Napthoquinones from Neocosmospora sp.—Antibiotic Activity against Acidovorax citrulli, the Causative Agent of Bacterial Fruit Blotch in Watermelon and Melon

Abstract

Bacterial fruit blotch (BFB) is a bacterial disease that devastates <i>Cucurbitaceae</i> crops worldwide, causing significant economic losses. Currently, there is no means to treat or control the disease. This study focused on exploring the antibacterial properties of endophytic fungi against <i>Acidovorax citrulli</i> (<i>Aac</i>), the causative agent of BFB. Based on disc diffusion, time kill and MIC microdilution broth assays, four endophytes showed promise in controlling <i>Aac</i>. Nonetheless, only one strain, <i>Neocosmospora</i> sp. MFLUCC 17-0253, reduced the severity of disease on watermelon and melon seedlings up to 80%. Structure analysis revealed production of several compounds by the fungus. Three of these secondary metabolites, including mixture of 2-methoxy-6-methyl-7-acetonyl-8-hydroxy-1,4-maphthalenedione and 5,8-dihydroxy-7-acetonyl-1,4-naphthalenedione, anhydrojavanicin, and fusarnaphthoquinones B exhibited antagonistic activity against <i>Aac</i>. The chemical profile data <i>in planta</i> experiment analyzed by LC-Q/TOF-MS suggested successful colonization of endophytic fungi in their host plant and different metabolic profiles between treated and untreated seedling. Biofilm assay also demonstrated that secondary metabolites of <i>Neocosmospora</i> sp. MFLUCC 17-0253 significantly inhibited biofilm development of <i>Aac</i>. To the best of our knowledge, secondary metabolites that provide significant growth inhibition of <i>Aac</i> are reported for the first time. Thus, <i>Neocosmospora</i> sp. MFLUCC 17-0253 possesses high potential as a biocontrol agent for BFB disease.