Distribution of Alternaria alternata isolates with resistance to quinone outside inhibitor (QoI) fungicides in Brazilian orchards of tangerines and their hybrids

Abstract

Alternaria brown spot (ABS), caused by Alternaria alternata, has been controlled with sprays of quinone outside inhibitors (QoI), demethylation inhibitors (DMI) and copper-based fungicides. QoI fungicides have been intensively used for citrus disease control in São Paulo (SP) since 2003, when ABS was first found in the state. In 2017, citrus growers from several regions in SP stated experiencing failures in ABS control with the use of QoI fungicides. This study was then proposed to assess the sensitivity of A. alternata isolates to QoI fungicides by conidial germination and resazurin microtiter assay, and also to determine the geographic distribution of sensitive and resistant isolates in areas cultivated with tangerines and their hybrids in SP. One hundred and twenty-eight A. alternata isolates were sampled in nine municipalities in different SP regions in 2017 and 2018. Isolates collected in 2003/2004, immediately after the first report of ABS in SP, were used as baseline. Effective concentrations of azoxystrobin and pyraclostrobin to reduce conidial germination by 50% (EC50) were estimated and A. alternata isolates were classified as resistant when the conidial germination rate was ≥50% at a discriminatory rate of 1 μg mL−1. Isolates resistant to QoI fungicides were widely found throughout SP regions, and no pattern in resistance distribution was observed. In five out of the nine municipalities, 100% of isolates were resistant to both fungicides, while in other four municipalities the percentage of resistant isolates to azoxystrobin (46%–100%) was higher than that of pyraclostrobin (15%–93%). The G143A mutation positively correlated with the resistance phenotype to azoxystrobin for all A. alternata isolates and to pyraclostrobin for most of them. The resazurin microtiter assay proved useful for rapid qualitative classification of sensitive and resistant A. alternata isolates. Our results suggest the need for anti-resistance strategies in the chemical management of the disease in the state.