Identification of cassava black stem and root rot agents in Thailand

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-alpha 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.