Isolates of Colletotrichum truncatum with Resistance to Multiple Fungicides from Soybean in Northern Thailand

In Thailand, four systemic fungicides-carbendazim (Car), azoxystrobin (Azo), difenoconazole (Dif), and penthiopyrad (Pen)-are commonly used to control soybean anthracnose caused by Colletotrichum truncatum; however, the pathogen has developed resistance. From 2019 to 2020, fungicide resistance in C. truncatum from fields in Chiang Rai and Chiang Mai was monitored. In tests of 85 C. truncatum isolates for resistance to multiple fungicides, 15.3% were Car(R)Azo(R), 34.1% were triple resistant (Car(R)Azo(R)Dif(R) or Car(R)Azo(R)Pen(R)), and 50.6% were Car(R)Azo(R)Dif(R)Pen(R). Surprisingly, all isolates tested had lost their sensitivity to one or more of the fungicides tested. The carbendazim-resistant isolates carried a point mutation in the beta-tubulin gene at codon 198 (E198A) or 200 (F200Y), and all azoxystrobin-resistant isolates had a mutation in the cytochrome b gene at codon 143 (G143A) or 129 (F129L). Moreover, a novel mutation at codon 208 (S208Y) in the gene encoding succinate dehydrogenase subunit B was detected in all of the isolates highly resistant to penthiopyrad. No mutation linked with difenoconazole resistance was detected in the genes encoding cytochrome P450 sterol 14 alpha-demethylase. To the best of our knowledge, this is the first report of C. truncatum isolates resistant to multiple fungicides and serves as a warning to take measures to prevent the occurrence and distribution of these multiple-fungicide-resistant populations in soybean fields.

Automated summary

The soybean anthracnose-causing pathogen, Colletotrichum truncatum, in Thailand has developed resistance to multiple fungicides commonly used for its control. The study monitored fungicide resistance in C. truncatum isolates from fields in Chiang Rai and Chiang Mai from 2019 to 2020. Results showed that a significant percentage of isolates exhibited resistance to carbendazim (Car), azoxystrobin (Azo), difenoconazole (Dif), and penthiopyrad (Pen). The resistant isolates carried specific mutations in target genes. This study serves as a warning about the occurrence and distribution of multiple-fungicide-resistant populations of C. truncatum in soybean fields.