Potential for biocontrol of monosporascus root rot/vine decline under greenhouse conditions using hypovirulent isolates of Monosporascus cannonballus

Monosporascus root rot/vine decline (MRR/VD) causes root necrosis and sever e stunting of muskmelon and watermelon plants in several countries around t he world. MRR/VD is caused by the soilborne ascomycete fungus, Monosporascu s cannonballus. Currently, there are few options available for control of M RR/VD. This research describes experiments to test the possibility of using naturally occurring M. cannonballus isolates containing double-stranded RN A (dsRNA) for the biological control of MRR/VD. These isolates often develo p a degenerate phenotype characterized by slow growth and reduced ascospore production. In addition, these degenerate isolates are hypovirulent on mus kmelon. Plants co-inoculated with a hypovirulent, dsRNA(+) isolate (Tx93-44 9(+)) and a virulent, dsRNA(-) isolate (Az90-33(-)) at an inoculum ratio of 10 : 1 (hypovirulent : virulent) were indistinguishable from the uninocula ted plants in greenhouse pathogenicity trials. In vitro infection assays us ing fluorescence microscopy on aniline-stained muskmelon roots suggested th at although the hypovirulent dsRNA(+) isolate Tx93-449(+) penetrated and pa rtially colonized roots of the seedlings, it was not as efficient in coloni zing the roots as the virulent, dsRNA(-) isolate Az90-33(-). While more ext ensive experiments are needed, these data suggest that hypovirulent dsRNA() isolates of M. cannonballus have potential for development as biological control agents to reduce disease pressure associated with MRR/VD.