Larval Bradysia impatiens (Diptera: Sciaridae) potential for vectoring Pythium root rot pathogens

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 spp. 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.