Pestalotiopsis macadamiae

The primary symptom of the disease is the appearance of small, brown to dark-gray spots on the leaves, which gradually expand and become irregular in shape.

As the infection progresses, tiny black specks appear on the surface of the lesions. These are the acervuli, the fungal fruiting bodies that indicate the pathogen is actively producing spores.

On twigs and young stems, the infection manifests as elongated, sunken lesions that can eventually girdle the branch, causing dieback and sudden wilting of the distal parts.

Fruit damage is also common, characterized by dark, depressed spots on the husks. Severe infections can lead to premature drop of the developing nuts or rotting of the husk tissue.

• necrotic brown leaf spots;
• black fungal fruiting bodies on lesions;
• dieback of young shoots;
• premature nut drop and fruit rotting.

The disease is caused by the fungus Pestalotiopsis macadamiae, an opportunistic pathogen that specifically targets macadamia trees. It is a filamentous fungus belonging to the Coelomycetes group, known for its ability to persist in plant tissue.

This pathogen survives primarily in infected plant debris, such as fallen leaves, old twigs, and necrotic fruit husks. It can remain dormant until weather conditions favor the production of reproductive spores.

The fungus produces asexual spores called conidia within specialized structures known as acervuli. These spores are released in large numbers and are easily disseminated by wind and water splash from rain or overhead irrigation.

Once the spores land on susceptible host tissue, they germinate under high humidity and penetrate the plant through stomata or mechanical injuries on the bark and leaves.

The colonization of host tissues happens intercellularly, where the fungal hyphae secrete enzymes that break down plant cell walls, leading to the characteristic necrotic lesions associated with this pathogen.

High humidity and moisture are the most critical factors for the infection process. Pestalotiopsis macadamiae thrives when moisture persists on the canopy for extended periods.

The optimal temperature range for spore germination and mycelial growth is between 22°C and 28°C, making warm, rainy springs particularly dangerous for macadamia orchards.

Orchards with poor ventilation due to excessive tree density are prone to higher disease pressure, as the lack of airflow keeps the foliage damp for longer durations.

Trees that are already stressed due to drought, nutritional deficiencies, or pest infestations are significantly more susceptible to successful colonization by the fungus.

Mechanical damage caused by pruning tools or environmental factors creates ideal entry points for the pathogen, significantly increasing the probability of new infections.

The pathogen causes significant economic damage by reducing photosynthetic leaf area, which weakens the tree and stunts its growth in severe cases.

Frequent leaf drop leads to a reduced ability of the tree to accumulate energy, ultimately resulting in smaller nut sizes and lower total yield for the harvesting season.

Damage to the husks not only reduces the overall quality of the nut but can also invite secondary infections from other molds and decay-causing microorganisms.

If the infection is left uncontrolled, the dieback can affect the structural integrity of the canopy, requiring extensive pruning and long-term recovery efforts for the tree.

For nursery production, the disease can be devastating, as it can kill young seedlings before they are ready for planting, leading to total loss of investment in the plant material.

Effective management begins with strict sanitation: all infected twigs, leaves, and fallen husks must be removed from the orchard and destroyed to reduce inoculum load.

Preventive fungicide sprays, particularly those containing copper, should be applied during periods of high rain to protect new growth and healthy leaf tissues.

Pruning strategies should focus on maintaining open canopies to ensure adequate sunlight penetration and airflow, which helps to keep foliage dry and inhibits fungal growth.

Balanced fertilization and good irrigation practices are essential to maintain tree vigor, allowing the plant to better resist initial infection attempts by the fungus.

Regular orchard monitoring is crucial; detecting the first signs of the disease allows for localized treatment and prevents the rapid spread of the pathogen throughout the farm.