Brown Root Rot Disease (BRRD) is one of the most devastating urban tree diseases in tropical and subtropical areas, including Hong Kong. It can result in tree death in a few months and is difficult to detect in the early stages of development. Fungal isolation and PCR methods are currently the most widely adopted methods to diagnose the disease. However, they are both time and technically demanding. Loop-mediated isothermal amplification (LAMP) is a superior molecular-based diagnostic method with great specificity, accessibility, and effectivity. In this study, 15 BRRD-positive and 15 BRRD-negative trees were sampled from 19 roadside slopes in Hong Kong from the end of 2020 to the middle of 2021. The wood tissues were isolated and cultivated in PN3 and PDA agars for the disease diagnosis. The mycelium samples in PDA were directly conducted in LAMP kits (mLAMP) to substitute the purified DNA materials. Wood tissues were also used in LAMP kits (wLAMP) as impurified and highly contaminated samples. The results of mLAMP and wLAMP were compared with the results of isolation to evaluate the specificity and sensitivity of LAMP method. The results showed that mLAMP had 100% sensitivity and 73.3% specificity. For wLAMP, both the sensitivity and specificity were 73.3%. For symptomless trees, 85.7% and 64.3% congruencies were found in mLAMP and wLAMP, respectively. Based on the results of this study, the co-application of LAMP in the current tree management work was also discussed. We envisaged LAMP is a sensitive, prompt, and user-friendly method to diagnose BRRD and it could favor the BRRD diagnosis in fields by accelerating and promoting large-scale screening.