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In the present study, analyses of SSR molecular markers were performed to investigate the genetic diversity of 133 tuber mustard cultivars. Eighty-one pairs of SSR primers from a total of 600 in Brassica produced stable amplified bands. In addition, 810 bands were detected among the cultivars, and 724 of those were polymorphic (89.38 %). The average number of bands per locus was 10.0 with a range from 5 to 16. Shannon’s information index for each SSR locus varied from 0.52 to 3.72, with an average of 2.74. The coefficients of genetic similarity in the SSR marker patterns among the 133 cultivars ranged from 0.77 to 0.91, with an average of 0.85. The cluster analysis showed that the cultivars could be classified into six clusters when the genetic similarity was 0.83, with 90.23 % of the cultivars included in Clusters 5 and 6. Principal component analysis was carried based on the SSR data. The results showed that the first three principal components could explain the genetic variation with 85.47, 0.67, and 0.61 %, and the 133 cultivars could be divided into six clusters according to the nearest phylogenetic relationship. It was indicated that the similarity was high and the genetic diversity was narrow among the 133 mustard tuber cultivars. 360 individuals from 24 cultivars were analyzed to reveal the genetic structure and genetic diversity within cultivars. A total of 925 alleles were detected in the 24 cultivars. Estimates of the mean number of alleles ‘A’, the effective allelic number ‘Ae’, the observed heterozygosity ‘Ho’, and expected heterozygosity ‘He’ were 6.0, 3.6, 0.64, and 0.37, respectively. An obvious genetic deviation from Hardy–Weinberg expectation was observed both among and within cultivars and a considerable genetic variation was revealed within rather than among cultivars. It is necessary to broaden the genetic basis of the breeding germplasm in tuber mustard. Based on their geographical distributions, the tuber mustard cultivars in this study can be divided into up-Yangtze river, mid-Yangtze river, and down-Yangtze river groups. Genetic diversity was highest in mid-Yangtze river group, followed by up-Yangtze river group, and then down-Yangtze river group. It was presumed that the origin center or genetic diversity center of tuber mustard was mid-Yangtze river, and the crop was transmitted along the Yangtze river in both directions.

The entomopathogenic fungus L. lecanii has been developed as biopesticides and used widely for biological control of several insects in agricultural practice. Due to the lack of isolation/count methods for L. lecanii in soil, the persistence of this fungus in soil appears to have attracted no attention. A selective medium and count method for L. lecanii in soil based on cetyl trimethyl ammonium bromide (CTAB) was developed, and then the persistence and viability of this fungus in soil were investigated under field conditions between 2012 and 2014. The results showed that the rate of recovery for L. lecanii in soil on the selective CTAB medium was satisfactory. The minimum CFUs for L. lecanii on the selective medium (0.5 g/L CTAB) was about 102 conidia/g soil. The L. lecanii density in soil declined quickly in the first month after inoculation with fungal conidia, kept stable for 6 to 10 months, and then decreased gradually until undetectable. L. lecanii could persist for at least 14 months in the agricultural soil of northern China. The colony growth, conidia yield and germination rate on plates, as well as the median lethal concentration or times (LC50 or LT50) to aphids, mycelium growth in aphids and sporulation on aphids of L. lecanii did not change significantly during the persistence in soil. In general, the count method developed here was a very useful tool for monitoring the dynamics of natural or introduced L. lecanii populations in soil, and the data on the persistence of L. lecanii in soil reported here were helpful for biological control and environmental risk assessment.