Husk spot
Registered against on 1 crops
Husk spot, caused by the fungus Pseudocercospora macadamiae, is one of the most economically significant diseases affecting Australian macadamia orchards. The pathogen targets the green outer husk of developing nuts, leading to premature nut drop before the kernel has fully matured and accumulated oil.
Where it matters in Australia
Husk spot is a major fungal threat across Australia's main macadamia producing regions, particularly in the warm, humid growing districts of the Northern Rivers in New South Wales, South East Queensland, and Bundaberg. Disease pressure is highest in orchards where spring rainfall coincides with early nut development.
Symptoms
Initial symptoms appear on green husks as small, circular, bright yellow to chlorotic spots. As the infection progresses, these lesions expand into distinct circular brown or black spots, often retaining a yellow halo. Under humid conditions, a velvety grey-brown spore layer may become visible in the centre of the lesion.
Severe infections cause the husk tissue to die and turn hard and woody, prompting the tree to drop the nut prematurely. Unlike healthy natural nut drop, husk spot causes premature fall of immature nuts with low kernel recovery. Husk spot can be distinguished from anthracnose by its defined circular spots and grey spore mat, whereas anthracnose typically produces darker, spreading decay often accompanied by bright salmon-pink spore masses in wet weather.
Life cycle
The pathogen overwinters primarily in old, diseased husks that remain hanging in the tree canopy (known as sticktights) or on the orchard floor. During periods of warm, wet weather in spring and early summer, mature spores are released from these reserves and spread short distances by rain splash and wind drift to young, developing green husks.
Infection typically occurs during the early stage of nut growth, from pea-sized nut set through to shell hardening. However, the fungus exhibits a long latent period, often taking several weeks to months after initial infection for visible spots to appear. Once established, infected husks dry out prematurely and trigger an abscission layer, causing the nuts to drop early. Many infected husks fail to detach cleanly and remain in the canopy as sticktights, providing inoculum for the following season.
Monitoring
Scouting should begin prior to flowering by evaluating the block history and assessing the canopy for lingering sticktights from previous seasons. Orchards with high sticktight counts represent a high inoculum risk.
Direct nut monitoring begins at nut set (around matchhead to pea size) and continues through early summer until shell hardening. Inspect green husks in the lower and mid-canopy for early yellow flecks. Collect and inspect early fallen nuts to differentiate between natural crop thinning and premature drop caused by husk spot infection. Keep detailed spray diaries and orchard records to track disease pressure across individual blocks.
Management
Managing husk spot relies on an integrated approach combining cultural sanitation, canopy management, and strategic targeted fungicide sprays.
Cultural management focuses on reducing inoculum and improving airflow. Prune trees after harvest to open the canopy, encouraging faster drying of foliage and husks following rain. Mechanically remove or knock down sticktights during winter pruning to break the disease cycle. Ensure orchard floor management promotes the breakdown of fallen husk material.
Chemical management requires preventive cover during the peak infection window between nut set and shell hardening. Rotate between different fungicide mode-of-action groups (FRAC codes) to manage resistance risks. Always consult the approved APVMA product label for specific application timing, withholding periods, and maximum allowable sprays per season.
Crops with registered control options
Common questions
- Why does husk spot cause premature nut drop so long after initial infection?
- The pathogen Pseudocercospora macadamiae has a long latent period. Infection takes place during early nut development when husks are young, but visible lesions and subsequent tissue necrosis develop weeks or months later. The dead husk tissue triggers the tree to form an abscission zone, causing immature nuts to drop.
- How do I choose between the registered fungicide options listed for macadamias?
- Select products based on your orchard's resistance management strategy, crop stage, and planned harvest interval. Ensure you alternate between different FRAC groups across the season. Always check the individual product label for specific application directions, registered uses, and re-entry or harvest withholding periods.
- Why is removing sticktights from the canopy so critical for control?
- Old infected husks hanging in the tree serve as the primary carryover source for fungal spores from one season to the next. Removing sticktights significantly reduces the initial spore load in spring, making protective spray programs far more effective.
- How can I distinguish husk spot from anthracnose on damaged nuts?
- Husk spot typically causes distinct, circular tan-to-brown spots with a yellow margin and may show a velvety grey spore mass in moist conditions. Anthracnose creates darker, rapidly spreading dark spots or rots that frequently develop salmon-pink spore masses during wet weather.
- Where can I find withholding periods (WHP) and maximum application rates for husk spot sprays?
- Specific application rates, spray volumes, withholding periods, and seasonal application limits are legally defined on the APVMA-approved chemical label affixed to the product container. Always read and follow the label instructions prior to mixing and application.