Complex sentence: An important post-harvest disease affecting mangosteen especially in Thailand is called Diplodia fruit rot ("Diplodia theobromae") which, as a secondary pathogen, enters the host plant through wounds.
Split into simpler sentences: An important post-harvest disease affecting mangosteen especially in Thailand is called Diplodia fruit rot ("Diplodia theobromae"). As a secondary pathogen, D. theobromae enters the host plant through wounds.