Late Blight
Phytophthora infestans
The pathogen behind the Irish potato famine. Water-soaked patches on tomato and potato leaves, collapsing the plant within days in cool wet weather.
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Identify from a photoBackground
Late blight, caused by the oomycete Phytophthora infestans, is a devastating disease of potatoes and tomatoes. In the right cool wet weather it can destroy an entire planting in 5-10 days. Lesions start as water-soaked greyish patches on leaves, rapidly browning and spreading. In humid weather a white fuzzy growth appears on leaf undersides at the margin of lesions.
Australia sees periodic serious outbreaks in potato-growing regions, especially in Tasmania, the Adelaide Hills, and cool upland Victoria.
How to identify
- Water-soaked greyish-green blotches on leaves, rapidly browning
- White downy fungal growth on leaf underside at lesion margin in humid weather
- Brown lesions on petioles and stems, can girdle the stem
- Tomato fruit: dark greasy patches that rapidly rot; potato tubers: reddish-brown sunken lesions with firm rot below
Life cycle
Sporangia airborne in cool damp weather, germinate directly or release swimming zoospores. Infection to sporulation in 3-5 days under ideal conditions.
Weather triggers
- Temperature: 10-22°C
- Humidity: >90%
- Rainfall trigger: >10mm
- Classic conditions: nights with high humidity and leaf wetness > 10 hours, days of 15-20C. Prolonged cool wet spells are peak risk.
Peak season (southern hemisphere)
Red = active season · Dark red outlined = this month
Affected vegetables & crops
Click any crop to see current prices and growing info.
Climate zones at risk
Organic & low-impact control
- Copper-based sprays as a protectant before weather risk periods
- Destroy affected plants promptly. Do not compost
- Remove volunteer potatoes which act as inoculum reservoirs
Chemical control
- Protectant fungicides (mancozeb, chlorothalonil) and systemic fungicides (metalaxyl) used commercially
- Home gardeners should focus on prevention and copper
Always read product labels. Registrations change.
Prevention
- Plant certified disease-free seed potato
- Avoid overhead watering, especially in cool weather
- Space plants generously for airflow
- Remove volunteer potatoes that overwinter in the compost or garden
Companion planting
No reliable companion prevents late blight.
Biosecurity
New aggressive genotypes (A2 mating type) have appeared in parts of Australia. Remain vigilant.
Know when Late Blight is actually about
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