Microbe-mediated germination of ascospores of Monosporascus cannonballus

Ascospores of Monosporascus cannonballus germinated readily in the rhizosph ere of cantaloupe plants growing in field soil. However, little or no germi nation occurred in the rhizosphere of melon plants growing in field soil th at was autoclaved prior to infestation with ascospores. The latter data sug gested that root exudates alone do not stimulate ascospore germination and that the soil microflora may be involved in the induction of ascospore germ ination. Amending field soil with streptomycin (which inhibits gram-negativ e microorganisms) did not suppress ascospore germination in the rhizosphere of cantaloupe plants. However, amending the soil with penicillin (which in hibits gram-positive microorganisms) did suppress ascospore germination. Pe ntachloronitrobenzene (PCNB), which inhibits the gram-positive actinomycete s but does not inhibit gram-positive or gram-negative bacteria, also suppre ssed ascospore germination. These results suggest that actinomycetes, eithe r directly or indirectly, are involved in the induction of ascospore germin ation in field soil in the presence of exudates from cantaloupe roots. Opti mum germination occurred at temperatures ranging from 25 to 35 degrees C, a nd data indicate that a high percentage (greater than or equal to 72%) of t he ascospore population within 500 mu m of a root are capable of germinatio n and subsequent penetration of cantaloupe roots.