Fungus gnat (Bradysia impatiens) feeding and mechanical wounding inhibit Pythium aphanidermatum infection of geranium seedlings (Pelargonium x hortorum)

Authors: BRAUN, SARAH - Cornell University; SANDERSON, JOHN - Cornell University; NELSON, ERIC - Cornell University; DAUGHTREY, MARGERY - Cornell University; Wraight, Stephen

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2009
Publication Date: 12/1/2009
Citation: Braun, S.E., Sanderson, J.P., Nelson, E.B., Daughtrey, M.L., Wraight, S.P. 2009. Fungus gnat (Bradysia impatiens) feeding and mechanical wounding inhibit Pythium aphanidermatum infection of geranium seedlings (Pelargonium x hortorum). Phytopathology. 99:1421-1428.

Interpretive Summary: Fungus gnat larvae are important greenhouse pests, damaging plants by their feeding/tunneling activities, especially in the roots and stems of young seedlings or cuttings. Fungus gnat infestations are also commonly associated with plant disease epidemics, and it has long been speculated that these insects play a significant role in disease outbreaks by either vectoring plant pathogens or predisposing plants to infection. Numerous studies have demonstrated that fungus gnats have the capacity to transmit plant pathogenic microorganisms, but potentially important effects of fungus gnat feeding on host-plant susceptibility to disease have not been thoroughly investigated. In this study, we tested the hypothesis that plants damaged by fungus gnat larvae would be predisposed to infection by the root rot pathogen Pythium aphanidermatum. Geranium seedlings were exposed to fungus gnat larvae or were mechanically wounded by severing the root tip and then inoculated with the pathogen. Surprisingly, seedlings injured by either fungus gnat feeding or mechanical cut suffered nearly 50 percent less mortality due to Pythium infection than uninjured seedlings. Results suggest that wounding activated a systemic defense that made the plants more resistant to infection. The unexpected finding of a potentially beneficial effect of fungus gnat damage reveals that the role of these pests in Pythium infection is more complex than previously understood and underscores a need for additional research, including testing of larger plants. Large plants are less susceptible to feeding damage by fungus gnats and therefore might be more likely to benefit from induced resistance to disease. Such findings could affect development of economic thresholds and management programs for this pest.

Technical Abstract: A series of laboratory tests were conducted to investigate potential effects of fungus gnat (Bradysia impatiens) feeding damage on susceptibility of geranium seedlings (Pelargonium x hortorum) to infection by the root rot pathogen Pythium aphanidermatum. Effects were compared to those from similar tests in which the seedlings were mechanically wounded by severing the root tip with a scalpel. Assays of geranium seedlings in Petri dishes revealed a pronounced negative fungus gnat x Pythium interaction, with exposure to fungus gnat larvae 24 h prior to inoculation with P. aphanidermatum zoospores resulting in up to 47% fewer seedling deaths than would have been expected if the two agents had acted independently. Similar results were observed when seedlings were subjected to mechanical wounding 24 h prior to zoospore inoculation. In contrast, no interaction occurred when seedlings were mechanically wounded immediately prior to inoculation. The degree of plant damage inflicted by the feeding activities of the larval fungus gnats had no significant effect on the combined damage from fungus gnats and Pythium in Petri dishes. Ancillary studies showed that Pythium development on V-8 agar was not inhibited by presence of fungus gnat-associated microorganisms nor were seedlings inoculated with these microbes less susceptible to Pythium infection. The precise mechanism or mechanisms underlying the observed interactions were not elucidated; however, the results strongly suggest that both fungus gnat feeding and mechanical wounding activated systemic defenses that made the seedlings more resistant to Pythium infection.