Fungicide Resistance in Ascochyta Rabiei in the Pacific Northwest

Ascochyta blight, caused by the fungal pathogen Ascochyta rabiei, is a major disease of chickpea (Cicer arietinum L) in the Pacific Northwest and worldwide. Management of this disease relies heavily on fungicide applications, namely demethylation inhibitors (DMI), succinate dehydrogenase inhibitors (SDHI) and quinone outside inhibitors (QoI). However, repeated use of fungicides over time can select for resistant pathogen populations, diminishing the efficacy of these fungicides and threatening disease control. This study evaluated the sensitivity of 171 A. rabiei isolates collected from symptomatic chickpeas from Washington, Idaho, Oregon and California to the QoI fungicides azoxystrobin (AZO) and pyraclostrobin (PYR). Isolates included both historic collections from 1984 to 1996 (prior to widespread use of systemic fungicide) and contemporary collections from 2020 and 2022. In vitro conidial germination assays were conducted on a subset of 35 and 32 A. rabiei isolates for AZO and PYR, respectively, representing both historic and contemporary collection years. Results showed markedly reduced sensitivity among contemporary isolates, with ECsubscript 50 values (the effective fungicide concentration required to inhibit fungal growth or germination by 50%) for AZO and PYR, indicating a 331-fold and 978-fold decrease in sensitivity, respectively, compared to historic baseline isolates. A discriminatory dose assay using 1 μg/ml of the two fungicides was then applied to assess 127 and 92 A. rabiei isolates for sensitivity to AZO and PYR, respectively. Isolates with >60% conidial germination at this dose were classified as resistant, while isolates with 60% conidial germination, indicating widespread QoI resistance. Greenhouse trials confirmed that resistant isolates caused significantly more Ascochyta blight on AZO-treated chickpea plants, with even the highest concentration of AZO (100 μg/ml) proving ineffective at controlling disease caused by QoI resistant isolates. Molecular assays revealed that all resistant A. rabiei isolates carried the G143A mutation in the cytochrome b gene, which is known to confer total QoI resistance to fungal pathogens, further validating the QoI fungicide resistance among the isolates. In contrast, mycelial growth inhibition assays with the SDHI fungicide fluxapyroxad (FLU) and the DMI fungicide mefentrifluconazole (MEF) on a subset of 60 isolates from contemporary and historic collections showed a narrow range of low ECsubscript 50 values (FLU: 0.0092 – 0.0795 μg/ml; MEF: 0.0115 – 0.0913 μg/ml) and no significant decreases in sensitivity between historic and contemporary collections. These results suggest that while the two tested QoI fungicides may no longer be effective against A. rabiei in the PNW, SDHI and DMI fungicides remain viable options for disease management.