Commercial fruit crops and cut flower cultivations are widespread in Northern Thailand. Severe root rot disease onset has been observed since 2020, in commercial crops viz. citrus, durian, orchids, passion fruit, etc. samples received at the Chiang Mai University plant clinic. Further collections and investigations were done at the original farm and greenhouse sites. The laboratory observations showed the majority of these root rot disease symptoms were similar to Pythiaceae root rot symptoms. The symptomatic plant parts, the adjacent soil, and the growth material were used for the Oomycete isolation through baiting techniques. To verify the fungal isolate, morphological studies combined with phylogeny with the internal transcribed spacers (ITS) and mitochondrial gene Cytochrome c oxidase subunit I (COX1) were conducted. The gene regions were amplified with primer pairs ITS5/ITS4 and OomCoxI- Levup/ Fm85mod respectively. A pathogenicity assay was performed to fulfill Koch’s postulates to prove the pathogenicity of each isolate. The current study shows a high diversity of Pythiaceae associated with commercial crops in Northern Thailand and the species can be concerned as immerging pathogens. The plant clinic at the Department of Entomology and Plant Pathology of Chiang Mai University is an extension service providing efficient plant disease detection through modern techniques and disease control recommendations for the farmers and related organizations. Further, this service provides plant pathology students a platform to practice plant disease detection and get involved in extension services.