Novel diversity and virulence patterns found in new isolates of Cydia pomonella granulovirus from China

Cydia pomonella granulovirus (CpGV) is successfully used worldwide as a biocontrol agent of the codling moth (CM) (Cydia pomonella). The occurrence of CM populations with different modes of resistance against commercial CpGV preparations in Europe, as well as the invasiveness of CM in China, threatening major apple production areas there, requires the development of new control options. Utilizing the naturally occurring genetic diversity of CpGV can improve such control strategies. Here, we report the identification of seven new CpGV isolates that were collected from infected CM larvae in northwest China. Resistance testing using a discriminating CpGV concentration and the determination of the median lethal concentration (LC₅₀) were performed to characterize their levels of virulence against susceptible and resistant CM larvae. The isolates were further screened for the presence of the 2 × 12-bp-repeat insertion in CpGV gene pe38 (open reading frame 24 [ORF24]), which was shown to be the target of type I resistance. It was found that three isolates, CpGV-JQ,-KS1, and-ZY2, could break type I resistance, although delayed mortality was observed in the infection process. All isolates followed the pe38 model of breaking type I resistance, except for CpGV-WW, which harbored the genetic factor but failed to overcome type I resistance. However, CpGV-WW was able to overcome type II and type III resistance. The bioassay results and sequencing data of pe38 support previous findings that pe38 is the major target for type I resistance. The new isolates show some distinct virulence characteristics when infection of different CM strains is considered. CpGV is a highly virulent pathogen of the codling moth (CM). It is registered and widely applied as a biocontrol agent in nearly all apple-growing countries worldwide. The emergence of CpGV resistance and the increasing lack of chemical control options require improvements to current control strategies. Natural CpGV isolates, as well as resistance-breaking isolates selected in resistant CM strains, have provided resources for improved resistance-breaking CpGV products. Here, we report novel CpGV isolates collected in China, which have new resistance-breaking capacities and may be an important asset for future application in the biological control of codling moths.