Phytophthora root rot of macadamia

The disease is characterized as a root and collar rot. Unlike other generalist species of the genus, Phytophthora constricta exhibits significant aggression toward macadamia trees, leading to severe systemic vascular dysfunction.

The biological cycle of the pathogen is heavily dependent on moisture. Zoospores require free water in the soil to swim and locate the host's root tips, making the pathogen particularly active during wet seasons.

Once inside the plant, the mycelium invades the cortical tissues and cambium, creating necrotic lesions. These lesions eventually girdle the trunk or root system, interrupting the tree's ability to transport essential nutrients and water.

The pathogen persists in the environment via chlamydospores or oospores, which are dormant structures capable of surviving in the soil for several years without the presence of a host plant.

Initial symptoms are often subtle, appearing as a general decline in tree vigor. The foliage may show chlorosis (yellowing), wilting, and premature leaf drop, making the canopy appear sparse.

A diagnostic sign is the presence of lesions on the crown or the base of the trunk. Removing the outer bark reveals a reddish-brown or black necrosis in the underlying cambial tissue.

The root system becomes severely compromised. Fine feeder roots are typically the first to rot, leading to a reduced root mass and an inability to sustain the tree during dry weather.

As the disease advances, tree dieback occurs. You may notice dead twigs and branches at the top of the canopy, and in acute cases, the entire tree may collapse suddenly.

Because subterranean symptoms are not visible until advanced stages, trees may appear healthy while the root system is already significantly damaged, making early detection extremely difficult.

The development of Phytophthora root rot is favored by excessive soil moisture and poor drainage. Waterlogged soil conditions provide the necessary environment for zoospore motility.

Temperature is a key driver, with the pathogen thriving in soil temperatures ranging from +18°C to +25°C. These temperatures are common in many macadamia-growing regions.

Heavy soils, such as those with high clay content, exacerbate the disease because they hold moisture longer, creating a sustained environment for the pathogen to thrive and spread.

Physical injuries to the lower trunk or roots, often caused by heavy machinery, cultivation equipment, or herbicide spray, serve as entry points for the pathogen to infect the host tissue.

The movement of contaminated soil or water via runoff from infected areas to healthy blocks is a primary pathway for the spread of Phytophthora constricta within an orchard.

This disease is a major threat to macadamia production globally. It can lead to complete tree loss, resulting in significant financial setbacks for orchard owners.

The reduction in the tree’s root system directly correlates to a decrease in nut yield. Infected trees produce fewer and smaller nuts, which significantly impacts the market value of the harvest.

Orchard management becomes more costly and complex, as infected blocks require intensive monitoring, quarantine measures, and the potential removal of productive trees.

If not managed, the disease can spread throughout an entire plantation, turning once-productive land into unsuitable ground for replanting macadamia for many years.

The economic impact is further magnified by the high cost of site remediation and the long waiting period required to bring new replacement trees to a productive age.

The primary control measure is the use of high-quality, certified nursery stock to ensure that the orchard starts free of Phytophthora constricta.

Orchard design should emphasize excellent soil drainage. Implementing raised beds or installing subterranean drainage systems can help prevent water stagnation in the root zone.

Chemical control involving systemic fungicides, such as phosphites, is standard practice. These are applied either as a foliar spray or through trunk injection to protect the tree’s root system.

Strict sanitation protocols must be followed. Tools, footwear, and machinery should be sanitized between blocks to prevent cross-contamination of spores via soil particles.

If an infected tree is identified, it should be removed along with its root system and surrounding soil. The area must be left fallow or treated with approved soil amendments before any attempt to replant.