ARS | Publication request: Effect of fungus gnat Bradysia impatiens (Diptera: Sciaridae) feeding on subsequent Pythium aphanidermatum infection of geranium seedlings (Pelargonium x hortorum)

Submitted to: Phytopathology
Publication Type: Abstract Only
Publication Acceptance Date: June 16, 2008
Publication Date: July 26, 2008
Citation: Arnold, S.A., Nelson, E.B., Sanderson, J.P., Daughtrey, M.L., Wraight, S.P. 2008. Effect of fungus gnat Bradysia impatiens (Diptera: Sciaridae) feeding on subsequent Pythium aphanidermatum infection of geranium seedlings (Pelargonium x hortorum). Phytopathology. 98(Supplement):S14.

Technical Abstract: Dark-winged fungus gnats in the genus Bradysia (Diptera: Sciaridae) and root rot pathogens in the genus Pythium (Oomycetes) are important pests of greenhouse floriculture. Observations have pointed to a possible correlation between Pythium root rot disease and fungus gnat infestations; however, interactions among fungus gnats, Pythium spp. and floral crops have not been thoroughly investigated. The goal of this study was to determine the effects of plant feeding by fungus gnat larvae on subsequent Pythium aphanidermatum (Pa) infection of geranium seedlings (Pelargonium x hortorum). A 2 x 2 factorial experiment with fungus gnat feeding damage and Pa inoculation as treatments was conducted using a laboratory bioassay. Geranium seeds were geminated on water-saturated filter paper in small Petri dishes (1 seed/dish). When the seedlings were 7 days old, 4th-instar fungus gnat larvae were placed in the dishes (3/dish) and allowed to feed on the seedlings. The larvae were removed after 24 h, and the dishes were inoculated by pipetting 0.4 ml of Pa zoospore suspension onto the filter paper. A total of 13 assays were conducted, using 80–100 seedlings/assay and inocula with ca. 25–500 zoospores/ml. Mortality of seedlings recorded 7 days post-inoculation increased significantly in the presence of Pa and with increasing zoospore concentration. In the fungus gnat + Pa treatments, there were highly significant reductions (up to 37.5¬ percentage points) in geranium seedling mortality compared to Pa inoculation alone. Further experiments are warranted to elucidate the mechanism of this interaction. These findings enhance our understanding of the association between fungus gnats and Pythium spp. in greenhouse floriculture.