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

Abstract

Ambrosia beetles, a significant concern in maintaining cocoa productivity, are now considered an emerging pest on cacao in some areas of South Sulawesi, Indonesia. <italic>Xylosandrus compactus</italic>, 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 <italic>X</italic>. <italic>compactus</italic> 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 <italic>X</italic>. <italic>compactus</italic> 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 <italic>Fusarium</italic> isolates, one <italic>Lasiodiplodia</italic> isolate<italic>,</italic> and one <italic>Colletotrichum</italic> isolate. The leaf disk test showed that the <italic>Lasiodiplodia</italic> isolate was the most virulent, while the other fungal isolates were less aggressive. Pathogenicity tests were performed on rootstock seedlings of cacao, with a <italic>Lasiodiplodia</italic> isolate producing longer vascular streaking than the other fungi. Temperature tests showed that <italic>Lasiodiplodia</italic> grew well and faster under two ambient Sulawesi's prominent temperatures, 30 and 35 °C. These findings corroborate our previous study regarding <italic>X</italic>. <italic>compactus</italic> 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.

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