Characterization and Pathogenicity of <i>Colletotrichum truncatum</i> Causing <i>Hylocereus undatus</i> Anthracnose through the Changes of Cell Wall-Degrading Enzymes and Components in Fruits

Anthracnose is one of the destructive diseases of pitaya that seriously affects the plant growth and fruit quality and causes significant yield and economic losses worldwide. However, information regarding the species of pathogens that cause anthracnose in pitaya (<i>Hylocereus undatus</i>) fruits in Gansu Province, China, and its pathogenic mechanism is unknown. Thus, the purposes of our present study were to identify the species of pathogens causing <i>H. undatus</i> fruits anthracnose based on the morphological and molecular characteristics and determine its pathogenic mechanism by physiological and biochemical methods. In our present study, forty-six isolates were isolated from the collected samples of diseased <i>H. undatus</i> fruits and classified as three types (named as H-1, H-2, and H-3), according to the colony and conidium morphological characteristics. The isolation frequencies of H-1, H-2, and H-3 types were 63.04%, 21.74%, and 15.22%, respectively. The representative single-spore isolate of HLGTJ-1 in H-1 type has significant pathogenicity, and finally we identified <i>Colletotrichum truncatum</i> as the pathogen based on the morphological characteristics as well as multi-locus sequence analysis. Moreover, the <i>H. undatus</i> fruits inoculated with <i>C. truncatum</i> had a significantly increased activity of cell wall-degrading enzymes (CWDEs) cellulase (Cx), β-glucosidase (β-Glu), polygalacturonase (PG), and pectin methylgalacturonase (PMG), while having a decreased level of cell wall components of original pectin and cellulose in comparison to control. The average increased activities of Cx, β-Glu, PG, and PMG were 30.73%, 40.40%, 51.55%, and 32.23% from day 0 to 6 after inoculation, respectively. In contrast, the average decreased contents of original pectin and cellulose were 1.82% and 16.47%, respectively, whereas the average increased soluble pectin content was 38.31% in comparison to control. Our results indicate that <i>C. truncatum</i> infection increased the activities of CWDEs in <i>H. undatus</i> fruits to disassemble their cell wall components, finally leading to the fruits’ decay and deterioration. Thus, our findings will provide significant evidence in the controlling of pitaya anthracnose in the future.