University of Maryland Extension

Late Blight FAQ's

late blight of tomato

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Late Blight FAQs

Q. What is it?

A. Late blight is a disease caused by a fungus-like microorganism that infects and kills tomato and potato plants. The pathogen, Phytophthora infestans, was responsible for the Irish potato famine of the 1840’s.

Late blight can occur at anytime during the growing season when the weather is cool and wet. The disease is slowed down by a return to hot, dry weather. Infections occur when the late blight pathogen and host plants (tomato and potato) are present in the same place, AND weather conditions are cool and wet.

Late blight is brought into the garden from infected transplants and seed potatoes or may be blown in from neighboring areas. However, more virulent strains of the late blight fungus may over-winter in soils and plant residues. Remove weeds in the tomato/potato family (e.g. horsenettle, Eastern black nightshade, jimsonweed) because they can also be infected.

Q. How do I know if I have it?

A. 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. The undersides of the lesions may be covered with a white fuzzy growth that contains the spores of the pathogen. On the stems, late blight lesions appear brown to almost black. Infected tomato fruits develop shiny, dark or olive-colored lesions which may cover large areas. Potato leaves and stems will show the same symptoms. Infected potato tubers develop a dry, corky rot that often shows up in storage.

Q. How does it spread?

A. Spores are spread short distances by rain and very long distances by wind.

Q. What’s the difference between early blight and late blight?

A. Early blight, Alternaria solani, is a common fungal leaf spot disease encountered by most Maryland gardeners. Early blight can infect tomato leaves anytime during the growing season and is favored by wet, humid conditions and moderate temperatures. Early blight rarely affect stems or fruits. Septoria leaf spot is another fungal leaf spot disease, like early blight, that can cause leaves to turn brown and die. As you can see in the photos below, early blight and late blight cause very different leaf symptoms. Late blight can occur at anytime during the growing season when the weather is cool and wet. The disease is slowed down by a return to hot, dry weather.

Q. What should I do if my plants have late blight?

A. You cannot “cure” this disease once you see the symptoms. You should immediately 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 to “bake” for a few days before putting it in a trash can. This will help to kill the pathogen and prevent it from infecting your garden next season. Do not attempt to compost infected potato or tomato plants.

Q. Can I eat harvest and eat tomatoes and potatoes from my plants if they get late blight?

A. Yes, but harvest and eat tubers right away. Don’t store them.

Q. Can I save seeds from my tomatoes?

A. The pathogen is not carried on seeds, but if infected fruits rot before seeds are mature, there won’t be any viable seeds to harvest.

Q. How do I prevent the problem?

A.     Buy locally grown tomato transplants or start your own at home.

  • Allow extra room between potato and tomato plants to encourage rapid drying of leaves.
  • Water around the base of plants and avoid wetting foliage in early evening.
  • Prevent weed growth; weeds can limit air circulation around plants and weeds in the tomato/potato family (e.g. Eastern black nightshade) can be infected by late blight.
  • Do not plant store-bought potatoes or tubers harvested from blighted plants during the previous season. Always purchase new seed potatoes that are certified, "disease-free".
  • ‘Kennebec’ and ‘Elba’ are potato cultivars that show some resistance to late blight. 'Iron Lady', 'Mountain Magic', 'Jasper', 'Matt's Wild Cherry', 'Legend', and 'Plum regal' are resistant tomato cultivars.
  • Keep developing tubers covered with soil to protect them from late blight.
    Do not compost store-bought potatoes.

Q. Should I spray my tomato and potato plants with a fungicide?

A.  In years when late blight is detected in your area, fungicides like chlorothalonil and mancozeb can help protect “clean”, un-infected plants if sprayed on all plant parts on a regular basis. Fixed copper is less effective but probably the best choice for organic gardeners. In all cases, you must apply the fungicide prior to infection for it to be effective. Always follow label instructions when applying fungicides.

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