Tomato and potato late blight

In 2009, the disease known as late blight affected many tomato and potato crops throughout the northern states. The disease was thought to have started in tomato transplants. This is a new strain of the pathogen not previously reported in the United States. The disease is an oomycete similar to downy mildew of many herbaceous species and is known as Phytophthora infestans. The new genotype is known as US-22 and causes late blight on tomatoes and potatoes. The pathogen can only survive in live tissue such as potato tubers and does not overwinter on tomato debris. In years when we have had a serious late blight epidemic, it is often followed by sporadic outbreaks in following years. The source in following years is usually potato tubers that have survived the winter in fields; known as volunteer tubers.

Once the disease has taken hold of the plant, there is very little that can be done to halt its progress to other plants other than destruction of the affected plants. Here is some information on the disease and some tips for growers for 2010.

  • The new strain of this disease can infect and kill plants within 10 days.
  • The disease can spread from tomato to potato plants and to surrounding gardens and commercial fields. The spores of this disease can be carried in the air.
  • In the past, losses of potatoes due to late blight have caused farming businesses to close throughout Michigan.

What can you do?

Tomatoes and potatoes

  • Plant transplants and potato seed with a certification label obtained from a reputable dealer.
  • Do not use home-saved tomato or potato seed.
  • Discard transplants that have discolored areas with white, threadlike growth and potato seed that have blemishes or appear to have sunken or purpled in areas on the surface.
  • Leave adequate space between transplants and seed tubers to allow maximum airflow for the growing plant.
  • Fungicides are available for small growers and gardeners that can prevent the disease from spreading between plants and crops. Such products are Daconil or Manzate (home gardeners) or an alternative would be Echo 720 (farmers).
  • Water plants in the evening to allow a drying period during the day.
  • Fungicides should be applied every seven days or as directed on the label. Apply in water at a rate of 50 gal/A.
  • Copper fungicides are less effective than those mentioned above but will help on plants that are not infected (can mix coppers with Daconil/Echo).
  • If symptoms develop (see images) pull up the diseased plant from the roots, place in a plastic bag, seal the bag and dispose of the bag without reopening it. The spores of this disease can be carried in the air.
  • Check plants daily and dispose of diseased plants immediately.
  • Destroy affected plants with Reglone or other like product (farmers) if removal is not possible.
  • Leave diseased tubers and waste tomatoes on the surface of the ground to freeze over winter.


  • Allow potato sprouts on seed to grow to about ½ inch before you plant.
  • Do not plant seed tubers into cold, wet soils.
  • Plant the seed tubers at a maximum depth of three inches and then increase hill size after emergence when plants are at least six inches tall but do not re-cover plants.
  • After harvest, check tubers regularly as infected tubers can quickly decompose. Discard infected tubers in a sealed plastic bag.

Professional/commercial growers have difficulty growing disease-free crops. Please help to maintain businesses and jobs in your area by following the above guidelines.

The Michigan Department of Agriculture has the authority to declare an area a nuisance and to eliminate the source of late blight inoculum in order to prevent the spread of disease to farmer’s crops.

For further assistance, contact your local MSU Extension Office.

US-22 genotype of Phytophthora infestans is shown in the following photos:

Tomato late blight.
Left, Late blight on green tomato fruit. Right, Late blight on ripe
tomato fruit.

Tomato late blight.
Late blight stem lesion on tomato.

Left, Late blight on tomato. Right, Late blight foliar lesion on tomato.

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