Association between the fungus Acremonium curvulum and Eurasian water milfoil, Myriophyllum spicatum

Abstract

Acremonium curvulum W. Gams was isolated during three growing seasons from the stems, leaves, and roots of Eurasian water milfoil (Myriophyllum spicatum L.) that grew submerged in University Bay of Lake Mendota, Wisconsin. Despite substantial variation, an overall trend suggested more frequent incidence of this fungal-plant association as the season progressed. Surveys of eight other Wisconsin lakes located in three drainage basins showed this relationship to be of general occurrence. Acremonium curvulum occurred with seeds and, more commonly, with vegetative fragments of milfoil. Anatomical evidence from sectioned or fractured milfoil cells, scanning electron microscopy of stem and leaf surfaces, and plating of surface-disinfested tissue pieces indicated that Acremonium grew both epiphytically and endophytically. Healthy plants harbored the fungus, which was apparently eliminated by immersion of shoots in a solution of the fungicide thiabendazole. If milfoil was stressed by environmental or mechanical manipulations, proliferation of A. curvulum accompanied decline of the plant. Healthy, Acremonium-free, cultured milfoil immersed in a conidial suspension developed systemic basal necrosis that spread acropetally to involve about two-thirds of the tissue within 1 week. Dry weight and shoot elongation of inoculated, diseased milfoil were slightly less than those of control plants, although new growth was usually initiated from adventitious buds within 3–4 weeks. Recovery of infected plants was promoted somewhat by thiabendazole.