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Tomato Leaf Spot Season Begins

Michelle Grabowski, UMN Extension Educator

M.Grabowski, UMN Extension

Photo 1: Leaf spot diseases starting on the lower leaves of a tomato plant

Warm wet weather throughout Minnesota has provided excellent conditions for tomato leaf spot pathogens to grow and spread. The first leaf spots of many tomato diseases have recently been observed.

Both fungal and bacterial leaf spot diseases are commonly found on tomatoes in Minnesota. Septoria Leaf Spot, Early Blight, and Bacterial Spot are all common problems on garden tomatoes. All of these diseases overwinter on infected plant debris in the soil. Rain or irrigation can splash spores or bacteria up onto the lower leaves of the plant to start new infections.

If you are growing tomatoes this year, take the time now to examine the lower leaves of your plants. Look for black to brown spots on the leaves closest to the ground. For help identifying which disease is affecting your plant visit the 'What's wrong with my plant?' online diagnostic tool.

Do not be disheartened if you do find disease on your plants. There are still a few things that can be done to prevent the spread of disease and end up with a good harvest of tomatoes.

  • On the next dry day, go to the garden and pinch off all leaves with leaf spots on them. Start with the worst infected leaves and stop when you have removed 1/3 of the plant's leaves (removing more than this hurts the plant).
  • Remove the infected leaves from the garden. Infected leaves can be composted if the compost gets hot or buried so that soil microorganisms will begin to break down the plant material that shelters the pathogen.
  • Stake plants to improve air movement around the leaves.
  • Avoid spraying leaves with water. Use drip irrigation or direct water at the base of the plant. If sprinkler irrigation is the only option, water early on a sunny day so leaves dry quickly.
  • Mulch the soil around the plant with an organic mulch like straw or woodchips or with plastic mulch to prevent spores from splashing up from last year's plant debris.
  • Remove any weeds that might be crowding the plant.
  • If you choose to spray a fungicide to protect leaves from Septoria Leaf Spot or Early Blight, sprays should begin now. Read all label instructions prior to applying the fungicide. Tomato MUST BE LISTED on the pesticide label and ALL INSTRUCTIONS must be followed or the tomatoes will not be safe to eat.
Remember that we grow tomatoes for the fruit not for the leaves. It is an acceptable practice to use the cultural control practices above to reduce leaf spot disease problems and tolerate some leaf spots. One study found that yield in tomatoes was not reduced until over 50% of the plant's leaf area was infected by Septoria Leaf Spot. That means plants will continue to produce the same amount of tomatoes until diseases reaches very severe levels.
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