Macadamia phytophthora root rot

Phytophthora constricta is a pathogenic oomycete responsible for a devastating root and collar rot disease affecting macadamia trees. It is classified within the Peronosporaceae family and is recognized as a major limiting factor in macadamia production, particularly in regions with conducive environmental conditions.

The primary host for this pathogen is the macadamia tree (Macadamia integrifolia, M. tetraphylla). The fungus specifically targets the crown and root tissues, disrupting the vascular system and preventing the uptake of water and essential nutrients, which is fatal if left untreated.

Clinical signs of an infection often include the formation of distinct lesions or constricted areas at the base of the trunk near the soil line, which is why the pathogen is named constricta. Above-ground symptoms manifest as leaf yellowing, wilting of the canopy, and dieback of branches, eventually leading to the rapid decline of the entire tree.

The development and spread of Phytophthora constricta are heavily dependent on water, as the motile zoospores require free water to reach the roots of the host plant. Over-irrigation, poor drainage, and movement of infested soil or contaminated farm tools are the primary pathways for the disease to spread across orchard blocks.

Integrated disease management is essential to prevent losses:

• Selection of well-drained planting sites to avoid waterlogging.
• Using pathogen-free nursery stock for orchard establishment.
• Implementation of strict sanitation practices for agricultural machinery.
• Application of systemic fungicides like phosphonates to boost host defense.
• Routine inspection of the crown area for early detection of lesions.