Late Blight of Tomato and Potato

Key points about late blight

• Late blight (Phytophthora infestans) is the devastating water mold disease that caused the Irish potato famine in the 1840s.
• The disease affects all parts of tomato and potato plants.
• It can become active in cool, wet weather.
• It rarely occurs in Maryland.
• USABlight is a national project to monitor for late bright outbreaks. 
• If you suspect late blight on your tomato or potato plants, please submit close-up photos through the Ask Extension service.

What is late blight and how does it spread? 

Late blight is a potentially destructive water mold or fungus-like disease that can affect tomatoes and potatoes in Maryland fields and gardens. This is the same disease that caused the Irish potato famine in the 1840s. Late blight can occur at any time during the growing season when the weather is cool and wet. Disease spores are spread short distances by rain and very long distances by wind. The disease is slowed down by a return to hot, dry weather.

Gardeners in cooler areas of Maryland are more likely to see late blight symptoms. Late blight is sometimes brought into the garden from infected transplants and seed potatoes. The most recent widespread infections in home gardens occurred in 1994 and 2009.

Symptoms of late blight

When plants are infected with late blight:

• Lesions develop on leaves and stems as dark, water-soaked spots. These spots enlarge until the entire leaf or stem turns brown and dies. Dead leaves typically remain attached to stems.
• Under favorable conditions, a white fuzzy mold will appear on leaf undersides that contain the spores of the pathogen.
• Stem lesions appear brown to almost black.
• Infected tomato fruits develop shiny, dark, or olive-colored lesions which may cover large areas.
• Infected potato tubers develop a dry, corky rot that often shows up in storage. When the infection is severe, a foul odor is produced in both crops.
• Infected tomato and potato plants often decline rapidly and die.

Late blight in Maryland

2009 outbreak

A number of strains of Phytophthora infestans were identified from different east coast states in 2009. The number of strains present is not as important as whether or not two mating types occur. For sexual reproduction to occur, two mating types- A1 and A2- must be present. The presence of the two mating types allows the disease to overwinter in the soil without tomato or potato plant tissues. The most prevalent strain in Maryland and the Eastern U.S. is US-23, an A1 mating type that can only overwinter on live plant tissue (e.g., potato tubers left in the ground), good news for farmers and gardeners.

Management of late blight in a garden

For tomatoes

1. Buy locally grown transplants or start your own at home.
2. Select late blight-resistant varieties if late blight is a concern in your area.
3. Keep foliage dry; avoid overhead watering; avoid crowding your plants.
4. Learn to distinguish the symptoms of late blight from those of other common tomato diseases, like early blight and Septoria leaf spot.
5. Pull out and remove plants with late blight symptoms. This will protect your neighbors’ gardens and local farmers. Put plants in a large plastic bag, seal the bag and leave it out in the sunshine for a week before putting it out with the household trash for pick-up. Do not attempt to compost infected plants.
6. You cannot “cure” this disease once you see the symptoms. Protectant fungicides, like chlorothalonil, mancozeb, and fixed liquid copper, can help protect foliage if applied prior to infection. Copper sprays are acceptable for organic gardening.
7. Remove weeds in the tomato/potato family (e.g. horsenettle, Eastern black nightshade, jimsonweed) that can also serve as host plants.

For potatoes

1. Always purchase new seed potatoes that are certified, "disease-free". Don’t plant store-bought potatoes or tubers harvested from blighted plants during the previous season.
2. Select late blight-resistant varieties if late blight is a concern in your area.
3. Keep developing tubers covered with soil.
4. Protectant fungicides, like chlorothalonil and fixed copper, can help protect foliage if applied prior to infection.
5. Pull out and remove plants with late blight symptoms. This will protect your neighbors’ gardens and local farmers. Put plants in a large plastic bag, seal the bag and leave it out in the sunshine for a week before putting it out with the household trash for pick-up. Do not attempt to compost infected plants.
6. Harvest and eat tubers from infected plants right away. Don’t store them.
7. At the end of the season, dig up and remove all potatoes (including small pieces). 
8. Don’t attempt to compost store-bought potatoes that go bad.
9. Remove weeds in the tomato/potato family (e.g. horsenettle, Eastern black nightshade, jimsonweed) that can also serve as host plants.

Additional resources

USABlight: A National Project on Tomato & Potato Late Blight

Late Blight: A Serious Disease of Potatoes and Tomatoes