|Braun, Sarah -|
|Sanderson, John -|
Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 9, 2011
Publication Date: March 1, 2012
Citation: Braun, S.E., Sanderson, J.P., Wraight, S.P. 2012. Larval Bradysia impatiens (Diptera: Sciaridae) potential for vectoring Pythium root rot pathogens. Environmental Entomology. 102:283-289. Interpretive Summary: Fungus gnat larvae are important greenhouse pests, causing damage by feeding/tunneling in plant roots and stems. These insects are also commonly associated with diseased plants, and it has long been speculated that they are important vectors of root-rot pathogens, including Pythium species. Previous laboratory studies based on Petri dish assays have revealed that fungus gnat larvae readily ingest Pythium spores and that ingested spores can survive passage through the larval gut and produce high rates of plant infection. However, little is known of the factors potentially affecting Pythium transmission by fungus gnat larvae under greenhouse conditions. Assays revealed that vectoring of Pythium to geranium seedlings varied greatly with the assay substrate and also with the number and nature of ingested spores. Transmission was high (65%) in Petri dishes with larvae fed a strain of Pythium that produced abundant thick-walled resting spores (oospores), but transmission of this strain was much lower (<6%) in plug cells with a commercial peat-based potting mix. Larvae were less efficient at vectoring Pythium strains that produced few oospores, and no transmission was observed with two non-oospore-producing strains. Passage through the larval guts significantly increased the viability (germinability) of Pythium oospores but greatly reduced the viability of other types of spores. These results indicate that fungus gnat larvae are less efficient vectors of Pythium pathogens than previously believed and will be useful to researchers developing IPM strategies for greenhouse pest and disease management.
Technical Abstract: A series of laboratory experiments were conducted to investigate the capacity of Bradysia impatiens (Johannsen) larvae to ingest propagules from two strains each of Pythium aphanidermatum (Edson) Fitzp. and P. ultimum Trow and transmit the pathogens to healthy geranium seedlings on a filter-paper substrate in Petri dishes. The capacity of fungus gnat larvae to transmit P. aphanidermatum to seedlings rooted in a commercial peat-based potting mix and germination of Pythium oospores and hyphal swellings before and after passage through the guts of larval fungus gnats were also examined. Assays revealed that Pythium¬ transmission by larval fungus gnats varied greatly with the assay substrate and also with the number and nature of ingested propagules. Transmission was highest (65%) in the Petri dish assays testing larvae fed P. aphanidermatum K-13, a strain that produced abundant oospores. Transmission of strain K-13 was much lower (< 6%) in plug cells with potting mix. Larvae were less efficient at vectoring P. ultimum strain PSN-1, which produced few oospores, and no transmission was observed with two non-oospore-producing strains: P. aphanidermatum Pa58 and P. ultimum P4. Passage of P. aphanidermatum K-13 through larval guts significantly increased oospore germination; however, decreased germination of hyphal swellings was observed following larval gut passage for strains of P. ultimum. These results expand previous studies suggesting that larval fungus gnats may vector Pythium spp.