As growers, we strive for healthy plants for maximum fruit production, long-term success, and healthy soil. It can sometimes be too late once symptoms are identified as fungal disease, often causing rapid decline, failure to thrive, and even plant death. Due to the ease at which fungal diseases travel, tomato blight is a disease that can cause serious issues for small-scale and market gardeners alike. 

Ensuring your soil is sterile and amended correctly is crucial to producing healthy seedlings and crop fields. Soilborne diseases can run rampant for years, so grower knowledge, scouting, and swift action are critical. Increase your gardening confidence by knowing the signs to look out for and effective treatment plans to keep your gardens healthy. Read on to learn five easy ways to sterilize your soil to keep your garden tomato blight-free to achieve maximum fruit production. 

Understanding Tomato Blight and Its Causes

Tomato blight can be very dangerous to your crops, so it’s essential to know the symptoms, how to prevent it, and what to do if you suspect it in your soil. 

What is Tomato Blight?

Tomato blight, late blight, and potato late blight all refer to the same disease. Many growers use these interchangeably, but I’ve pinpointed some key differences between early and late blight. Tomato, potatoes, and other hairy nightshades can become infected. It runs especially rampant in the enclosed spaces of high tunnels. 

Early Blight: 

• Cause: Pathogens Alternaria tomatophila and Alternaria solani

• Prevalent where springs are cool and wet, and humidity is high 

• It affects fruits, leaves, and stems but does not typically kill the plant. Symptoms affect older parts of the plant first and work their way up. However, yields can drastically decrease, and pathogen populations increase.  

• After plants experience defoliation, you may notice sun-damaged tomatoes.

• Develops when temperatures are between 59 and 80°F (15 and 26°C) and is most active at 82 to 86°F (27 and 30°C).

• Pathogens spread in water splash up, wind, human contact, tools, equipment, and heavy dew.

• It can overwinter on plant debris, soil, infected seeds, or plants brought in.

• We recommend using early blight-resistant cultivars. Use filters in seed catalogs or refer to Cornell University’s log. 

Early blight is often confused with bacterial spot or Septoria leaf spot. As the disease progresses, small spots will become more ring-like in early blight, clarifying the distinction. If you’re unsure which disease you see, snap a picture or send a leaf sample to your local extension office for confirmation. 

Late Blight: 

• Cause: Pathogen Phytophthora infestans, a water mold

• It’s called late blight because it typically appears later in the season than early blight. Spores travel along the coastline through weather patterns and precipitation. Severity varies depending on the season’s weather. Some say the disease is inevitable as no true resistant variety is available. Ideal conditions are cool and damp, 60 to 70°F (15.5 to 21°C).

• Why you’ve heard of it: It caused the Irish Potato Famine of the 1840s.

• Symptoms include dark brown spots on leaves and stalks, shriveling, lesions on stems, soft tubers, and decaying fruit and tubers. 

• Pathogens can overwinter in the soil. 

• Early detection may allow you to save your plot. Destroying infected plants by burning them or tying them up in a plastic bag is the only way to prevent the disease from spreading. Scouting regularly, proper sanitation, healthy watering practices, and ample spacing are preventative measures. 

• Fungicides can treat late blight.

Both tomatoes and potatoes infected with tomato blight suffer from rotting fruit or tubers. While blight is not curable, it is preventable and treatable if the timing is right. If you do not identify and treat the disease swiftly, ideal conditions can cause total decimation overnight. Even with treatment, yields may still be affected. Do not consume infected fruits.  

Causes of Tomato Blight

Two primary fungal pathogens cause tomato blight: Alternaria solani and Phytophthora infestans. Both spread quickly through infected soil, plants from outside your farm, and all types of moisture, including irrigation, rain, water splash, dew, and condensation. Pollinators, animals, and humans also help spread pathogens. Practice crop rotation and greenhouse sanitation to reduce the risk of contracting blight. 

Why Soil Plays a Key Role

Soilborne diseases strike year after year by lying dormant in the soil until optimal conditions arise. Soil is a medium that holds the disease in place. Then, when conditions are just right, they move up through roots, irrigation, and rain splash to infect new plants. 

Like most things in growing, healthy soil is critical to healthy crops. Overapplication of amendments, too much organic matter (yes, there can be too much!), and not practicing crop rotation increase the risk of soilborne diseases. If you suspect a blight infection, treating it before planting new seedlings is critical. Otherwise, you’re setting yourself and your plants up for failure. 

Related: How to Test Soil pH Before Planting — and Why You Should

5 Ways to Sterilize Soil for Tomato Blight

Here are five ways to sterilize soil against tomato blight. 

1. Solarization: A Natural and Effective Method

This method utilizes clear UV-stable plastic about three mils thick to cover the soil and trap the sun’s heat underneath.

• Moisten the soil you want to solarize until it behaves like a crumbly cookie batter but is not soggy. 

• Secure the plastic over the plot with weight and bury the edges so it remains taut. 

• Leave the plastic on for four to six weeks for maximum effects. The ground beneath the plastic becomes extremely hot in the sun, killing pathogens and pests in the soil. 

• Solarization also kills some good bacteria and soil microorganisms, so once you remove the plastic, you’ll need to replenish the soil before planting. Use a soil test to determine your needs. 

According to the University of California Agriculture & Natural Resources IPM Program, this chemical-free and inexpensive method can control many fungal and bacterial pathogens, “including those that cause Verticillium wilt, Fusarium wilt, Phytophthora root rot, Southern blight, damping-off, crown gall disease, tomato canker, potato scab.” For this same reason, it’s crucial to ensure compost from external farming sources is adequately heated so as not to spread pathogens onto your farm. 

2. Using Heat Treatments for Soil/Sterilization 

Soil steaming is said to have developed in Germany in 1888 and has been used to rid soils of pathogens until pesticides took hold of the growing market to fight disease and weeds. Growers are gaining interest in this safer method since studies have shown the danger pesticides present to the biodiversity of soil, pollinators, and people, including this large peer-reviewed study, Pesticides and Soil Invertebrates: A Hazard Assessment, from 2021 involving researchers from the Center for Biological Diversity, Friends of the Earth, and the University of Maryland. All studies examined found that soil invertebrates are gravely endangered by pesticide use. 

While some pathogens can withstand extreme heat, tomato blight pathogens simply can’t so heat treatments effectively destroy them. Baking the soil to kill pathogens is typically practiced on a small scale, primarily by home gardeners. Here’s the process: 

• Moisten the soil. 

• Add three to four inches of soil to a baking pan and cover it with foil. 

• Put the pan in a preheated oven at 200°F (93°C) for 30 minutes or until the soil reaches an internal temperature of 180°F (82°C). This process might stink up your house, so plan to open windows or turn fans on. 

• Let the soil cool before using it. Store it in an airtight container in a cool, dry place so it remains sterile. 

Do not extend the recommended time or temperatures, which can lead to phytotoxicity. Phytotoxicity can cause adverse effects such as a lack of germination, discoloration, stunted growth, stress, failure to thrive, or death in seedlings. 

Home gardeners can also sterilize their soil by steaming it in a pressure cooker:

• Add three cups of water to the cooker

• Fill mason jars with about four inches of leveled soil

• Cover the jars with aluminum foil

• Set the jars inside the cooker on the rack, which will hold them above the water

• Cover with the cooker lid and leave the steam valve just slightly open

• Heat jars at 10 pounds for 15 to 30 minutes

Another way to sterilize old potting soil is to moisten it, add about two pounds to a gallon-size, microwaveable plastic bag, and nuke it in a 600-watt microwave for 90 seconds or until the soil reaches 180 to 200°F (82 to 93°C). Then, date and label the bag, seal it, and store it for later use. 

After sterilizing, add worm castings, coco coir, compost, or vermiculite to enhance the structure and boost microbes. Grow bags give home gardeners great control over soil health and help them produce delicious and healthy tomatoes. We’ll discuss larger-scale options for market gardeners later. 

3. Integrating Antifungal Soil Amendments

Similar to the human gut, there are beneficial and harmful microbes in the soil, and it’s up to us to keep them balanced. Adding antifungal amendments is a new technique farmers use to help control soil pathogens, colonize roots, reduce plant stress, and kill root pathogens harmful to their crops, particularly root/stem rot and damping off. Trichoderma harzianum 1295-22, or T-22, was created by Professor Gary Harman of Cornell. Its advantage is that it thrives under field conditions where, usually, native microbes would simply take over. Here’s what we know about the product:

• It’s an inexpensive and chemical-free bio-fungicide. The ROI potential is high on large-seeded summer crops like beans, tomatoes, potatoes, and sweet corn, as learned from Trichoderma for Control of Soil Pathogens and the University of Connecticut. 

• T-22 increases a plant’s immunity and, thus, its ability to defend against harmful viruses, bacteria, and fungi.

• Proper timing for application is crucial for success. It is most effective in culture when temperatures are above 60°F (15°C).

• Apply T-22 to seeds to get more bang for your buck. A small amount is adequate, and once the seed germinates, T-22 will reproduce independently without help. 

• For peak performance, the soil should not be overly wet or dry. 

• T-22 is eco-friendly and rapidly colonizes, forming symbiotic relationships with plants.

This product type is crucial as farmers attempt to grow enough food for our growing population. 

Trichoderma, as stated in Trichoderma: The “Secrets” of a Multitalented Biocontrol Agent, possesses the “induction of rapid plant development and production, an increase in nutrient absorption, rhizosphere modification and tolerance improvement to both biotic and abiotic stresses.”

Use T-22 preventatively by applying it directly to the soil in powder or liquid form or drenching the soil around existing plants. Mix it with your growing soil and compost to use in the seed-starting process, or blend it with compost and use it as a side-dress for a boost during the growing season. Follow the instructions on the package of the selected products for best results. 

4. Proper Crop Rotation Practices

Crop rotation suggestions significantly impact the risk for disease and should be practiced by growers of all sizes across regions. Moving crops to new fields every few seasons breaks pest and disease lifecycles by removing their preferred hosts. This vital practice reduces the negative impacts of pest pressure and soilborne diseases. Early tomato blight spores can sometimes live dormant in the soil for up to ten years, whereas late blight requires plant debris or live plants. 

Most experts recommend rotating crops based on their family every three to six years, but you should do what works best in your system. Test your soil annually to ensure nutrient levels are good following heavy feeders like tomatoes and peppers. Regularly rotate these six different crop families: 

• Alliums: onions, garlic, leeks, shallots

• Nightshades: tomatoes, potatoes, peppers, eggplant

• Cucurbits: cucumbers, squash, melons, pumpkins

• Brassicas: kale, cabbage, cauliflower, broccoli, collard greens, mustard greens

• Legumes: beans, peas

• Umbellifers: parsley, dill, carrots, parsnips, fennel

Rotating crops in a home garden may seem daunting, especially when space is limited. Start small, draw maps for your records, and note the positive outcomes. Color-code your maps to help you track next season’s gardens. Sustainable Agriculture Research and Education (SARE) has a color-coded rotation plan you can review and adjust to meet your needs. 

5. Scaling Solutions for Market Gardeners

When it became clear that steaming soil was effective but needed to work on a market gardener scale, large-scale soil steamers came onto the scene. Market gardeners can invest in a soil-steaming machine or rent one to steam their soil once per season, as experts recommend. Becky Maden of the University of Vermont Extension explains the soil preparation and process in the video below. The whole process takes about an hour and a half, which changes based on the size of your tunnel. Check with your local conservation district to inquire about rental availability. Add soil steaming to your farm’s plans so you can steam each tunnel when it’s sitting fallow or before the season begins. 

VIDEO: Soil Steaming Overview with Becky Maden, UVM Extension OR 

Setting up a Soil Steamer with Becky Maden

UVM presented the process of steaming a high tunnel, including costs, external resources, and tips and tricks to save on cost. Check out the presentation Soil Steaming in High Tunnels, and click on the steam calculator to determine how long it might take to steam your high tunnel beds. 

Home gardeners can scale this method down and steam one garden bed at a time, as it is the fastest, safest, and most effective way to kill pathogens without damaging the soil health. While there are commercial soil treatment products that prevent fungal growth, they may present long-term effects like resistance and mutations. Unfortunately, large-scale growers that feed the masses rely on treatments like copper fungicides, azoles, morpholine, and QuI, to name a few, to prevent crop failure. Scientists are constantly searching for effective treatments to use on large scales. Read more about them in Antifungal Agents in Agriculture: Friends and Foes of Public Health. 

Related: Growing Tomatoes For The Urban Farm

Seed Starting Tips to Reduce Tomato Blight Risk

Here are some things you can do to prevent tomato blight from taking over your farm. 

Choosing Blight-Resistant Tomato Varieties

When flipping through seed catalogs or perusing them online, look for blight resistance codes EB, AB, and LB. Here are some options broken down by tomato type. 

Cherry tomatoes: ‘Apple Yellow F1,’ ‘Cherry Bomb,’ ‘Geranium Kiss,’ ‘Green Doctors Frosted,’ ‘Jasper F1’ ‘Mountain Magic,’ ‘Pink Beauty’ ‘Supersweet 100 F1,’ ‘Tommy Toe’

Grape tomatoes: ‘Fantastico F1,’ ‘Honey Bunch,’ ‘Juliet’

Heirloom tomatoes: ‘Brandywine,’ ‘Damesel,’ ‘Green Zebra,’ ‘Marnero,’ ‘Roma,’ ‘Rose de Berne’

Plum tomatoes: ‘Indigo Rose,’ ‘Plum Perfect F1,’ ‘Plum Regal,’ ‘Verona’

Slicing tomatoes: ‘Abigail F1,’ ‘Better Boy F1,’ ‘Bigdena F1,’ ‘Brandywine,’ ‘Defiant,’ ‘Galahad F1,’ ‘Gold Medal,’ ‘Iron Lady,’ ‘Mountain Merit,’ ‘Purple Zebra,’ ‘Stellar F1,’

If you’re up for a new challenge and have patience, consider grafting tomatoes using resistant rootstock like ‘Arnold’ and ‘Submarine.’ When growing in a high tunnel, space plants accordingly, rotate often, and take action if disease is suspected. Trellis your tomato plants adequately with tomato roller hooks. If this is your first time growing indeterminate tomatoes in a high tunnel or hoop house, check out our complete trellising system. Field tomatoes typically become infected with blight earlier due to wind and precipitation.  

For further details on soil health basics, read Beginners Guide to Soil, Media, Amendment, and Fertility, and check out some of our favorite seed-starting products below.  

5" reusable seed starting pots
Air prune propagation tray - 72-cell
1020 mesh deep trays

Using Sterile Seedling Mix

Use one of the methods discussed in detail earlier to prepare a sterile seedling mix. Store it in an air-tight container until seed-starting or rooting. You’ll do the same whether using brand-new bagged soil or reusing soil you have on the farm already. Note that while purchased soil isn’t sterile, you shouldn’t worry about pathogens if you get your products from a trusted, reputable source. You must also sterilize equipment, surfaces, and tools to prevent contamination during greenhouse work. Let’s discuss ways to ensure this before starting seeds. 

Ensuring Hygiene and Equipment Sterilization

Follow these simple yet crucial tips to ensure your seed-starting area and supplies remain disease-free. 

• Clean surfaces with disinfecting spray after every use, including storage areas, seed-starting tables, workstations, and hardening-off areas. 

• Disinfect trays immediately after emptying them to rid them of potential pathogens. It doesn’t take much time and makes a huge difference in general sanitation. Plus, plants are at their most vulnerable stage when they’re young seedlings, so taking this extra step could be the difference between thriving and diseased plants. 

• Combine 3% hydrogen peroxide and water at a 2:1 ratio. Spray the solution on the tools and let it stand for 10-15 minutes. Rinse and let dry. 

After pruning and harvesting, clean hand tools with antibacterial soap and water. After each use, brush off dirt and debris to keep them in good working order and prevent rust from forming. If needed, spray them off with the hose and hang them to drip dry. Take them apart one to two times annually to ensure pathogens aren’t hiding and to keep them in tip-top shape. 

If you suspect or have confirmed disease, disinfect tools immediately using one of the solutions below to avoid cross-contamination. Also, remove clothing and wash it to prevent disease from spreading to other garden areas. 

• Mix a 10% bleach solution with a 1:9 bleach-to-water solution. Dip or spray spades, shoves, rakes, trellises, pots, and stakes. Wipe them thoroughly and allow them to dry before storing or using them. 

• Boil solid metal tools for about 15 minutes and allow them to dry thoroughly. Avoid tools with plastic or glue on them to prevent them from melting. 

• 70% isopropyl alcohol is easy and cost-effective. Do not dilute the alcohol before using it; wipe the tools dry before storing. 

When using harsh chemicals and disinfectants, read the caution labels and always use them as directed to avoid injury. Keep these solutions out of reach of children and pets and discard any extra. 

Read our article, How to Wash and Care for Seedling Trays, for more details on why and how to sanitize seedling trays.

Setting Plants Up for Success After Sterilization

Now that your soil is sterile take these additional steps to ensure peak garden performance. 

Ensuring Proper Soil Drainage

Overwatering seedlings is a sure way to cause root rot, inhibition of root growth, lack of oxygen, and damping off. Since blight spores travel through moisture, which their reproduction requires, the risk increases with standing water and fungal growth. Proper watering techniques and drainage are crucial during seed-starting, growing in the greenhouse, and once plants are growing in garden beds. Read Why Organic Matter is Key to Sustainable Soil Management for tips on adjusting soil texture and increasing organic matter to help it drain properly. 

Mulching for Protection

Mulching will prevent soil from splashing onto plant stems and lower leaves, reducing the risk of blight spores spreading. Mulch will also help the soil retain moisture, keep the soil at a more consistent temperature, and reduce fungal activity on the surface. Consider organic straw, woodchips, bark, leaf mulch, compost, or a tarp white side up for mulch. Drip irrigation will limit the amount of water splashed onto foliage possible. 

Adequate Plant Spacing and Airflow

Always follow spacing recommendations for tomatoes, especially when growing them in the protected space of a high tunnel, where diseases and pests can spread quickly and cause damage. 

Proper spacing, trellising, vents, and regular pruning will help create a healthy balance of airflow, ventilation, and fresh air circulation, reducing the chance of disease. Plant small-fruited tomato varieties at 12 to 24 inches with one main leader. Increase spacing when using a multi-leader system. Space larger-fruited tomatoes like heirlooms and beefsteaks at 18-36 inches. Use the Florida weave trellising method to maintain healthy and upright plants in open fields. 

FAQs About Treating Tomato Blight and Soil Sterilization

What are the early signs of tomato blight in plants?

Monitor lower, older leaves, where blight appears first. Prune and remove suckers often to maintain ample air circulation and scout for cankers along the stem near the soil line. Fruit may develop dark, water-logged spots and begin to decay. 

Related: Pruning Indeterminate Tomatoes for Maximum Production in a Hoop House

Can I reuse soil affected by blight without sterilizing it?

Early blight spores live in infected soil and will affect future plants. If blight is confirmed, sterilize the soil before planting into it. 

How does crop rotation prevent tomato blight from returning?

Rotating crops by removing blight’s preferred host plants disrupts its lifecycle and reduces the chance of blight striking again. While it can continue to live in the soil for many years, breaking the cycle lowers the risk and the disease populations. 

Are there chemical alternatives to soil sterilization for market growers?

Soil steaming is the best non-chemical alternative to the most common chemical used in the past, methyl bromide. Since the Environmental Protection Agency (EPA) banned methyl bromide for its ozone-damaging properties, other chemicals on the market have been used to control soil pests and pathogens. See the list of them here. 

Should I use fungicides even after sterilizing the soil?

Sterilizing the soil removes the need for growers to use fungicides. Fungicidal application, when disease is not apparent, will do unnecessary harm to microbes in the soil. 

How do I prevent tomato blight in high tunnels differently than in-ground planting?

Proper air ventilation, circulation, and proper nitrogen levels are crucial to preventing blight in high tunnels. Overapplication of nitrogen increases the chances of blight. Annual soil tests will ensure healthy levels. Avoid using amendments like Epsom salts, which can make matters worse. 

Are there risks to beneficial soil organisms when sterilizing soil?

There is always a risk that some beneficial organisms will die. Follow proper heating instructions, do not exceed the advised temperature, and sterilize soil only once annually. Some growers add beneficial microbes and compost to help increase populations to healthy levels. Contact your local extension office if you’re unsure about any part of the process. 

Sterilizing Soil Will Keep Your Tomatoes Fungus Free. 

Growers of all scales can control early and late blight by sterilizing their soil. Coat untreated seeds at seed-starting time to prevent blight contraction and help keep populations down. Ensuring your soil is blight-free will increase your yields and overall plant health. If you’ve struggled with blight in the past or suspect spores are present in your soil, try one or more of these blight-fighting methods this season to reduce the risk and help you have an abundant tomato season.