University of Maryland Extension

Late Blight of Tomato - Potato

tomato in container with late blight

Author: Jon Traunfeld

Will Late Blight be a Problem for Vegetable Gardeners this Season?

We hope not, but nobody knows the answer to this question right now. The late blight outbreak of 2009 was devastating for many backyard and community gardeners in Maryland and other East Coast states who witnessed the rapid decline and death of infected tomato and potato plants. In current years there have been scattered outbreaks of late blight but nothing as serious as what occurred in 2009.
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 soil. The most prevalent strain in Maryland seems to be US-23, an A1 mating type that can only overwinter on live plant tissue (e.g., potato tubers left in the ground). The reason for the severe outbreak in 2009 was the early introduction of inoculum on tomato transplants sold through retail stores, along with conducive weather (wet with cool nights and warm days).

Description

Late blight of tomato and potato is a potentially destructive fungal disease in Maryland home 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. The disease is slowed down by a return to hot, dry weather. Gardeners in cooler areas of Maryland may see late blight symptoms more regularly. The most recent widespread infection in home gardens occurred in 1994 and 2009.

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 mold will appear on leaf undersides. 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 infection is severe, a foul odor is produced in both crops.

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.

late blight fruit foliar symptoms
Foliage and fruit symptoms

late blight foliage
Early foliage symptoms

late blight advanced foliar symptoms
Advanced foliage symptoms on tomato 

 late blight fruit symptoms
Early fruit symptoms

late blight advanced fruit symptoms
Advanced fruit symptoms

Management

This is a destructive disease. Contact horticulture consultants at HGIC using the website portal and if you have any questions about your tomato and potato plants. The following steps can be taken to minimize late blight problems:

Tomato

  1. Buy locally grown transplants or start your own at home.
  2. Keep foliage dry; avoid overhead watering; avoid crowding your plants.
  3. Learn to distinguish the symptoms of late blight from those of other common tomato diseases, like early blight and Septoria leaf spot.
  4. 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.
  5. You cannot “cure” this disease once you see the symptoms. Again, remove the infected plants.
  6. Protectant fungicides, like chlorothalonil and fixed copper, can help protect foliage if applied prior to infection.

late blight fruit foliar symptoms
Foliage and fruit symptoms

tomato plant with advanced late blight
Advanced foliage and fruit symptoms

Potato

  1. Always purchase new seed potatoes that are certified, "disease-free".
  2. Keep developing tubers covered with soil.
  3. Protectant fungicides, like chlorothalonil and fixed copper, can help protect foliage if applied prior to infection.
  4. Do not compost store-bought potatoes and don't store infected tubers.

What can I do now?

  • Be observant and vigilant! Monitor your tomato and potato plants closely for symptoms of the disease.
  • At the end of the season, be sure to dig up and remove all potatoes (including small pieces).  Next spring plant only certified disease-free potato seed pieces.  Don't plant supermarket potatoes.
  • Contact the Home and Garden Information Center (HGIC) via Ask an Expert if you believe you have late blight. If possible, include a close-up photo of suspected late blight infections.

Additional Resources

Video

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