Abstract An alginate formulation of Alternaria eichhorniae (isolate Ae5) was evaluated in greenhouse trials as a bioherbicide to control waterhyacinth ( Eichhornia crassipes ). Pellet and powder formulations, the latter used in combination with hydrophilic substances, were tested. Four weeks after applying an alginate-pellet formulation of Ae5 to waterhyacinth plants in a greenhouse, the treated leaves became diseased and died, and the biomass of treated plants decreased by 29% from the initial weight. Nontreated, fungus-free plants gained weight and weighed 60% more than the fungus-treated plants after 3-4 weeks ( P = 0.05). Fungus-treated plants produced on average 5% new leaves whereas the control plants produced 81% new leaves. Water loss decreased by 31% over a 6-week period in plots containing plants treated with a powdered alginate formulation of Ae5 compared to fungus-free, waterhyacinth-containing plots. The water loss from plots without waterhyacinth (open surface) was 47% less than the loss from plots containing fungus-free, waterhyacinth plants. In a detached-leaf assay, the level of disease initiated by the powdered alginate formulation of AeB supplemented with a hydrophilic substance was higher (disease rating of >5.1) compared to the same formulation without a hydrophilic supplement (rating of 3.3). Two months after four sequential applications of the formulation supplemented with a hydrophilic polyacrylamide, disease severity on fungus-treated plants increased by 93% and the biomass (fresh weight) decreased by 89% compared to the control. Thus, A. eichhorniae curtailed waterhyacinth growth in the greenhouse, but multiple applications appear necessary to obtain a practical results in the field.