Molecular detection of fungi carried by Bradysia difformis (Sciaridae: Diptera) in South African forestry nurseries

Bradysia difformis (Sciaridae: Diptera) has recently been identified from South African forestry nurseries, and is thought to have been introduced into the country. Fungus gnats, including Bradysia spp., are known to transmit various fungal pathogens. It has thus been hypothesised that B. difformis might be responsible for the rapid spread of the pathogen Fusarium circinatum within South African forestry nurseries. Previous studies have, however, failed to confirm this assumption. In this study we attempted to determine the association between B. difformis and the two nursery pathogens F circinatum and Botrytis cinerea, using sensitive DNA-based markers. A total of 60 fungus gnats and four combined collections of 25-30 fungus gnats were obtained from four of the major forestry nurseries in South Africa. The species-specific primers CIRC1A and CIRC4A and C729+ and C729- were used in an attempt to detect F circinatum and Bo. cinerea, respectively. The sensitivity of these primers when fungal DNA was mixed with fungus gnat DNA was tested at various concentrations. General fungal primers were used to detect any other fungi on B. difformis. Neither F circinatum, Bo. cinerea nor any other fungal pathogens were detected on B. difformis. This is despite the ability of CIRC1A and CIRC4A to detect F circinatum at a minimum of 13.4pg and a ratio of 1:3 727 fungus to fungus gnat DNA, and the ability of C729+ and C729- to detect Bo. cinerea at a minimum of 13.4pg and a ratio of 1:14 691 fungus to fungus gnat DNA. Other fungi were detected using the general fungal primers, but none of these fungi were pathogens. We conclude that B. difformis does not play a major role in the movement of these or other fungal pathogens in South African forestry nurseries.