Plectosporium tabacinum, a pathogen of the invasive aquatic weed Hydrilla verticillata in Florida

Plectosporium tabacinum, the anamorph of Plectosphaerella cucumerina, was i solated in 1996 from Hydrilla verticillata (hydrilla), an invasive aquatic weed in Florida. P. tabacinum, applied as a suspension of conidia, was path ogenic to hydrilla shoots maintained in aqueous solutions in test tubes. Ko ch's postulates were fulfilled in several repeated experiments. Infected sh oots became slightly chlorotic within 24 h after inoculation. Infected leav es remained intact and were supported by water pressure but collapsed upon removal from water. Histological studies of leaves stained with malachite g reen and acid fuchsin revealed fungal hyphae within plant cells. The diseas e developed over a range of temperatures from 15 to 30 degrees C. At 25 deg rees C, symptoms were most severe in 5% Hoagland's solution, followed by ri ver water, deionized water, 0.5% Hoagland's, tap water, and spring water. D isease severity increased as inoculum concentration wits increased from 10( 5) to 10(7) conidia ml(-1). This is the first report of P. tabacinum as a p athogen of hydrilla, a fully submerged aquatic plant species.