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Seven steps to prevent late blight of tomatoes in 2018

M. Grabowski, UMN Extension Educator

In 2017, a disease of tomatoes and potatoes, called late blight, appeared in Minnesota farms and gardens near the end of summer. This devastating disease rots leaves, stems, fruit and tubers of potatoes and tomatoes.

Many gardeners lost most of their home grown tomatoes just as they were ripening last year and want to know what can be done to prevent late blight from striking again.

Tomatoes infected with late blight. M. Grabowski, UMN Extension

Late blight killed my tomato plants last year. Can I grow tomatoes in my garden this year?

Yes. The good news is that the late blight pathogen does not survive Minnesota’s winters very well. It can only survive on potato culls or tomato fruit that are sheltered in soil or a warm compost pile. Other common leaf spot diseases of tomato, like early blight and Septoria leaf spot, do survive in plant debris in the soil, however so it is a good idea to move tomatoes a minimum of 8 to 10 feet away from where they were grown last year.

Will late blight be a problem this year? 

It’s possible that late blight could be a problem in Minnesota in 2018 but not guaranteed. Since the late blight pathogen does not survive Minnesota winters very well it needs to be brought into the state for disease to occur. It can come in on infected tomato transplants, infected potato seed, or through wind-blown spores from warmer states.

Once it arrives, disease will only be a problem if the weather is wet and rainy. Late blight is caused by a water mold and needs high levels of moisture to thrive.

What can I do to prevent late blight this year?

  1. Pull any tomato plants or potato plants that came up where you did not intentionally plant them. These volunteer plants arise from potato tubers or tomato fruit that were left in the garden or in the compost pile last year.

    Although the late blight pathogen does not survive Minnesota’s winters on its own, it can be protected by a warm compost pile or in tubers or fruit buried in the soil. If these left over tomatoes and potatoes sprout in your garden, they may be carrying the late blight pathogen. Pull them as soon as you see them and bury them in a compost pile that heats up to 148 F.
  2. Inspect tomato transplants prior to purchase. Leaves and stems should be green and firm. Avoid any plants with dark rotten areas.
  3. Look for late blight resistant tomato varieties. Check seed catalogs and plant labels to find disease resistant varieties.
  4. Plant only seed potatoes from a reliable seed supplier. Do not use potatoes from the grocery store for seed. Seed potatoes from a seed supplier are grown using practices to prevent plant disease and tubers are inspected at harvest. Some potato seed is certified disease free. Do not use tubers saved from potato plants that were infected by late blight last year as seed.
  5. Provide ample space between tomatoes and stake the plants to promote good air circulation around the plant. This will help them dry off after rain or irrigation. Late blight and other fungal diseases need moisture on the plant surface to start new infections.
  6. Check plants regularly for disease and identify any problems you find with the help of UMN Extension’s online diagnostic tool.
  7. If late blight does show up, infected plants should be buried to prevent movement of airborne spores. Some gardeners prefer to place the infected plant in a closed plastic bag and allow it to 'cook' in the sun for several days before burying the plant in the compost or garden soil to make sure the pathogen is dead.

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