Summer Scholarship: Can insects spread cotton plant pathogens in Australia?

Abstract

Providing Australian cotton protection from biotic threats and environmental stresses requires informed surveillance and management. One such threat is the fungal disease, Verticillium wilt, caused by the pathogen Verticillium dahliae. Verticillium wilt remains a severe disease in Australian cotton, with more virulent strains causing significant yield losses (Chapman et al. 2016). This disease would be better managed if potential pathways for transmission of Verticillium could be identified. Overseas publications report other species of Verticillium that can be transmitted by insects (Kalb et al. 1986, Huang & Harper 1985, Price 1975). The aim of this study was to identify if any insects could digest and then transmit V. dahliae.

A pilot study (to optimise the methodology required) and four experiments (Experiment 1, 2 and 3 exposing insects to a pure fungal culture of V. dahliae, and Experiment 4 exposing insects to the diseased cotton plant tissue (leaves with symptoms of Verticillium wilt)) were undertaken to assess if insects found on or in the soil around the cotton plant could transmit V. dahliae. Results showed that the pathogen was transferred externally on the insects: Corticaria subtilissima (minute brown scavenger beetle), Anthicus australis (ant- like beetle), Elateridae (wireworm), Dicranolaius bellulus (red and blue beetle) and Creontiades dilutes (green mirid). In addition, the pathogen survived the digestive tract of several insects: Dicranolaius bellulus (red and blue beetle), Iridomyrmex (small meat ant), Corticaria subtilissima (minute brown scavenger beetle), Anthicus australis (ant-like beetle) and Elateridae (wireworm). The faecal matter isolations and subsequent recovery of V. dahliae from these insects are evidence that they are capable of transmitting the V. dahliae pathogen.

The findings suggest that insects are a potential source of pathogen transmission within Australian cotton crops. While insects could spread V.dahliae, additional research is needed to establish if they can be inoculated by Verticillium wilt plants, and to fully understand the relationship between the pathogen and these potential vectors to inform the surveillance and management of this key biotic threat.