Controlled environment studies on the infection of Emex australis by Phomopsis emicis

The effect of temperature, dew period, inoculum concentration and host maturity on the growth and development of Emex australis after inoculation with Phomopsis emicis was studied in controlled environments. Disease was assessed by recording changes in the length of stems, number of fully expanded leaves, dry weights, number of fruits and disease severity index. Disease was as severe at 18°C as it was at 28°C. There was a trend of increased disease with longer dew period. Inoculum concentrations of 1·10⁶ and 1·10⁷ conidia per ml resulted in a significant net reduction in stem growth compared with lower concentrations. Progressively higher disease ratings occurred as the inoculum concentration increased to 1·10⁷ conidia per ml. There were no significant effects of inoculation with P. emicis on either seedlings, rosettes or flowering rosettes, but 10-week-old plants were highly susceptible to infection and stem collapse. The implications of these results for the potential development of P. emicis as a mycoherbicide are discussed.