Plant samples (leaves, branches, and roots) were collected from mandarin rootstocks and trees. Seven mandarin orchards of two main different agricultural practices, four conventional mandarin farms (CF; high chemical fertilizers input with synthetic pesticides application, mainly organophosphate and carbamate groups) and three sustainable mandarin farms (SF; minimum chemical fertilizer, optimum organic fertilizer input with natural control of pests), were selected. The sites were located in the main mandarin growing areas: Fang and Mae-ai District, Chiang Mai Province. From each site, three to five composite samples of leaves, young branches, and roots randomly collected from healthy plants were obtained. The root samples were mainly collected from the mandarin rootstocks that were produced by each farm. The method of Gordon and Weber (1951) was modified and used to determine IAA production.Representative actinomycetes isolates from each genus were cultured in nutrient broth (NB) containing tryptophan (2.0 mg/mL) and incubated with shaking (120 rpm) at room temperature (28-30 ℃) for 7 d. The culture broth was filtrated (Whatman No. 5), and 1 mL of the culture filtrate was mixed with 2 mL of Solawaski's reagent [50 mL 35% perchloric acid; 1 mL 0.5 iron (III) chloride (FeCl₃)] and incubated in the dark for 30 min; development of a pink color indicates IAA production. The absorbance was measured at 530 nm using a spectrophotometer (Hitachi U-1000). The concentration of IAA produced was calculated using a standard IAA curve.