Cassava (Manihot esculenta Crantz) black stem and root rot (BSRR) caused by complex fungi is one of the
most serious fungal disease of cassava in Thailand. The objective of this study was to identify the causal agent
of BSRR disease of cassava in Nakhon Ratchasima province, Thailand. From June 2015 to May 2018, the
outbreak areas in three districts of Nakhon Ratchasima province were surveyed for cassava plants with the
characteristic symptoms. From 1800 diseased samples, 139 fungal pathogens were isolated and separated by
morphological traits. Among them, 33 fungal isolates were subsequently tested for pathogenicity on detached
stems and roots of CMR 43-08-89, a susceptible cassava cultivar, and proved to be pathogenic with different
levels of aggressiveness. Next, PCR amplification of DNA of eight pathogenic isolates with high
aggressiveness using two sets of universal primers ITS1/ITS4 and Ef1-688F/Ef1-1251R, revealed that the
TEF1-α gene region could be used for the identification and classification at the species level. The analysis
results fit well with that of the morphological studies on growth and colonial characteristics of the eight
isolates. Based on the GenBank database, they were identified as Lasiodiplodia theobromae, L.
euphorbiaceicola and Neoscytalidium dimidiatum.