Phytophthora drechsleri Tucker and Pythium spp. as pathogens of safflower in New South Wales

Abstract

Pythiaceous fungi associated with a widespread root rot disease of safflower (Carthamus tinctorius) in New South Wales include Phytophthora drechsleri, Pythium myriotylum, Pythium ultimum Pythium irregulare, Pythium acanthicum, and a number of sterile Pythium spp. Pathogenicity of these fungi was tested against safflower seedlings and older plants growing either in soil with moisture content at field capacity or in flooded soil. Phytophthora drechsleri, Pythium myriotylum, Pythium ultimum and Pythiumirregulare caused severe pre-emergence damping off. When safflower plants, three and eight weeks old, were inoculated with each of the fungi, only Phytophthora drechsleriand Pythium myriotylum killed a high proportion of plants. The pathogenicity of Phytophthora drechsleri, Pythium myriotylum, Pythium ultimum, and Pythium irregulare was increased by flooding the soil for ninety hours after inoculation. Phytophthora drechsleri was isolated from naturally infected saffron thistle (Carthamus lanatus). Cross inoculation tests with Phytophthora drechsleri isolates from safflower and saffron thistle showed that both isolates could infect each host. Saffron thistle could be an important alternative host of Phytophthora drechsleri in the field. It is concluded that Phytophthora drechsleri is the major cause of root rot of safflower in New South Wales.