Overview:
- What causes late blight
- Symptoms of late blight
- Disease cycle of late blight
- How to manage late blight
Late blight is a highly destructive disease of potatoes and tomatoes caused by the oomycete Phytophthora infestans. This fungus-like pathogen attacks foliage, as well as tubers and fruits, causing them to rot and making them inedible.
Potato late blight is infamous for having been responsible for the Irish potato famine in the middle of the 19th century, causing the death of around one million people. Today, the disease continues to cause severe losses; for potatoes, these are estimated at around 20% annually worldwide.
Thankfully, biological pest control can help prevent and manage late blight, reducing the need for harmful chemical methods.
What causes late blight?
Late blight is caused by the oomycete (a fungus-like organism) Phytophthora infestans. It mostly attacks potatoes but can also affect tomatoes at any stage of crop development.
In wet conditions, late blight spreads very quickly across foliage and tubers/fruits and can cause the death of an entire plant in 7 to 10 days, if untreated.
The pathogen can also survive between seasons and can spread through infected seed tubers/tomato seedlings, or any infected plant material that remains after harvest (like potato-waste dumps).
What are the symptoms of late blight?
As late blight can progress very quickly, the earlier you spot the symptoms, the quicker you can implement management practices.
Watch out for the following symptoms:
- Discoloured areas on leaves: early signs of infection are pale-green areas. These rapidly develop into larger brown, watery-rot spots (or lesions), which rapidly expand to cover large areas of the leaf. The leaf eventually collapses, shrivels, and turns brown.
- Brown/black lesions on stems: these are often irregular and look greasy.
- Patches of discolouration on fruits/tubers:
- In potato tubers: patches of brown to purple discolouration on the skin which become darker and sunken with time. Red-brown decay can be found under the skin, which softens as the infection progresses. Some lighter attacks might not be visible on the tubers, but the potatoes can rot during storage.
- In tomato fruits: large sunken yellow or brown spots with distinct rings
- White fuzzy growth can sometimes be spotted on the underside of leaves, at the edge of lesion, in humid conditions.
Disease cycle of late blight
Phytophthora infestans, the microorganism causing late blight, reproduces similarly to a fungus. This means that it creates sporangia (‘spores’) to reproduce. These spores are very light and are easily dispersed by wind or rain.
Infected plants can infect new plants through the dispersion of these spores. Once spores come into contact with a plant leaf, they germinate and penetrate the plant, leading to the development of lesions. In the new plant, the pathogen spreads within the plant tissues. Under wet conditions, the pathogen produces fresh spores, and the cycle repeats.
It takes 3 to 5 days for spores to germinate and start penetrating the plant, and only 12 hours of moisture for an infection to occur. In ideal conditions, namely wet with daytime temperature of 15-21°C and nighttime of 10-15°, the whole disease cycle can be completed in just 5 days.
How to manage late blight
Prevention
Prevention is the best way to deal with late blight, as a late blight infections are far more difficult to cure once established. To prevent late blight from occurring in your plants, or to minimize damage if an infection occurs, there are several practices that you can implement.
- Use high-quality seeds and avoid reusing planting material e.g. tubers from a previous year
- Plant late blight-resistant or tolerant varieties
- Keep leaves as dry as possible:
- Space rows to ensure good air circulation
- Water early in the morning so foliage can dry during the day
- Avoid overhead watering
- Avoid planting in a wet or shaded area
- Don’t leave any unmonitored plants in the area
- Destroy volunteer plants
- Destroy (or bury) waste (no open cull piles during the growing season)
- Practice good sanitation:
- Crop rotation to prevent the buildup of the late blight pathogen in the soil
- Sanitize equipment
Control
Keep a close eye on the weather forecast to identify optimal conditions for late blight. If a late blight infection is detected, you need to act quickly.
Good sanitation is essential:
- Remove and destroy infected plants immediately, if feasible
- Do not compost diseased material
Biological products can be used to limit late blight spread and protect uninfected plants:
- Microbials: most biological plant protection products for use against late blight contain bacteria that are able to fight the pathogen. For example, Puelche and Serenade ASO are two products containing Bacillus species of bacteria.
- Natural substances can also act as fungicides and help to manage late blight infections, for instance products containing dipotassium phosphite such as Agrisolutions Confine Extra and Alude, or tea tree extract, such as Timorex Gold 238 EC
In areas where outbreaks of late blight are common, fungicides can be applied preventively when environmental conditions are favorable for infections.
- To identify products that are available and registered in your region for late blight control, use the CABI BioProtection Portal and select ‘late blight’ and your crop.
Harvest and storage
Proper harvest, handling and storage of potatoes and tomatoes is essential to prevent further late blight losses. Good practices include:
- Avoiding harvesting when conditions are wet
- Removing any damaged tubers/fruits
- Ventilating and drying tubers/fruits immediately after harvest
- Storing tubers/fruits in a cold, dry room and not letting them become wet
- Monitoring regularly for soft rot or dark patches that could indicate a secondary late blight infection
By integrating the above practices growers can significantly reduce the risk and impact of late blight post-harvest.
You can find more information on late blight management below:
- In Spanish: Manejo Integrado del Tizon tardio and Enfermedades de la papa: plataforma de evaluación de riesgo sanitario
- In English: Managing the risk of late blight
