Botrytis (grey mould)
Registered against on 9 crops
Botrytis (grey mould), caused by the fungal pathogen Botrytis cinerea, is one of the most widespread and costly diseases affecting Australian horticulture. It attacks a broad range of high-value crops, causing pre-harvest fruit rot, flower blights, stem cankers, and severe post-harvest decay. The disease is particularly destructive in wine and table grapes, tomatoes, macadamias, pome fruit (apples and pears), stone fruit (cherries), and citrus crops (lemons, limes, and oranges).
Economic loss occurs through direct yield reduction, degraded fruit quality, and rejected consignments. In wine grapes, Botrytis contamination damages berry skin integrity and introduces off-flavours and oxidizing enzymes (laccase) that ruin juice quality. In macadamias, flower raceme blight reduces nut set, while in fresh produce like tomatoes, apples, and cherries, latent infections lead to rapid breakdown during transport and storage.
Because Botrytis thrives in high humidity and free moisture, crop losses can escalate rapidly if microclimates inside the plant canopy are unmanaged or if cool, wet weather coincides with vulnerable growth stages such as flowering and harvest.
Where it matters in Australia
Botrytis is a major pathogen across diverse Australian agricultural regions. It presents severe risks in viticulture across southern Australia (including the Yarra Valley, Adelaide Hills, Tasmania, Sunraysia, and the Riverland), pome fruit orchards in the Goulburn Valley and Batlow, macadamia plantations in the Northern Rivers and Bundaberg, and field/protected tomato production in Bundaberg and southern growing districts. Wet, humid conditions during bloom or harvest trigger localized outbreaks regardless of latitude.
Symptoms
Symptoms vary slightly by host crop but share a defining characteristic: dense, velvety grey-to-brown fungal spore mats under humid conditions.
- Grapes: Early infection shows as translucent, water-soaked spots on berries ("slip-skin" stage), followed by soft brown rot and grey fuzz covering entire bunches.
- Tomatoes: Small, pale ring spots with light halos ("ghost spots") appear on green fruit from aborted spore infections. Mature fruit displays soft rot, while lower stems develop sunken brown cankers covered in grey mould.
- Macadamias: Blighting of flower racemes, where buds turn brown, wither, and drop prematurely with visible grey fungal growth on pedicels.
- Apples, Pears, Cherries & Citrus: Soft, watery brown lesions on calyxes or fruit surfaces, developing nest-rot in clusters or post-harvest storage.
Botrytis is distinguished from Sclerotinia (which produces cottony white mycelium and large black sclerotia without dense grey spore mats) and Penicillium (which produces blue-green powdery mould).
Life cycle
The pathogen overwinters as sclerotia (hardened fungal masses) or saprophytic mycelium in crop residues, vine canes, prunings, fallen leaves, and mummified fruit left in the block or orchard. In spring, under damp conditions, these structures produce millions of airborne conidia (spores) that travel on wind currents and rain splash.
Primary infections occur when spores land on susceptible, moist tissues—especially senescing flower petals, floral caps, or fresh wounds. Botrytis frequently establishes latent (quiescent) infections during flowering, remaining dormant inside developing young fruit without showing visible symptoms. As fruit matures, sugars rise, and acidity drops, the fungus reactivates, causing rapid tissue breakdown. Secondary spread occurs rapidly throughout the canopy during humid periods as newly sporulating lesions infect neighboring healthy fruit clusters.
Monitoring
Begin monitoring early in the season prior to flowering (e.g. capfall in grapes or early bloom in tree crops) and continue through cluster closure, fruit ripening, and pre-harvest. Inspect at least 20 to 30 plants or trees per block, focusing on sheltered microclimates, low-lying frost pockets, dense foliage zones, and rows downwind of prevailing wet weather.
Look for early signs of flower blight, water-soaked lesions, or senescing petals sticking to developing fruit. Perform a "slip-skin" check on maturing grape berries by gently rubbing the skin to see if it slides off the pulp easily. Keep close track of local weather forecasts; continuous leaf wetness combined with high relative humidity indicates immediate risk, requiring preventative action before heavy rain events.
Management
Effective Botrytis control relies on an integrated strategy combining canopy management, sanitation, targeted biologicals, and fungicide timing:
- Cultural Practices: Open up tree and vine canopies through canopy trimming, shoot positioning, and leaf plucking around fruit zones to maximize light penetration, improve airflow, and speed up drying times after rain or irrigation. Manage nitrogen applications to avoid excessively lush foliage. Maintain strict orchard hygiene by removing prunings, trash, and mummified fruit from the block.
- Biological Controls: Apply registered bio-fungicides (such as antagonist bacteria or fungi) during bloom to colonize flower parts and outcompete Botrytis spores on infection sites.
- Chemical Control & Resistance: Apply protectant or systemic fungicides strategically during critical vulnerability windows—such as flowering, cluster closure, veraison, and pre-harvest. Botrytis carries a very high risk of developing fungicide resistance. Always rotate between different chemical activity groups (FRAC groups) according to CropLife Australia resistance management guidelines, avoiding back-to-back sprays of the same mode of action. Consult approved product labels for WHP, re-entry intervals, and crop safety rules.
Crops with registered control options
Common questions
- Why is flowering considered a critical management window for Botrytis?
- Botrytis readily colonizes senescing floral tissues like caps and stamens during bloom. The fungus can enter a dormant phase inside the young fruit tissue and remain hidden until fruit sugars increase near harvest, leading to unexpected pre-harvest rot.
- How does canopy architecture influence grey mould severity?
- Dense canopies trap humidity and block sunlight, creating a humid microclimate where fungal spores germinate rapidly. Opening the canopy via leaf plucking or shoot positioning speeds up foliage drying and improves fungicide spray penetration into fruit clusters.
- How should I approach fungicide resistance management for Botrytis?
- Botrytis cinerea adapts quickly to fungicides. To preserve chemistry efficacy, restrict the number of applications of any single FRAC group per season, rotate between non-cross-resistant activity groups, and integrate cultural controls to keep overall disease pressure low.
- Where do I check withholding periods (WHP) for fresh market versus processing crops?
- withholding periods vary depending on whether the crop is destined for fresh market sales, processing, or export destinations. Always read the approved product label attached to your specific chemical container before spraying, as WHPs differ across crop end-uses.
- Why do grape and tomato registrations outnumber citrus and stone fruit for Botrytis?
- The registered-use table reflects historical chemical registration investments and crop susceptibility. Grapes and tomatoes experience severe, direct fruit rot pressure requiring multi-site and systemic options, whereas Botrytis in citrus is primarily an opportunistic or post-harvest concern. Always ensure the specific crop and pest combination is printed on the product label before use.