Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/12/2004
Publication Date: 6/28/2004
Citation: Wheeler, M.H., Bruton, B.D., Puckhaber, L.S., Zhang, J., Stipanovic, R.D. 2004. Identification of 1, 8-dihydroxynaphthalene melanin in Monosporascus cannonballus and the analysis of hexaketide and pentaketide compounds produced by wild-type and pigmented isolates of the fungus. Journal of Agricultural and Food Chemistry. 52(13):4113-4120. Interpretive Summary: The fungus Monosporascus cannonballus causes disease and severe production losses to muskmelon and watermelon in the U.S. and other countries around the world. Normally this fungus does not produce appreciable amounts of pigment except for a black substance known as melanin. Occasionally, however, the fungus undergoes a number of changes. For example, its ability to cause disease is greatly diminished, it is no longer able to produce the black substance melanin, and in its altered fungal forms it produces large amounts of yellow-to-brown pigments that are not produced by healthy forms of the fungus. Recently, five of these pigments that are produced by an unhealthy, altered form of the fungus were identified. In the present investigation, the same five pigments were shown to be commonly produced by other altered forms of the fungus, whereas they were usually absent in healthy forms of the fungus. Also, in the present investigation, the type melanin in healthy forms of the fungus was identified and was found to be identical to that previously identified in several other fungi known to cause diseases in plants and animals. The present investigation suggests that the production of pigments and the loss of melanin are responsible for preventing disease in muskmelon and watermelon plants, when they are challenged by altered forms of M. cannonballus.
Technical Abstract: Monosporascus cannonballus is a fungal pathogen that causes root rot and vine decline in muskmelons and watermelons in several countries. Wild types of the fungus often undergo degenerative changes, which include production of yellow to brown pigments, decreased hyphal melaninization, development of hypovirulence, and a reduction in perithecia and ascospores. Cultures of degenerative yellow to brown isolates of the fungus accumulated five related hexaketides. In contrast, these hexaketides were present only in minute amounts in wild-type cultures unless grown on NaCl amended media. Tricyclazole blocked melanin biosynthesis in the wild types, resulting in accumulation of metabolites related to 1,8-dihydroxynaphthalene melanin. Melanin was absent in the pigmented isolates of M. cannonballus, even without the presence of tricyclazole, and they accumulated melanin-related metabolites. dsRNA may be responsible for the degenerative changes that occurred in wild-type isolates, including loss of fungal melaninization which may be associated with loss of virulence in those isolates.