Our diagnostic help desk has been deluged with disease – tomato diseases specifically. 

With all the questions rolling in, and after listening to an excellent presentation by Marion Murray, I reached out to her to provide some information on a tomato disease blog post.  Many thanks Marion for sharing your disease expertise with us!

Tomato Disease Blog Post – Marion Murray, USU Extension Integrated Pest Management (IPM) Project Leader

Identifying problems with your tomato plants can be tricky.   The typical symptoms—curling leaves and brown/yellow spotting—are a result of a myriad of problems.  Sometimes the problem is a plant pathogen, such as a fungus, bacteria, or virus, and sometimes the problem is the growth environment, such as lack of nutrients, water, or even herbicide injury.  Often the Salt Lake County Extension office can help with diagnosis, and if needed, plant samples can be sent to the Utah Plant Pest Diagnostic Lab in Logan.

If diseases have plagued your tomatoes in the past, be sure to follow these best practices from here on out:

  • Disinfect pruning shears.
  • Stake tomatoes to improve airflow.
  • Monitor plant health on a regular basis.
  • Remove weeds, especially volunteer tomato plants.
  • Use organic mulch.
  • Remove diseased plants immediately and do not compost them.
  • Remove all debris after harvest.
  • Rotate out of a tomato-related crop every other year.

tomatoes at meals plus garden

Here are 5 bad actors to be on the look-out for:

#1: Curly Top Virus

We have been seeing a lot of curly top this year, due in part to the increasing populations of the vector of this disease, beet leafhopper.  Key characteristics of curly top are:

  • Thick leaves that curl upward
  • Purple leaf veins
  • Stunted growth
  • Early ripening fruit
  • Affects younger transplants
  • Symptoms are evenly dispersed on the entire plant

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The best management is to remove infected plants immediately, and remove nearby weeds and volunteer tomato plants.  There is no cure for infected plants.

More info on curly top: USU Extension Curly Top Fact Sheet

#2: Early Blight

Early blight is caused by a fungus, and occurs during periods of warm, wet weather (or if irrigation water hits foliage).  This disease primarily affects foliage only in Utah.  Key characteristics of early blight are:

  • Dark irregular leaf spots, often surrounded by a yellow halo
  • The leaf spots are made up of concentric rings, looking like a bull’s-eye
  • Older leaves are affected first
  • Symptoms start appearing in late summer
Early blight on tomato plant

Photo Credit: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org

Intervention with a copper-based fungicide is only required if the entire bottom third of the plant is affected.

More info on early blight: USU Extension Early Blight Fact Sheet

#3: Late Blight

Late blight is caused by a fungus-like organism, and occurs during periods of cool, wet weather (or irrigation).  Once established, this can be a devastating disease as it moves rapidly within and between plants.  Leaves, stems, and fruit can all become infected, and plants can be quickly killed.  Key characteristics of early blight are:

  • Olive-colored, greasy-looking leaf spots (early infections)
  • Brown to black blotches on foliage and stems (older infections)
  • Firm, brown “rings” on fruit, starting at the stem end
  • Rapid dieback of the plant
  • Succulent leaves are usually affected first
  • Symptoms start appearing in late spring to mid-summer

Late Blight Affecting tomato plant

Once the disease is detected, all infected plants should be removed immediately.  A copper-based fungicide can be used on the remaining plants to prevent further infection.

More info on late blight:  Utah Pests News

#4: Powdery mildew

One does not usually think of powdery mildew on tomato, but we have seen it on occasion in Utah.  This fungus cannot overwinter in northern Utah.  Spores are blown in from warmer areas, resulting in infections that may happen toward the end of the season.  White mycelium is rarely seen.  Key characteristics of powdery mildew are:

  • Blotchy brown lesions with a yellow halo
  • General yellowing of foliage
  • Older leaves affected first
  • Symptoms start appearing in late summer

powdery mildew on tomato plant

Remove any affected foliage.  Fungicides are not warranted for this disease.

More info on tomato powdery mildew: USU Extension Powdery Mildew Fact Sheet

#5: Verticillium Wilt

Verticillium wilt is caused by a soil-dwelling fungus that enters through the roots and clogs the plant’s water-conducting tissue.  When we see this disease in Utah, the most common theme is that the crops on the farm are not rotated.  When a single crop is grown in the same soil for many years in a row, fungi like verticillium can build up because they have a ready supply of the host material.  Key characteristics of verticillium wilt are:

  • Dieback of leaves starts at the bottom of the plant
  • Dieback of leaflets follows a V-shaped pattern from the outer leaflet in
  • General wilting of entire plant with recovery in the evening
  • Brown streaking inside the vascular tissue at the crown of the plant
Verticillium wilt on potato

Verticillium wilt on potato

Rotating crops is the simple solution to preventing most soil-borne pathogens.  When verticillium has been identified, the soil must be out of tomato-potato-pepper-eggplant production for at least 5 years.

More info on verticillium wilt:  University of Minnesota Extension Verticillium Wilt Fact Sheet