Salinity Tolerance of Two Isolates of Lagenidium Giganteum (Oomycetes: Lagenidiales), a Fungal Pathogen of Mosquito Larvae1

Abstract

Laboratory experiments were conducted to determine the salinity range over which North Carolina (NC) and Louisiana (LA) isolates of the fungus Lagenidium giganteum (Couch) grow vegetatively and infect mosquito larvae. The mycelial growth rates of the 2 isolates on nutrient agar were increased with 2.5 and 5.0 parts per thousand (ppt) of NaCl added and reduced with 7.5 ppt or more NaCl. The LA and NC isolates did not grow on agar containing 20 and 30 ppt NaCl, respectively. The concentrations of NaCl for 50% inhibition (IC50) of mycelial growth of the LA and NC isolates (10.9 and 12.0 ppt, respectively) were not significantly different (P > 0.05). The ability of the 2 isolates of L. giganteum to infect larvae of Aedes taeniorhynchus decreased as salinity of the water increased. The IC50 values for the inhibition of infection in mosquito larvae by the LA and NC isolates (0.52 and 0.55 ppt NaCl, respectively) were not significantly different (P > 0.05). Microscopic examination of the fungus in saline and distilled water showed that NaCl inhibited the production of zoospores. In water containing 1.5 ppt NaCl there was complete inhibition of zoosporogenesis and mosquito infection in each isolate of L. giganteum. Zoosporogenesis was ca. 22 times more sensitive to salinity than was mycelial growth.