Late blight is a fungal disease that can infect tomato and potato plants. The disease spreads rapidly and will likely destroy the infected plant in a few days. Wind- and water-borne fungal spores can quickly spread to other plants—including eggplants, and peppers.
Late blight commonly strikes when nights are cool and moist and days are warm and humid.
There is no reliable way to stop late blight once it takes hold in a garden. The best course is always to destroy infected plants immediately so that the disease does not spread.
Here are basics for identifying and treating late blight:
Target Plants: Tomato and potato
Tomatoes leaves: water-soaked spots show first on lower leaves; spots start out pale green diffuse irregular spots on upper side of leaf, usually near the edges of tips of leaves; spots turn brown to purplish-black and velvety with pale green border on underside of leaf; spots appear on young leaves at the top of the plant; spots look water-soaked; no concentric rings or defined border around necrotic spots. In humid, wet conditions, a fuzzy ring of mold around spot appears on the undersides of leaves. Leaves shrivel, turn brown, and die. Disease spreads rapidly.
Tomato stems: black and brown lesions appear on stem and petioles and spread; when petioles or leaf stalks are affected, entire leaf can collapse; entire vine or plant can quickly collapse and die in period of high humidity.
Tomato fruit: spots beneath the skin develop into reddish-brown dry, leathery, firm spots appear on the tops and upper sides of green fruit; spots grow larger; the skin wrinkles and darkens to chocolate brown; white mold forms in damp, humid conditions; soft rot sets in.
Potato leaves: small irregular pale to dark green spots appear—spots are surrounded by yellow rings; spots can grow across leaf veins; spots grow rapidly and enlarge in cool, moist weather becoming brown to purplish black. Entire leaflets may die; disease can grow down leaf stem to main stem. White fungal growth appears on underside of leaves.
Potato vines and stems: disease spreads from leaves across petioles to vine or main stem
Potato tuber: skin becomes discolored from brown to purplish; brownish dry, granular rot enters tuber extending ½ to ¾ inch into tuber. Rotting tubers have unpleasant odor.
How Spread: Late blight happens where there is high humidity and temperatures are between 60° and 80°F. It often strikes when nights are cool and moist and days are warm and humid. Late blight fungi spores are wind-, rain-, and soil-borne. Infection often occurs before plants blossom. Spores germinate readily at about 70°F.
Organic Control and Prevention:
- Keep plant foliage dry at all times; shelter plants in a clear plastic tent if weather is wet.
- Do not overhead irrigate. Water only at soil level.
- Check for symptoms every day when the weather is wet and humid.
- Treat plants with preventive sprays of compost tea or Bacillus subtilis.
- Pull and destroy the plant as soon as you recognize the disease; even if only a few leaves are infected. Dispose of infected material in a sealed plastic bag.
- Wash clothing and tools after handling infected plants.
- Hill potatoes to keep spores from reaching tubers.
- Remove all foliage from plants before harvesting potato tubers.
- Harvest potatoes only in dry weather.
- Do not store infected tubers or fruits.
- Select resistant varieties to plant in future; plant only certified disease-free seed potatoes.
- Do not plant tomatoes and potatoes close to one another. Plant crops in a different location each year. Rotate when plants have been infected.
- Clean all tools and clothing after working with infected plants.
- Copper fungal sprays can protect against infection, but spray must be thorough and repeated often.
- Apply fungicide containing mancozeb every seven days, starting when shoots are 6 inches tall and continue as long as vines are green.
Disease Range: United States and southern Canada, particularly in humid regions.
Scientific name: Phytophthora infestans
Prevention of Fungal Diseases:
- Improve air circulation around plants.
- Mulch around the base of plants to reduce water splashing.
- Do not use overhead watering.
- Control weeds.
- Rotate crops.