Improvement of the propamocarb-tolerance of Lecanicillium lecanii through UV-light radiation-based mutagenesis

Lecanicillium lecanii is an important entomopathogenic fungus which is used as a biopesticide for control of sucking insects. However, low fungicide-tolerance limits its application in the field. To improve the propamocarb-tolerance of L. lecanii, an optimized UV-light radiation-based mutagenesis system was established. The radiation time was optimized to 60 s for L. lecanii in the first round of UV-light radiation when a 20 Watt germicidal lamp was placed at a distance of 45 cm from plates. Both the positive mutation rate and EC50 value of the UV-mutant were the highest when using this radiation time. The tolerance ratio (TR) was always lower than 1.13 in the first round of UV-light radiation. To further increase fungal propamocarb-tolerance, the highest propamocarb-tolerant mutant in the first round of UV-light radiation was used as a starting strain for multiple rounds of UV-light radiation in which a same germicidal lamp and radiation parameter were applied as described in the first round of UV-light radiation. TR was largely accumulated to 2.13 on the fourth round of radiation, and then began to level off and remained constant. A higher propamocarb-tolerant mutant was obtained in the sixth-round UV-light radiation, whose median effective concentration (EC50) value was increased 2.15-fold compared to the wild-type strain, and it was mitotically stable in 20 passages on PDAY medium. This mutant did not show any significant changes of conidial yield on plates and virulence on aphids, although its colony growth was significantly lower than that of the wild-type strain. After the application of propamocarb at a recommended field dose of 550 μg/ml, the interval time of using L. lecanii was significantly shortened in this mutant compared to the wild-type strain. The results indicated that this improved strain could be potentially applied in combination with propamocarb in the field.