What is Late Blight?
Late Blight is one of the most destructive and fast-moving plant diseases a home gardener can face. Infamous for causing the Irish Potato Famine in the 1840s, this disease can wipe out an entire tomato or potato crop in less than two weeks if conditions are right. It is a community disease; because its spores travel so easily on the wind, an infection in one garden can quickly threaten an entire neighborhood.
The culprit is Phytophthora infestans, a water mold or oomycete. Despite its fungus-like behavior, it is more closely related to algae. This is important because it thrives in very specific environmental conditions: cool, damp weather with high humidity and leaf wetness. It is most common in the northern United States but can appear anywhere when temperatures and moisture are favorable.
The primary targets in the home garden are:
- Tomatoes
- Potatoes
- (Occasionally) Other nightshade family members like petunias or eggplant
Life Cycle
Late Blight is a master of survival and rapid spread. The pathogen needs a living host to survive the winter. Its primary hideout is in infected potato tubers that were left in the ground ("volunteers") or discarded in compost or cull piles. When these infected tubers sprout in the spring, the pathogen grows with them.
Once active on a plant, during periods of high humidity and cool nights (50-60°F), the water mold produces millions of microscopic spores on the leaf surfaces. During the day, as temperatures rise (60-70°F), these spores are released and easily carried by wind and rain splash to neighboring plants. When a spore lands on a wet leaf, it can cause a new infection in just a few hours. This cycle of spore production, spread, and infection can repeat every 5-7 days, leading to an explosive and devastating outbreak.
Symptoms
Late Blight progresses with terrifying speed. Learning to spot the first signs is critical for any chance of saving nearby plants.
Early Stage
- Water-soaked spots on leaves: The first signs are often large (dime to quarter-sized), irregular, grayish-green or water-soaked looking spots on the leaves.
- Pale halo: These initial spots may have a pale green or yellowish ring around their edge.
Middle Stage
- Rapid lesion growth: The spots enlarge very quickly, turning dark brown to black and feeling greasy or leathery.
- White fuzzy growth: This is the key diagnostic sign. In humid conditions, especially in the early morning, a fuzzy white or grayish ring of spores will appear on the undersides of the leaves, right at the edge of the diseased spot.
- Stem infections: Dark brown to black, often elongated lesions appear on stems and leaf stalks. Stems can become brittle and snap easily.
Late Stage
- Widespread blight: Entire leaves and stems turn brown and shrivel, making the plant look like it was hit by a hard frost.
- Distinctive odor: A characteristic musty, decaying odor often accompanies a severe infection.
- Fruit and tuber rot: On tomatoes, firm, greasy-looking, dark brown blotches appear, usually on the shoulders of the fruit. The rot extends deep inside. On potatoes, the skin develops a reddish-brown discoloration, and a dry, granular rot forms just beneath the surface.
How to Control Disease Progression
Late Blight is a garden emergency. Once a plant is infected, it cannot be cured. The goal is immediate and ruthless containment to protect other plants in your garden and your neighbors'.
- Destroy Infected Plants Immediately: At the very first confirmed sign, carefully cut the plant at the base. Place it, along with any fallen debris, directly into a large plastic bag and seal it. Dispose of the bag in your municipal trash. Do not compost infected plants; the pathogen can survive and spread.
- Monitor Nearby Plants: Inspect all other tomato and potato plants in your garden daily for any new symptoms. Be prepared to remove them as well.
- Hold Off on Replanting: Do not plant new tomatoes or potatoes in that area for the rest of the season.
- Sanitize Tools: Thoroughly clean and disinfect any tools, stakes, or cages that came into contact with the infected plant using a 10% bleach solution or rubbing alcohol.
Treatment Options
All available treatments for Late Blight are preventative. They are designed to protect healthy plants from becoming infected. They will not cure a plant that already has the disease.
Organic Treatment Methods
- Copper-based fungicides: This is the primary organic control. Products containing copper sulfate or similar formulations create a protective barrier on the leaf surface that kills spores before they can infect the plant. They must be applied before the disease arrives.
Chemical Treatment Options
- Fungicides containing the active ingredient chlorothalonil are highly effective protectants against Late Blight. These are widely available at garden centers. Always read and follow the label directions precisely.
Step-by-Step Treatment Procedure
- Watch the Weather: Start your spray program when conditions are favorable for Late Blight (forecasts of cool, wet weather).
- Spray Before Infection: Apply your chosen fungicide to healthy, uninfected tomato and potato plants.
- Ensure Thorough Coverage: Coat all parts of the plant, including the tops and undersides of leaves and the stems.
- Reapply Regularly: Follow the product label for reapplication intervals, typically every 7 to 10 days during high-risk weather. You must reapply after any significant rainfall, as it will wash the product off the leaves.
Expected Timeline for Recovery
An infected plant will not recover. A successful treatment program is one where your healthy plants remain disease-free through the end of the season. Prevention is the only path to a successful harvest when Late Blight is present.
Prevention Strategies
Because control is so difficult, prevention is everything with Late Blight. A proactive approach is your best defense.
- Start Clean, End Clean: Always purchase certified disease-free seed potatoes. Never save potatoes from a crop that might have had blight. At the end of the season, meticulously remove all tomato and potato vines and hunt for any stray potatoes in the soil.
- Eliminate Volunteers: Be vigilant about removing any "volunteer" potato or tomato plants that sprout up in your garden from the previous year. These are the most common starting points for an outbreak.
- Choose Resistant Varieties: Many modern tomato hybrids offer excellent resistance to Late Blight. Look for varieties like 'Defiant PhR', 'Mountain Magic', 'Celebrity Plus', or 'Iron Lady'. For potatoes, varieties like 'Kennebec' or 'Elba' have some resistance.
- Maximize Airflow: Space your plants farther apart than the minimum recommendation. Stake, trellis, or cage your tomatoes to lift them off the ground. Prune lower leaves to improve air circulation around the base of the plant.
- Water Wisely: Use soaker hoses or drip irrigation to apply water directly to the soil. This keeps the foliage dry and makes it much harder for spores to germinate. If you must use a sprinkler, water early in the morning so the sun can dry the leaves quickly.
- Mulch Your Plants: A layer of straw or wood chip mulch can help prevent any soil-borne spores from splashing up onto the lower leaves of your plants.
- Stay Informed: Check with your local cooperative extension service or use online tools like the USAblight tracking map to see if Late Blight has been reported in your area. If it has, start your preventative spray program immediately.
