Lack of Pythium aphanidermatum transmission by adult fungus gnats (Bradysia impatiens)

Studies have provided evidence for transmission of plant pathogens by greenhouse-inhabiting fungus gnats (Bradysia spp.). However, little research has been done on the association of fungus gnats with Pythium spp., though these Oomycetes are among the most important pathogens of greenhouse crops. The goal of this study was to determine if adult fungus gnats are vectors of Pythium aphanidermatum. In the first of a series of laboratory experiments, 10 adult gnats were released into small chambers containing two P. aphanidermatum-infected and two healthy geranium seedlings. None of the healthy plants became infected. In a second experiment, adult gnats held by the wings were dragged across P. aphanidermatum culture plates and immediately immersed in solidifying (40ºC) water agar in a small Petri dish or released into a dish with solidified water agar. Pythium colonies developed in 11% of the plates with fungus gnats immersed in agar, but in only 1% of plates where gnats were allowed to move freely on an agar surface. In a third experiment, 50 adult female gnats were placed in containers with potting mix and either 35 infected or healthy seedlings. After 24 hours, 20 of the 50 fungus gnats were transferred from each container to a new container with 35 healthy seedlings and 10 were immersed, as described above, in water agar. None of the seedlings became infected, and no Pythium colonies formed in the agar dishes. Extensive replication of these experiments demonstrated that fungus gnats are unlikely aerial vectors of Pythium aphanidermatum.