Pathogenicity of Phytophthora multivora to Eucalyptus gomphocephala and Eucalyptus marginata

Phytophthora multivora is associated with the rhizosphere of declining Eucalyptus gomphocephala, Eucalyptus marginata and Agonis flexuosa. Two pathogenicity experiments were conducted. The first experiment examined the pathogenicity of five P.similar to multivora isolates and one Phytophthora cinnamomi isolate on the root systems of E.similar to gomphocephala and one P.similar to multivora isolate on the root system of E.similar to marginata. In the second experiment, the pathogenicity of P.similar to multivora to E.similar to gomphocephala and E.similar to marginata saplings was measured using under-bark stem inoculation. In Experiment 1, the P.similar to cinnamomi isolate was more aggressive than all P.similar to multivora isolates causing significant loss of fine roots and plant death. Two P.similar to multivora isolates and the P.similar to cinnamomi isolate caused significant losses of E.similar to gomphocephala fine roots 02 mm in diameter and significantly reduced the surface area of roots 01 mm in diameter. One P.similar to multivora and the P.similar to cinnamomi isolate significantly reduced the surface area of roots 12 mm in diameter. Two of the P.similar to multivora isolates significantly reduced the number of E.similar to gomphocephala root tips. In E.similar to marginata, the length and surface area of roots 01 mm in diameter and number of root tips were significantly reduced by P.similar to multivora infestation. Rhizosphere infestation with the P.similar to multivora isolates and P.similar to cinnamomi isolate on E.similar to gomphocephala, and one P.similar to multivora isolate on E.similar to marginata, did not significantly influence the foliar nutrient concentrations. In Experiment 2, under-bark inoculation with P.similar to multivora caused significant lesion extension in E.similar to gomphocephala and E.similar to marginata saplings, compared to the control. We propose that P.similar to multivora is inciting E.similar to gomphocephala and E.similar to marginata decline by causing fine root loss and subsequently interfering with nutrient cycling throughout the plant. The impact of fine root loss on the physiology of plants in sites infested with P.similar to multivora requires further research.