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Tomato Plant Diseases and How to Stop Them

Feb 21, 2017

Written by Kayla Harless, People’s Garden Intern

Almost everyone who gardens grows tomato plants. We are passionate about our tomatoes and savor that ripe, fresh fruit. However, several diseases love our tomato plants just as much as we do. The People’s Garden Workshop topic this week was tomato blights and spots, and Dr. Martin Draper, a plant pathologist through the USDA National Institute of Food and Agriculture, taught us how to identify them and what we can do to treat and prevent them.

He discussed in detail three pathogenic diseases: septoria leaf spot, early blight, and late blight. Don’t let the names early blight and late blight mislead you, they can appear at any time during the year. Septoria leaf spot is characterized by small, dark circular spots that often have yellow halos around them; they appear on the lower leaves of the plant first. Early blight can set in the stems and leaves, and is identified by legions with target-like rings. Late blight affects large portions of the leaves. It looks white and fuzzy on the underside of the leaf, and destroys crops quickly. Early and late blight affect potatoes as well, in fact, late blight caused the Irish potato famine.

These diseases can be spread many ways, and knowing what they are can supply simple solutions. Moisture, especially on the leaves, provides great conditions for the spores of these diseases to make themselves at home. Watering at the base of the plant can help prevent this; if you do use a sprinkler system or similar method to water your tomatoes, do so in the morning to allow the plant an opportunity to dry throughout the day. It is also recommended to stake your tomato plants instead of caging them, and space them appropriately. This way, the space will make it slightly more difficult for diseases to spread quickly, and the airflow will keep the plants dry. Keep a watch on your garden, and pluck any leaves that show signs of disease and take out infected plants.

Some fungicides can be very effective; however, it is important to follow the label. A fungicide intended for different plants, not vegetables, won’t make a difference and may cause problems. If you garden organically, adding compost extracts or teas can be a treatment. To create a solution that prevents and treats disease, add a heaping tablespoon of baking soda, a teaspoon of vegetable oil, and a small amount of mild soap to a gallon of water and spray the tomato plants with this solution. This needs to be reapplied regularly to maintain its efficiency. Garden clean-up is another preventative key, as the diseases’ spores can overwinter on plants left in the garden from the previous year.

There are many USDA extension offices throughout the country and research facilities on every land-grand university, so if you have further questions or concerns about tomato diseases feel free to contact one of these places.

Dr. Martin Draper looked at samples the workshop attendees brought from their gardens and gave suggestions as to what can be done to bring the plants back to health.
Dr. Martin Draper looked at samples the workshop attendees brought from their gardens and gave suggestions as to what can be done to bring the plants back to health.

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S. Snyder
Jun 30, 2013

When our tomato plants start producing fruit the tomato starts to turn black on the bottom. Does anyone know what the problem is?

Jun 25, 2014

It is usually due to a lack of calcium in the soil - called blossom end rot.

Phungula SM
Mar 03, 2015

What is the cause of Septoria leaf spot? I am Plant Pathology undergraduate student.

Ben Weaver
Mar 04, 2015

@Phungula SM - thank you for asking. This is a complex question that is dependent on the crop and the origin of the problem. Septoria species fungi cause leaf spot diseases on many host plant species. Septoria fungi are typically residue-borne organisms that survive through periods of adverse environment on infected plant material. They reinfect susceptible green plant tissue during wet periods and the spores of the pathogen are spread from the residue by rainsplash. Septoria spp. causes important disease of celery where the disease is called late blight (Septoria apii/ S. apiicola); wheat - where it causes Septoria blotch (S. tritici); tomato – where it causes Septoria leaf spot (S. lycopersici). There are also Septoria leaf spotting diseases of blueberry, soybean, brambles, several flowering composites, and many, many other plant species.

Jun 26, 2018

What do you think of using commercial copper sprays to treat blight?

barbara purtell
Jun 14, 2020

How often do I spray the baking soda water solution on tomato plants? My tomato plants are approximately 12" tall as I buried them deep when planting. They have been in the ground just a little over a week. Would 1 tablespoon baking soda be too much for them? Also is this a better method than water and milk spray? Thank you.

Theodore Derby
Jun 30, 2020

The solution for septoria blight is what I was searching there any restrictions on temp, sunny/cloudy when applying the spray?

Jul 17, 2020

Thanks for the info. I'm trying the soda & soap.

Malka Plummer
Jun 02, 2021

Does it have to be applied to the underside of the leaves or just the tops?

Terry Dobson
Jul 24, 2021

Is there any thing you can do to the soil in the fall to eliminate the blight come spring

Arthur T Delaney
Jul 28, 2021

I've been getting septoria leaf spot for a few years now.

Thomas Emery
Aug 03, 2021

Will it affect the tomatoes if lots of the solution gets in the soil? I.E from a watering can rather than spraying?

Peter W
Aug 06, 2021

I had a tomato fungus last year. I pulled all the tomato plants and put them in a compost pile away from where the tomatoes have been planted. This year I moved my tomato plants about 40 feet away from where they were last year to a new garden. I again got a tomato fungus that is slowly killing my plants. How do I prevent this from happening next year.

Ben Weaver
Aug 12, 2021

@Peter W - thank you for your comment. There are many USDA Extension offices throughout the country and research facilities at every Land-grand University; if you have further questions or concerns about tomato diseases, feel free to contact one of these places. Visit to find a USDA Extension office near you.

Sue Kelly
Aug 12, 2021


Does adding my household vegetable & fruit compost to my vegetable garden cause septoria leaf spot on my tomatoes? I do not compost my tomato plants.

Thank you,

Jan 30, 2022

Hi, I tried your link: Visit to find a USDA Extension office near me. Both Macomb County & Oakland County Michigan links show "page not found" and showed Michigan State College (go green), not USDA as the source.
Can you give me info for an office in Michigan?