apple scab POMEWEST 

Timely reminder to keep an eye out

Recent reports confirm the presence of apple scab (black spot), caused by the fungus Venturia inaequalis, across several growing regions. Although the disease typically occurs at low levels in Western Australia, it’s prevalence has increased due to recent favourable weather patterns that support spore development and infection.

Words Susie Murphy White, Project Manager, Pomewest

APPLE scab can affect leaves, flowers, fruit, shoots and bud scales, thriving in cool, wet and humid conditions, with young leaves and fruit being the most vulnerable. Spots appearing in December often originate from primary infections established during the cool, wet periods of spring.

Secondary infections through summer

Secondary infections may continue throughout the summer wherever black spots are already established, and adequate moisture is also present. Heavy dews, prolonged or overhead irrigation, and rainfall can all trigger new infections.

Spores spread readily via wind, equipment, clothing, insects and birds, and once deposited, require high humidity (>95%) and free moisture to germinate.

Under suitable conditions (temperatures under 26ºC), lesions can develop within days, allowing rapid and repeated infection cycles throughout summer.

While leaves develop some resistance approximately 16 days after emergence, fruit remain susceptible until harvest. Later season infections may be subtle, taking 9 days for the visual lesions to appear at the optimal weather conditions, and can be often missed during picking, only to develop later as storage scab.

Inspection priorities for growers

Growers are advised to maintain regular, thorough monitoring across their orchards, with particular focus on susceptible varieties such as Cripps Pink, Cripps Red, and Granny Smith.

When assessing blocks, be sure to inspect:

• Both leaf surfaces and fruit

• Slow drying or shaded areas

• Edges near roadways and entry points

• Both inside and outside of canopies

• “Water trap” areas, where moisture lingers on leaf surfaces

Monitoring should continue after harvest, as older leaves become less resistant and Western Australia’s long, mild autumn favours disease spread.

Leaves often fall slowly, giving spores continued opportunities to infect new tissue.

If an infection is detected, growers can send samples to Dr Andrew Taylor at DPIRD Bunbury for testing through Curtin University’s Centre for Crop and Disease Management (CCDM). These samples will support Andrew’s research project on developing DNA based analysis and new monitoring tools to guide orchard management strategies that reduce fungicide resistance risk and assist plant breeders in identifying genetic selections for future tolerant varieties.

FIGURE 1. Life cycle of apple scab (black spot) caused by the fungus Venturia inaequalis.

Source: Compendium of apple and pear diseases and pests (2014)

Reducing winter inoculum

Winter orchard sanitation remains a crucial step in reducing spring infection pressure. Recommended practices include:

• Promoting leaf fall

• Applying foliar urea

• Mulching fallen leaves

These measures help break down infected material and reduce the reservoir of overwintering spores. Winter clean up should be treated as standard practice, similar to management approaches for pear scab, Mediterranean fruit fly, and Alternaria diseases.

Preventing spread between blocks

If apple scab is confirmed in an orchard, growers should immediately implement measures to limit its spread to neighbouring blocks. This includes controlling traffic from machinery, picking bins, visitors, contractors, and work crews. Spores can also be transported via leaves, fruit, bins, tools, and workers, though moisture is still required for infection to occur.

A review of current farm practices is recommended to identify risks and implement practical solutions.

Chemical management and weather monitoring

Weather forecasts and the decision support tool RIMpro should be monitored closely to anticipate infection events. Where infections have occurred without prior protection, curative sprays should be applied within the product’s recommended kick-back period.

Growers should review available chemical options and adhere to resistance management guidelines. A list of registered products for apple scab control is available in the Orchard Plant Protection Guide for Deciduous Fruit in NSW 2025–26.