Downy mildew
Registered against on 2 crops
Downy mildew, caused by the oomycete Plasmopara viticola in viticulture, is one of the most destructive wet-weather diseases affecting Australian grapevines. Related oomycete species can also impact solanaceous crops like tomatoes. Unlike true fungi, downy mildew pathogens are water moulds that thrive in warm, humid conditions and rely on free water on plant surfaces for spore germination and tissue infection.
An unmanaged outbreak can cause devastating yield losses within days. Severe early infection leads to flower cluster abortion, crop destruction, and extensive foliage damage. Severe defoliation late in the season weakens the vine, reducing cane maturation and sugar accumulation, which directly impacts fruit quality and lowers crop yield in subsequent seasons.
For commercial growers, the financial cost includes direct tonnage losses, downgraded fruit quality, and substantial expenditure on protective fungicide programs and canopy management interventions.
Where it matters in Australia
Downy mildew is a major disease across Australian grape-growing regions, particularly in warm, wet summer rainfall districts such as the Hunter Valley and Hastings River, as well as temperate regions subject to spring and summer rain events like the Yarra Valley, Mornington Peninsula, Riverina, Sunraysia, and the Barossa Valley. In tomatoes, downy mildew pressure sporadically affects field blocks in wet coastal and sub-tropical growing districts such as Bundaberg and Bowen when high relative humidity and warm conditions coincide.
Symptoms
On grapevines, primary infections appear on upper leaf surfaces as distinct, pale yellow, translucent spots known as 'oilspots'. Following warm, humid nights, a dense, cottony, white downy growth develops directly beneath these spots on the lower leaf surface.
As the infection progresses, oilspots turn brown, dry out, and cause premature leaf drop. Infected young flower clusters turn yellow-brown, wither, and abort. Infected berries turn grey-brown, shrivel, and drop off the bunch. In tomatoes, leaf symptoms present as pale green to yellow spots that rapidly turn dark brown or black, accompanied by fine, sparse growth on the leaf underside under high humidity.
Life cycle
Overwintering occurs primarily as thick-walled oospores in leaf litter and soil within the vineyard or block. In spring, when soil temperatures rise and sufficient rainfall occurs, resting oospores germinate to produce sporangia. Rain splash transfers these sporangia onto young, susceptible foliage and green tissue.
Once on the foliage, sporangia release swimming zoospores that enter open stomata in the presence of surface moisture. Following primary infection, the pathogen grows internally and produces fresh sporangiophores through leaf stomata during warm, dark, humid nights, appearing as dense white down on the leaf underside. These secondary spores are dispersed by wind and rain splash, driving rapid epidemic cycles through the canopy whenever warm, wet conditions persist.
Monitoring
Monitoring should commence early in the season around budburst and continue through to harvest, with maximum vigilance during rapid shoot growth, flowering, and bunch set. Track local weather forecasts and weather station data for primary infection criteria—specifically periods combining warm overnight temperatures and adequate rainfall.
Inspect vulnerable young foliage and dense canopy zones weekly, focusing on low-lying or sheltered patches in the block where leaf wetness lingers. Examine the upper surface of leaves for translucent oilspots and inspect the lower surface early in the morning for fresh white sporulation. Pre-infection forecasting and early detection of initial oilspots are critical decision points to trigger management interventions before secondary spread occurs.
Management
Effective downy mildew control relies on an integrated strategy combining canopy architecture management, weather-driven forecasting, and strategic chemical applications. Cultural controls focus on opening the canopy through shoot thinning, canopy trimming, and leaf plucking to increase airflow, accelerate leaf drying, and ensure spray coverage reaches vulnerable inner foliage. Managing mid-row cover crops and improving block drainage help reduce humidity within the crop zone.
Chemical management balances protectant fungicides (applied prior to rain to prevent spore germination) and systemic post-infection treatments (applied shortly after an infection event before symptoms develop). To manage fungicide resistance, growers must strictly adhere to CropLife Australia resistance strategies by rotating active ingredient groups, avoiding consecutive applications of single-site systemic products, and keeping detailed records in the block spray diary. Always consult the approved APVMA label for specific withholding periods, re-entry intervals, and application limits.
Crops with registered control options
Common questions
- Why does the registration table show hundreds of products for grapes but very few for tomatoes?
- The product numbers reflect active APVMA label registrations. Grapevines have a large market support network with numerous protectant and systemic formulations registered over decades. For tomatoes, downy mildew options on label are limited. Always review the target crop list on the approved APVMA label before selecting a chemical.
- How do I decide between a protectant fungicide and a systemic post-infection option?
- Protectant fungicides must be applied before weather events to coat vulnerable green tissue, making them the primary choice for preventative cover before rain. Systemic or post-infection fungicides are selected when rainfall prevents pre-infection spraying, acting on the pathogen inside leaf tissue shortly after an infection event occurs. Choose based on weather forecasts, canopy growth stage, and CropLife resistance rotation guidelines.
- How do I distinguish downy mildew from powdery mildew in the field?
- Downy mildew produces translucent, yellowish 'oilspots' on the upper leaf surface with dense, cottony white down strictly on the underside. Powdery mildew forms ash-grey or white powdery patches on both sides of leaves, shoots, and stems, and is often accompanied by a distinct musty odour.
- Why is overnight humidity critical to downy mildew development?
- Downy mildew relies on dark, highly humid conditions (typically above 98% relative humidity) combined with warm temperatures for secondary sporulation to emerge from stomata on the lower leaf surface. Dry or windy nights halt secondary spore production, preventing rapid canopy-wide spread.
- What key details should I verify on the chemical label before spraying?
- Check the registered crop status, maximum seasonal application limits, re-application intervals, WHP for harvest, export slaughter or spray diary requirements (such as AWRI Dog Book rules for wine grapes), and required buffer zones. Always read the physical APVMA label on the container.