Evidence for pathogenicity of Fusarium sporotrichioides to Eurasian water milfoil, Myriophyllum spicatum

Abstract

Fusarium sporotrichioides Sherb. (= F. tricinctum (Corda) Sacc. emend. Snyd. and Hans.), isolated from diseased Eurasian water milfoil (Myriophyllum spicatum L.), collected in University Bay of Lake Mendota, Wisconsin, was demonstrated to be pathogenic to this species. When inoculated to milfoil grown in flasks, in aquaria, or in the lake, the fungus induced localized elongate, necrotic stem lesions similar to those from which it had been isolated. Plants did not develop necrosis unless wounded by needle puncture or injected with inoculum. On wounded shoots immersed in a spore suspension, systemic apical chlorosis developed within 7–10 days, intensified to a yellow–orange color, and spread proximally to involve about half of the tissue within 2 weeks. The pathogen was reisolated consistently from symptomatic plants following surface sterilization. None of the control plants developed symptoms, and F. sporotrichioides was never isolated from them. The fungus was not highly pathogenic to milfoil. It apparently penetrated mainly, if not exclusively, through wounds and grew extensively in cortical lacunae although its spread was impeded by interruption of the lacunar canals at nodes.