Pathogenicity of fungi associated with seedling wilting under <i>Xylosandrus compactus</i> infestation on <i>Theobroma cacao</i> in South Sulawesi, Indonesia

Ambrosia beetles, a significant concern in maintaining cocoa productivity, are now considered an emerging pest on cacao in some areas of South Sulawesi, Indonesia. Xylosandrus compactus, the most prevalent species, has been found colonizing many cacao seedlings in a nursery. This study aimed to determine the incidence and impact of the X. compactus infestation in cacao seedlings, the pathogenic potential, and the growth of the fungal associates. From a commercial cacao nursery we randomly sampled 100 three-month-old rootstock seedlings. The beetles were identified morphologically, and then the associated fungi were isolated. The present study found that X. compactus attacked 24% of cacao rootstock seedling samples, and 8% of them showed wilting symptoms. Morphological examination identified four fungi recovered from infested galleries and larvae: two Fusarium isolates, one Lasiodiplodia isolate, and one Colletotrichum isolate. The leaf disk test showed that the Lasiodiplodia isolate was the most virulent, while the other fungal isolates were less aggressive. Pathogenicity tests were performed on rootstock seedlings of cacao, with a Lasiodiplodia isolate producing longer vascular streaking than the other fungi. Temperature tests showed that Lasiodiplodia grew well and faster under two ambient Sulawesi's prominent temperatures, 30 and 35 °C. These findings corroborate our previous study regarding X. compactus in cacao and provide insights into the potential threat of the beetle as a pest and a vector of plant pathogens in cacao seedlings. Also, these findings suggest that the use of infested seedlings can result in the spreading of the beetle.