Prevalence tendency and genetic laws of main pathogens of silkworm muscardine diseases in Guangxi

【Objective】To find out the incidence and prevalence laws of silkworm muscardine diseases in Guangxi, and to study the pathogenic characteristics and genetic law, so as to provide a scientific basis for precise control of silkworm muscardine disease.【Method】Based on the survey data of silkworm disease in Guangxi in the past 14 years, the incidence law was analyzed. Diseased silkworms, pests and biological control Beauveria bassiana strains from 5 sericulture areas(central Guangxi, south Guangxi, west Guangxi, northwest Guangxi and southeast Guangxi) were collected, and their pathogenicity on silkworm were tested through microscopic morphology identification and filter paper contact method after sporulation determination. PCR amplification was conducted based on ITS4/ITS5 primer, and Bidirectional sequencing results were imported into GenBank for BLAST analysis. According to the results of ISSR-PCR amplification, the electrophoresis pattern map was drawn by Quantity One, the genetic similarity coefficient was calculated by NTsys and the UPGMA cluster analysis was carried out, and the genetic diversity index of the tested materials was analyzed by PopGene32.【Result】From 2008 to 2021, the incidence of silkworm muscardine disease in Guangxi showed a fluctuating downward trend, with an average incidence of 5.19% for 14 consecutive years. May was the peak of the incidence of silkworm muscardine disease in Guangxi, with an average incidence of 13.37%, followed by April and October, with the morbidity as 7.91% and 6.87%, respectively. The incidence of silkworm stiff disease in mountainous area was generally higher than that in gentle hilly area, and the incidence of silkworm muscardine disease in gentle hilly area was ranked as follows:central Guangxi(13.55%) > northwest Guangxi(12.66%) > south Guangxi(11.56%) > west Guangxi(11.05%) > southeast Guangxi(9.53%). The incidence rate of silkworm stiff disease in mountainous silkworm area is ranked as:central Guangxi(18.91%) > northwest Guangxi(17.96%) > west Guangxi(16.01%) > south Guangxi(14.90%) > southeast Guangxi(11.61%). The pathogens of muscardine diseases in Guangxi belonged to the genera Beauveria, Aspergillus, Paecilomyces and Cordyceps, in which B. bassiana was the main pathogen, which accounted for 86.67%, and its ITS sequence was highly conserved with similarity ranging from 97.83% to 99.83%. The 72-hour pathogenicity test on silkworm showed that the cumulative mortality rate of B. bassiana(pesticide) from Chongzuo in Guangxi was 100.00%, which was significantly higher than that of the silkworm muscardine strains in the sericulture area(8.89%-93.33%). A total of 152 clear bands were amplified from 50 strains by 10 ISSR primers, of which 147 were polymorphic, and the polymorphic percentage was 87.5%-100.0%. The Nei's gene diversity index(H) of the 50 strains was 0.1740-0.3257, and the Shannon's information diversity index(I) was 0.2840-0.4880, showing extremely high genetic diversity. The UPGMA phylogenetic tree based on genetic distance showed that the isolates from southern, northwestern, southeastern and central sericulture areas of Guangxi had close genetic relationship with the isolates from pesticide.【Conclusion】May is the peak month of silkworm muscardine diseases in Guangxi and B. bassiana is the main pathogen, whose ITS sequence is highly conserved. The order of pathogenicity of B. bassiana from different sources to silkworm was pesticide source B. bassiana strain > pine forest B. bassiana strain > silkworm strain in silkworm area, and there is a very high genetic correlation among the 3 sources, suggesting that the pathogenic genetic background of silkworm muscardine disease in Guangxi is complex, and B. bassiana pesticides should be used carefully in the main sericulture areas.