Identify and manage six common tomato plant diseases with these tips.

How to identify and control tomato plant disease

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Tomato growers are a passionate bunch. Some of us spend long hours combing over seed catalogs and nursery benches full of plants to select the perfect tomato varieties for our garden. We plant, tend, prune, fertilize, stake, and otherwise care for our tomato plants with a dedication rivaled only by our dedication to our human family. But, even with all that care and attention, sometimes a tomato plant disease strikes our garden. Today, let’s review some of the most common tomato plant diseases and discuss ways to prevent and manage them, without resorting to synthetic chemicals for control.

Types of tomato diseases

Unfortunately, there are several pathogens that can cause tomato plant disease. I’m going to introduce you to several specific tomato diseases later in this article, but before I get to that, it’s important to talk briefly about the different types of pathogens and how to prevent them from striking your garden in the first place.

Some tomato disease pathogens are fungal organisms while others are bacterial or even viral. Different regions of North America are affected by different tomato pathogens, and rates of infection are dependant on many factors, including wind patterns, temperature, humidity, varietal resistance, and plant health, to name just a few. It’s important to remember that tomato plants that are healthy and properly cared for will often show more resistance to tomato plant disease, so ensuring your tomato crop has ample moisture and healthy, fertile soil is a must.

Preventing tomato disease requires a few special steps.

Preventing tomato diseases is a must, if you want to have productive plants.

Preventing tomato plant disease

Other than making sure your tomato plants are happy and healthy, there are a few other things you can do to help prevent tomato plant diseases. Here are nine tips to get you started on the road to disease-free, productive tomato plants:

  1. Rotate your crops. Since many tomato pathogens live in the soil, plant tomatoes in a different spot in the garden each year.
  2. Pinch off leaves with any signs of disease immediate and dispose of them in the trash to keep a possible infection from spreading.
  3. Don’t work in the garden when tomato foliage is wet or you may inadvertently spread pathogens from plant to plant.
  4. Choose disease-resistant varieties when selecting which types of tomatoes to grow.
  5. Remove all diseased tomato plant debris at the end of the growing season and burn it or toss it in the trash. Do not put diseased foliage in the compost pile.
  6. Provide adequate air circulation around each plant. Space tomato plants 5 to 6 feet apart.
  7. Mulch your tomato plants well at the start of the season. Two or three inches of compost, leaf mold, straw, or hay serves to keep soil-dwelling fungal spores from splashing up onto the lower leaves when it rains.
  8. Try to keep the foliage dry whenever possible. Hand irrigation or soaker hoses allow you to target the water on the root zone. The splash from overhead sprinklers can spread disease and wet foliage promotes fungal issues.
  9. Disinfect the empty pots if you grow your tomatoes in containers, using a 10% bleach solution at the end of the growing season and replace the spent potting soil with a new mix every spring.

    Prevention is key when it comes to tomato diseases.

    Follow every prevention tip you can to keep your tomato plants from being ravaged by diseases like this one.

6 Common tomato plant diseases

Despite your best efforts at preventing tomato diseases, they may still get a foothold in your garden from time to time. Here’s the low-down on six of the most common tomato plant diseases with information on identifying, preventing, and managing each of them.

Early blight

Identify: This common tomato plant disease appears as bulls-eye-shaped brown spots on the lower leaves of a plant. Often the tissue around the spots will turn yellow. Eventually, infected leaves will fall off the plant. In most cases, the tomatoes will continue to ripen, even as the disease symptoms progress up the plant.

Prevent: The early blight pathogen (Alternaria solani) lives in the soil and once a garden has shown signs of the early blight fungus, it’s there to stay because the organism easily overwinters in the soil, even in very cold climates. Fortunately, most tomatoes will continue to produce even with moderately severe cases of early blight. To prevent this tomato fungal disease, mulch plants with a layer of newspaper topped with untreated grass clippings, straw, leaf mold, or finished compost immediately after they are planted. This mulch forms a protective barrier, preventing the soil-dwelling spores from splashing up out of the soil and onto the plant.

Manage: Once the fungus strikes, organic fungicides based on Bacillus subtilis or copper can help prevent or stop the spread of this tomato plant disease. Bicarbonate fungicides are also effective (including BiCarb, GreenCure, etc).

Early blight tomato plant disease

Early blight often begins as irregularly shaped, bulls-eyed brown spots on the lower leaves of a tomato plant.

Fusarium wilt

Identify: The pathogen that causes Fusarium wilt (Fusarium oxysporum) is generally more common in warm, southern regions where this tomato plant disease can wipe out entire fields. Symptoms include drooping leaf stems. Sometimes an entire branch may wilt, often starting with the lower portion of the plant and then progressing upwards until the whole plant collapses. To confirm an infection, cut the main stem of the plant open and look for dark streaks running lengthwise through the stem. Sometimes there are also dark cankers at the base of the plant

Prevent: The spores of this tomato plant disease live in the soil and can survive for many years. They’re spread by equipment, water, plant debris, and even people and animals. The best method of prevention is to plant resistant varieties if you’ve had trouble with Fusarium wilt in the past. Also disinfect tomato cages and stakes with a 10% bleach solution at the end of every season.

Manage: Once this tomato plant disease strikes, there’s little you can do to control it. Instead, focus on preventing it for future years. Soil solarization can help kill fungal spores in the top few inches of soil, and crop rotation is key. There are also several biological fungicidal drenches that can be applied to soil (look for one based on the bacteria Streptomyces griseoviridis called MycoStop® or a granular one based on the fungus Trichoderma virens called Soil Guard®). These products may help prevent the infection from colonizing the roots of future crops.

Late blight

Identify: Late blight (Phytophthora infestans) is among the most destructive tomato plant diseases. Thankfully, it’s not very common, especially in the north where it doesn’t survive winter’s freezing temperatures without a host plant. Late blight is caused by a fungus, and it creates irregularly shaped splotches that are slimy and water-soaked. Often, the splotches occur on the top-most leaves and stems first. Eventually, entire stems “rot” on the vine, turning black and slimy. There may also be patches of white spores on the leaf undersides. In the north, the pathogen overwinters in buried potato tubers. In the south, it easily survives the winter.

Prevent: The spores of this disease are fast-spreading, moving on the wind for miles. If you live in the northern half of the continent, do not purchase potatoes and tomatoes that were grown in the south as you may inadvertently introduce late blight spores to your garden. This is not a common pathogen, but if late blight is reported in your area, there is little you can do to prevent the disease because the spores spread so rapidly. Plant only locally grown plants to help keep the pathogen out of your area.

Manage: Once late blight strikes, there is little you can do. Tear out the plants, put them in a garbage bag, and throw them out to keep the disease from spreading. Organic fungicides based on Bacillus subtilis are somewhat effective in preventing this tomato plant disease when it’s first discovered in your area.

Late blight tomato disease

Late blight is an extremely difficult tomato disease. It’s not common, but it is troublesome.

Septoria leaf spot

Identifiy: Appearing as tiny, round splotches on the leaves, this tomato disease (Septoria lycopersici) typically starts on the lowest leaves first. The spots have dark brown edges and lighter centers, and there are usually many spots on each leaf. Infected leaves eventually turn yellow and then brown, and fall off.

Prevent: Remove diseased tomato plants at the end of the season to prevent the spores from overwintering in the garden. Cut off and destroy infected leaves as soon as you spot them and disinfect pruning equipment before moving from one plant to another.

Manage: Organic fungicides based on copper or Bacillus subtilis are effective against septoria leaf spot, especially when used as a preventative measure.

Septoria leaf spot tomato disease

Septoria leaf spot is a tomato disease that produces splotches and spots on the foliage and can reduce yields.

Southern bacterial wilt

Identify: Unfortunately, once present, Southern bacterial wilt (Ralstonia solanacearum) is a tomato plant disease that spreads like wildfire. It’s soil-borne, but the bacteria that cause this tomato disease can travel by soil, water, plant debris, and even on clothes, tools, and skin. It’s naturally found in tropical regions and greenhouses, but it can arrive in the garden via infected plants that were purchased from other areas. Initial symptoms include the wilting of just a few leaves on a plant, while the rest of the foliage appears healthy. Over time, more and more leaves wilt and turn yellow until all the leaves succumb, though the stem remains upright. Slimy ooze threads out of the cut stems, and when they’re placed in water, milky streams of bacteria stream out of the cut.

Prevent: Southern bacterial wilt is soil borne and can survive for long periods in the soil on roots and plant debris. Like many other tomato diseases, it favors high temperatures and high humidity. The best way to prevent this disease is to purchase and plant only locally grown plants, or grow your own plants from seed. Southern bacterial wilt is more common in warmer regions, but has been found in Massachusetts and other northern regions as well.

Manage: There is no cure for this disease. Once confirmed, infected plants must immediately be removed and discarded in the trash.

Verticillium wilt

Identify: This fungal disease is caused by several soil-borne pathogens (Verticillium spp.). When present in a tomato plant, they block the vascular tissue in the plant and cause the leaves and stems to wilt. Symptoms progress slowly, often one stem at a time. Eventually, the entire plant yellows and withers. To confirm diagnosis, cut through the main stem of the plant and look for dark brown discoloration inside. Verticillum wilt is most problematic in late summer.

Prevent: Verticillium fungi can survive for many years in the soil and on plants. They thrive in slightly cooler summer temperatures (between 70 and 80 degrees F). Plant only resistant varieties.

Manage: Once verticillium wilt occurs, there’s little you can do to control the current year’s infection. Instead, focus on preventing this tomato plant disease in future years. Soil solarization will help kill the fungal spores in the top few inches of soil. Practice crop rotation: do not plant other members of the same plant family in that same planting area for at least four years after the infection.

With an eye toward prevention and employing early management practices as soon as a disease is spotted, you’ll be able to grow a terrific crop of tomatoes each and every season.

Book on growing tomatoes in containers

For more on growing great tomatoes, check out the following posts:
Our favorite cherry tomato varieties
The best heirloom tomato varieties
5 Tips for growing tomatoes in raised beds
Tomatoes not turning red? Try these 5 tricks
Top tomato varieties from the experts

Do you have a favorite tomato variety you grow every year? We’d love to hear about it in the comment section below!

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