotypes identified in our experiments are all S. oleracea. None of the two S. tetrandra and four S. turkestanica accessions screened had low levels of oxalate. Two accessions from Syria, PI 445782 (cultivar name Shami) and PI 445784 (cultivar name Baladi), consistently had low oxalate concentration. When expressed on a fresh weight basis, oxalate concentration may be affected by the moisture content of the plant. Oxalate concentration had little correlation with leaf types (flat or savoy) and leaf weight per plant. With the genetic variation and sources of low oxalate concentration found, breeding of spinach for a low level of oxalate seems feasible.
Leafminer (Liriomyza spp.) is a major insect pest of many important agricultural crops, including spinach (Spinacia oleracea). Genetic variability in leafminer resistance has not been studied for spinach. The purposes of the present experiments were to evaluate differences in leafminer damage among spinach genotypes, to compare results obtained from insect cage and field experiments, and to study the association among different resistant traits. We screened 345 accessions of the U.S. spinach collection for leafminer resistance in an outdoor insect cage and putative resistant genotypes were further tested in the cage and in the field over 2 years. Although no genotype was immune to leafminers, significant genotypic differences were found for leafminer stings per unit leaf area, mines per plant, and mines per 100 g plant weight. PI 274065 had the lowest sting density, whereas PI 174385 showed the fewest mines per unit plant weight among genotypes in the field. Rank order of stings per square centimeter leaf area did not significantly change for the genotypes in the cage and field tests, and the sting results from different tests were also highly correlated, suggesting that a cage test could be used to screen germplasm for fewer leafminer stings, and sting density is a reliable trait for the selection of leafminer feeding nonpreference. Stings per unit leaf area were not correlated with mines per plant or per 100 g plant weight, which suggests that feeding nonpreference does not necessarily mean oviposition–nonpreference for a spinach genotype and these two traits can be improved independently. Stings per square centimeter leaf and mines per 100 g plant weight had little correlation with plant weight in cage and field tests, suggesting that leafminer sting and mine densities are not associated with plant biomass, and it is possible to improve and combine the leafminer resistance and yield traits in a spinach cultivar. From these findings, the genetic improvement of spinach for leafminer resistance seems feasible.