Structure of the Surface Root System of Eucalyptus Marginata Sm. And Its Infection by Phytophthora Cinnamomi Rands

Abstract

The structure of the surface root system of jarrah (Eucalyptus marginata) trees was examined on a number of freely drained upland sites with different fire and management histories. The roots typically occurred in patches but in some stands formed extensive sheets. On excavation; this surface root system was composed of pads ranging in size from 10 cm to 1-3 m in diameter and c. 5 cm thick. The pads consist of short roots 1-3 mm long (which commonly form dense clusters around lateritic pebbles) which arise from small (n - 1)*th order laterals 0.5-1.5 cm long connected to (n - 2)*th order laterals 2-5 cm long and up to 0.7 mm in diameter. Mycorrhizal roots were common throughout the surface root pads. During the summer drought many of the short lateral roots die but the main framework of the roots of the surface pads is perennial. Following rains or irrigation, new, short lateral roots form rapidly from the framework of roots in the surface pads. Phytophthora cinnamomi was consistently recovered from short lateral roots and from the perennial roots (n - 1, n - 2) which form the framework of the root pads at a site in diseased forest where a high density of P. cinnamomi had been induced in the soil by irrigation. We hypothesize that the destruction of some of the perennial components of the root pads could explain why P. cinnamonzi can cause the decline and death of jarrah in an environment only marginally favourable for the fungus.