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Title: Transstadial transmission of Pythium in Bradysia impatiens and lack of adult vectoring capacity

Author
item BRAUN, SARAH - Cornell University
item CASTRILLO, LOUELA - Cornell University
item SANDERSON, JOHN - Cornell University
item DAUGHTREY, MARGERY - Cornell University
item Wraight, Stephen

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/5/2010
Publication Date: 12/6/2010
Citation: Braun, S.E., Castrillo, L.A., Sanderson, J.P., Daughtrey, M.L., Wraight, S.P. 2010. Transstadial transmission of Pythium in Bradysia impatiens and lack of adult vectoring capacity. Phytopathology. 100(12):1307-1314.

Interpretive Summary: Fungus gnat larvae are important greenhouse pests, causing damage by feeding/tunneling in plant roots and stems, especially those of young seedlings or cuttings. Fungus gnats are also commonly associated with diseased plants, and it has long been speculated that they play a significant role in development of disease epidemics by vectoring plant pathogens. Studies have revealed that adult fungus gnats can pick up and transmit the asexual spores (conidia) of many fungal pathogens that are produced in great numbers on the above-ground parts of infected plants; however, a significant role in the transmission of pathogens that do not produce such spores has not been demonstrated. This includes the many Pythium root rot pathogens that produce sexual spores called oospores primarily in plant roots where they are unlikely to be picked up by adult fungus gnats. In this laboratory study, we confined adult fungus gnats with geranium seedlings killed by three common Pythium pathogens and investigated the potential for these insects to pick up infectious inoculum from these plants and spread it to healthy plants. A DNA probe was developed to aid in the identification and tracking of Pythium in or on the bodies of the insects. No disease transmission was observed in any of our experiments. Pythium ingested by fungus gnat larvae was passed to the pupal stage, but not to the adult stage. Adult fungus gnats are short lived and rarely feed; these observations thus confirm that they are highly unlikely to acquire viable Pythium spores and are therefore not significant vectors of these pathogens. These findings will affect development of economic thresholds and management programs for this pest in many greenhouse crops.

Technical Abstract: Fungus gnats have been shown to transmit a variety of plant pathogenic fungi that produce aerial dispersal stages. However, few studies have examined potential interactions between fungus gnats and oomycetes, including Pythium spp. A series of laboratory experiments were conducted to determine if fungus gnat adults are vectors of several common greenhouse Pythium spp., including P. aphanidermatum, P. irregulare, and P. ultimum. An additional objective was to determine if P. aphanidermatum can be maintained transstadially in the gut of a fungus gnat larva through the pupal stadium to be transmitted by the subsequent adult. Adult fungus gnats did not pick up infectious Pythium propagules from diseased plants and transmit them to healthy plants in any experiment. Species-specific primers and a probe for real-time PCR were developed to detect the presence of P. aphanidermatum DNA in fungus gnat tissue samples. P. aphanidermatum DNA was detectable in the larval and pupal stages; however, none was detected in adult fungus gnats. These results are in agreement with previous studies that have suggested that adult fungus gnats are unlikely vectors of Pythium spp.