Blossom blight
Registered against on 3 crops
Blossom blight is a destructive disease complex caused by fungal pathogens that attack the flowering structures of fruit and tree nut crops during bloom. In cherry orchards, the primary agent is Monilinia fructicola (or M. laxa), causing brown rot blossom blight. In tree crops like macadamia and mango, pathogens such as Botrytis cinerea, Colletotrichum, and related species blights the delicate petals, racemes, and panicles whenever warm, humid weather coincides with flowering.
For Australian commercial growers, blossom blight strikes at the most critical stage of yield formation. Direct destruction of open flowers severely reduces potential fruit set, directly eroding crop yield and farmgate returns. Furthermore, infected floral tissues allow fungal pathogens to establish latent infections or move into spurs, racquets, and twigs, causing dieback and creating persistent inoculum sources that fuel severe post-harvest fruit rots later in the season.
Where it matters in Australia
Blossom blight is a major challenge across multiple distinct production regions in Australia. In cherry blocks across temperate growing districts such as Batlow, the Goulburn Valley, the Adelaide Hills, and Tasmania, wet spring conditions regularly drive Monilinia infections. In macadamia orchards situated in the Northern Rivers of New South Wales and around Bundaberg in Queensland, Botrytis blossom blight poses a high risk during damp flowering periods. In tropical mango production zones, including Bundaberg, the Atherton Tableland, the Burdekin, and Darwin/Katherine, floral blights disrupt panicle development and early fruit set.
Symptoms
In cherries, initial symptoms appear as small, water-soaked brown spots on petals that quickly spread to engulf the entire flower. Infected blossoms collapse, turn brown, and remain glued to the spur in sticky clusters, often developing visible tan-to-grey powdery fungal spore masses.
In macadamias, developing flower racemes turn brown to dark grey, collapse, and dry out into brown strings, frequently covered in velvety grey spore clusters (Botrytis blossom blight).
In mangoes, floral panicles display dark brown or black necrotic lesions, leading to premature blossom drop and blighted panicles that fail to set fruit.
Life cycle
The pathogens responsible for blossom blight carry over between seasons in mummified fruit, dead floral racquets, panicle remnants, infected twigs, and fallen canopy debris on the orchard floor. As temperatures rise in spring or during crop-specific flowering windows, moisture from rain, heavy dew, or overhead irrigation triggers the release of airborne or rain-splashed conidia.
Once spores land on susceptible petals, anthers, or pedicels, germination occurs rapidly under humid conditions. The fungus invades delicate floral tissues, leading to blossom necrosis. Under ongoing wet weather, infected blossoms produce massive secondary crops of spores within days, driving rapid secondary spread across the block.
Monitoring
Scouting should begin prior to flowering as flower buds, racemes, or panicles emerge, continuing through full bloom until petal fall. Inspect blocks at least weekly, increasing frequency to twice weekly during prolonged wet or foggy weather.
Examine open flowers, racemes, and panicles in both the upper and lower canopy across representative parts of the orchard. Look for early signs of petal browning, water-soaked spots, or premature flower drop. Track weather forecasts closely to anticipate rain events during key bloom stages, recording all field observations and control measures in the orchard spray diary.
Management
Managing blossom blight relies on an integrated approach combining orchard sanitation, canopy management, and carefully timed protectant fungicide sprays.
Prune trees during winter or post-harvest to open up the canopy, promoting rapid drying of flowers and foliage after rain. Remove and destroy mummified fruit, dead racemes, and diseased twigs from the block to minimise carryover inoculum. Avoid overhead irrigation during the flowering window to prevent artificial leaf and blossom wetness.
When chemical control is warranted, time protectant applications prior to rainfall events during high-risk flowering stages. Rotate between distinct mode-of-action groups in accordance with fungicide resistance management strategies. Always read the approved label for specific usage instructions, withholding periods (WHP), and application constraints.
Crops with registered control options
Common questions
- How do I distinguish fungal blossom blight from frost damage or bacterial blast?
- Bacterial blast often causes sudden dark systemic stem dieback and leaf spotting associated with cold frost events, without prolific spore growth. Blossom blight typically displays fluffy or velvety spore masses (grey, tan, or dark grey) on infected petals and pedicels during warm, humid or wet conditions during bloom.
- Why does the registered uses table show significantly more products for cherry than macadamia or mango?
- The registered product counts reflect differences in crop acreage, commercial market demand, chemical registration histories, and specific target pathogens across stone fruit, tropical fruit, and tree nuts. Always ensure a product is explicitly registered or permitted for your specific crop before use.
- When during the season is the crop most vulnerable to blossom blight infection?
- The key susceptibility window starts at early bloom (bud break or early raceme/panicle emergence) through to full bloom and early fruitlet set. Protectant applications are most effective when applied prior to predicted rain or high-humidity fog events during these flowering stages.
- How should resistance management be incorporated into a blossom blight spray strategy?
- Always rotate between different chemical activity groups (FRAC codes) rather than relying on consecutive sprays from the same group. Combine protectant fungicides with sound cultural sanitation to lower overall inoculum pressure and reduce selection pressure for resistant strains.
- Where can I find the exact withholding period and application rate for my specific block?
- Application rates, water volumes, re-entry intervals, and withholding periods (WHP) vary by product, crop, and region. Never estimate these figures; always read the approved product label attached to the container or consult the APVMA database before mixing.