Thailand is the leading producer and exporter of durians worldwide. Serious diseases in durians include root rot, stem rot, and fruit rot, which are caused by Phytophthora palmivora, P. nicotianae, and Pythium cucurbitacearum, respectively. Thai farmers have applied fungicides for more than 20 years to control rot, but it remains difficult to control. Thus, the monitoring of fungicide-resistance development in pathogens is important for disease management. Pathogens were isolated from naturally infected durians between 2016 and 2017 in southern Thailand. The sequences of the internal transcribed spacer (ITS) and 5.8S regions of rDNA were used for the identification of their species. Seventeen out of twenty isolates were confirmed to be P. palmivora. All the isolates were tested for mycelium-growth sensitivity to metalaxyl, azoxystrobin, and dimethomorph. The results showed that nine isolates were resistant to metalaxyl with the 50% effective concentration (EC₅₀) higher than 100 mg L⁻¹. By contrast, all the isolates were sensitive to both azoxystrobin and dimethomorph, with EC₅₀ < 1 mg L⁻¹. Metalaxyl-resistant isolates were not controlled (−25.6% to 22.2%) by the treatment of the detached leaves of ‘Monthong’ durian with 100 mg L⁻¹ metalaxyl prior to inoculation, but all the metalaxyl-sensitive and moderately metalaxyl-resistant isolates were better controlled (33.0% to 62.6%). These results clearly indicate that metalaxyl-resistant strains are present in the populations of P. palmivora in 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^RAzo^R, 34.1% were triple resistant (Car^RAzo^RDif^R or Car^RAzo^RPen^R), and 50.6% were Car^RAzo^RDif^RPen^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 β-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 14α-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.

Metalaxyl-M (Met-M) and dimethomorph (Dim) are two systemic fungicides commonly used in Thailand to treat durian rot diseases caused by Phytophthora palmivora, a pathogen that significantly affects durian production. Fungicides have long been used by Thai durian farmers to control this infection, but managing the disease has become increasingly challenging in recent years. Monitoring fungicide resistance evolution is essential for effective disease management. In 2020 and 2022, eighty-one and sixty isolates, respectively, were collected from naturally infected durians in southern and eastern Thailand. All isolates were identified as P. palmivora through sporangium formation. Furthermore, 23 out of the 141 isolates were confirmed as P. palmivora based on sequencing of the internal transcribed spacer (ITS) and 5.8S regions of rDNA. All isolates were tested for mycelium growth sensitivity to Met-M and Dim. EC50 values were used to classify sensitivity into three categories: sensitive (S) at EC50 < 1 mgL-1, moderately resistant (MR) at EC50 between 1 and 100 mgL-1, and resistant (R) at EC50 > 100 mgL-1. In southern Thailand, 58% of the isolates were Met-MRDimS, and 41.25% were Met-MSDimS. In eastern Thailand, 36.67% were Met-MMRDimS, and 41.67% were Met-MSDimS. These findings indicate that the populations of P. palmivora in the durian orchards in southern and eastern Thailand consisted of a mixture of sensitive, moderately resistant, and resistant strains.