Alternaria alternata is an important fungal pathogen causing leaf spot and fruit rot diseases in a variety of crops, including sweet cherry. The development of resistance to fungicides, especially quinone outside inhibitor (QoI) fungicides, has emerged as a significant concern in managing cherry diseases. In this study, 109 A. alternata isolates collected from four geographic regions were screened for sensitivity to QoI fungicide azoxystrobin and other fungicides. Results showed that 74 isolates (67.89%) were highly resistant to azoxystrobin. All isolates were highly resistant to MBC fungicide carbendazim, while resistance to DMI fungicide hexaconazole occurred at a lower frequency, and all isolates were sensitive to SDHI fungicide boscalid. For the tested isolates, positive cross-resistance was observed between azoxystrobin and pyraclostrobin, whereas no correlation in sensitivity was detected between azoxystrobin and other classes of fungicides. No significant differences were observed between azoxystrobin sensitive and resistant isolates in mycelial growth, sporulation and oxidative stress tolerance. However, resistant isolates showed significantly lower inhibition to osmotic stress caused by NaCl. The detached cherry fruit assay showed that azoxystrobin was highly effective against sensitive isolates but had greatly reduced control efficacy against resistant isolates. Nevertheless, the SDHI fungicide boscalid, effectively inhibited growth in both azoxystrobin-resistant and sensitive isolates. RT-qPCR analysis showed no significant difference in AaCyt b expression between sensitive and resistant isolates. Sequencing of the AaCyt b gene confirmed the presence of the G143A mutation in resistant isolates. These results demonstrate that this mutation confers high resistance with limited fitness penalties, and does not confer cross resistance to DMI, MBC, or SDHI fungicides. Finally, SDHI fungicides remain the best option to control A. alternata.
Hussain et al. (Thu,) studied this question.