How To Prevent 5 Tomato Diseases

Train your eye to identify which blights may be infecting your tomato plants, and act swiftly to save your harvest.

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by AdobeStock/romaset

Tomatoes are the stars of the home garden. A mealy, store-bought tomato will never compare to a sun-warmed heirloom straight off the vine. As a farmsteader, they’re one of my favorite crops, both for their multitude of culinary uses and their beautiful range of colors, sizes, and flavors.
Unfortunately, tomatoes are also susceptible to many diseases. Even for the experienced gardener, it can be tough to recognize tomato diseases, and even more challenging to treat them. I’ve found that prevention is key to a successful tomato harvest. Here are five common tomato diseases and how you can prevent them in your garden.

Common Diseases

Blossom-End Rot

It’s devastating when you grab what looks like a gorgeous tomato, only for your fingers to close on a dark, rotten spot on the bottom. Blossom-end rot is a common tomato disorder where the blossom end (or bottom) of the tomato rots. It may first appear as a water-soaked or bruised spot, but it’ll quickly develop into a sunken, dark-brown or black, leathery spot as the tissues decay.

If you’ve experienced blossom-end rot in your garden, you’ve probably heard that it’s caused by a calcium deficiency. While it’s true that a calcium deficiency can cause blossom-end rot, adding calcium to your soil may not always fix the problem. Your soil can have adequate calcium and you could still have tomatoes affected by blossom-end rot. If your soil test comes back with acceptable calcium levels, it’s time to consider other causes.

Calcium deficiency in tomatoes often only occurs in the fruit itself. Plants can fail to take up enough calcium or move it from other parts to the fruit, for a few reasons. Insufficient water while the fruit is developing can prevent the plants from taking up enough calcium from the soil and moving enough to the fruit. Root damage from fertilizer burn or close cultivation can also reduce water and nutrient uptake, mimicking drought conditions.

Conversely, waterlogged soils may be just as detrimental. Heavy, wet soils can prevent the plants from taking up adequate levels of nutrients. Over-fertilizing tomatoes early in the year may also lead to blossom-end rot. Rapid growth can cause plants to be unable to take up and move calcium quickly enough to keep up with leaf and fruit development.

Fusarium Wilt

green tomato plant with brown edged leaves

Just as the name suggests, one of the first things you may notice if your plant is affected by fusarium wilt is wilting. Plants will often begin to wilt during the day and then recover at night. As the disease progresses, sections of leaves will turn yellow. The yellowing can occur on whole sides of the plant or just on the leaflets on one side of a compound leaf. Fusarium wilt will kill the plant fairly quickly, and the entire plant will turn yellow and wilt; occasionally, the leaves will brown. A good way to further identify fusarium wilt is to peel the skin off the affected plant’s lower stem. If fusarium wilt has developed, you’ll notice discolored dark-red or brown vascular tissue.

Unfortunately, there’s not much you can do to save a plant that’s infected with fusarium wilt. It often enters the plant through root damage caused by nematodes and moves up into the plant through its roots.

Thankfully, fusarium wilt usually occurs only once each growing season, and it doesn’t normally spread from plant to plant or on the wind. Instead, it’s almost always brought in through infected soil or equipment. Plants growing in acidic soil tend to get the worst effects of this disease.

Early Blight

tomatoes in a bowl with black spots of early blight

Early blight is a frustrating fungal disease that’ll significantly reduce yields. Two fungi, Alternaria tomatophila and A. solani, cause this disease. When a plant is infected with early blight, you’ll first notice dark spots on older foliage growing near the ground. These spots are generally round, and large spots will develop concentric bull’s-eye rings.

The spots will progress to other foliage, the stem, and even the fruit. Tissue around the spots may become yellow, and affected leaves may turn brown and fall off. Spots on the fruit are generally sunken, dark, leathery spots, much like blossom-end rot; however, early blight spots will generally develop near the stem of the fruit, and one tomato can have multiple spots.

A plant becomes infected with early blight when the lower leaves come in contact with contaminated soil. This can happen by direct contact or by the soil splashing on the leaves while you’re watering or during heavy rains. This disease is more prevalent in hot, humid conditions. It can be spread to other soil by contact with humans or equipment or by wind-borne spores.

Late Blight

tomato leaves with brown spots

Perhaps the most infamous of the nightshade family fungal diseases, late blight was the disease responsible for the Irish Potato Famine in the late 1840s. In tomatoes, late blight can affect the plants’ leaves, stems, and fruit, and may cause total crop failure.
When infected, plants will develop large, brown leaf blotches with gray-green edges. As the disease progresses, whole sections of leaves will turn brown and die. Stem spots will be firm and dark-brown. Fruit spots will start the same way as stem spots, but they may grow mushy. In high humidity, white fungal growth may also occur on infected fruit and stems.

Late blight spreads the most in cool, damp weather. It may be introduced to gardens by wind-borne spores – even from 5 to 10 miles away or more – but it’s typically introduced by infected transplants or equipment that has come in contact with infected plants or soil.

Verticillium Wilt

Unlike fusarium wilt, verticillium wilt doesn’t usually cause plants to wilt as a first symptom. Typically, you’ll first notice yellow blotches on the lower leaves of an infected plant, and then the leaf veins may turn brown. As the disease progresses, the plant will develop brown spots on its leaves. Verticillium wilt is sometimes confused with early blight; a key difference is that early blight spots have definite edges and develop a concentric, bull’s-eye pattern, unlike verticillium wilt spots.

Eventually, as the fungus infects more of the plant, the leaves may begin to wilt, die, and drop off. This fungus grows in the plant’s xylem, or sap-conducting channels, and prevents the uptake of water and nutrients, leading to stunted plant growth. Fruit may develop yellow shoulders or sunburn from the lack of leaves. You may also notice that only the plant’s top leaves remain green.

Verticillium wilt can live in the soil for several years. It infects plants by entering the root hair. Plants with root damage from nematodes may be more susceptible. This disease thrives in cool, damp conditions.

Preventing Tomato Diseases

The most important steps to take to prevent tomato diseases are to maintain good soil conditions, choose and care for hardy cultivars, manage their water well, and deal with disease when it first shows itself.

Start with Your Soil

Get a soil test. Good gardens begin with healthy soil. One of the best things you can do for your garden is to test the soil. A soil test will let you know your soil’s pH level and essential nutrient levels. You’ll then be able to amend your soil accurately.

Rotate your crops. Rotation is important to keep all your plants healthy – but it’s essential for nightshade family members, including tomatoes, eggplants, peppers, and potatoes. I follow a three-year rotation, meaning I only plant nightshades in a bed after other crops or cover crops have grown there for at least two years. A longer rotation, such as 4 to 5 years, may be necessary to eliminate diseases that can persist in the soil, such as verticillium wilt.

Sow cover crops. Cover crops are an affordable way to add organic matter and build healthy soil, and they provide natural pest and disease suppression.
Provide your plants with adequate nutrition. I like to add finished compost to each transplant hole; you may also choose to add a bit of fertilizer. However, over-fertilizing can be harmful, so it’s wise to get your soil tested first.

Plant Care

Select disease-resistant cultivars. No one wants to give up their favorite heirloom varieties, but selecting a few disease-resistant plants can help make up the backbone of the homestead tomato harvest. You may have already noticed that many paste tomatoes can be hardier than the big slicers. They tend to be more disease-resistant and tolerant of drought and other poor growing conditions. Cherry tomatoes also tend to be forgiving.

You’ll want to read a little deeper into the seed catalog descriptions when dealing with fungal diseases. Many tomatoes, including some heirlooms, have shown resistance to specific diseases. Labels may vary a bit from seed company to seed company, but look for tomatoes marked for resistance, such as “EB” for early blight, “V” or “VW” for verticillium wilt, “LB” for late blight, “BER” for blossom-end rot, and “F” or “FW” for fusarium wilt.

Grow or purchase healthy seedlings. If you’re buying transplants, purchase them from a reputable grower, and check that they’re free from signs of disease. If you’re growing seedlings, use sterile potting mix, water plants appropriately, and pot them up as needed.

tomato trellises in a garden

Install trellises for healthy tomato growth. Trellises keep the plants off infected soil and allow maximum airflow around the foliage and stem. In our kitchen garden, I love using sturdy cattle panels as trellises. If you’re planting long rows of tomatoes, a trellising method using twine and metal T-posts called the Florida Weave might be a good choice. We’ve found that store-bought tomato cages aren’t large or sturdy enough to hold our plants; however, large ones can be homemade from welded-wire fencing and sturdy stakes or posts.

Prune your plants. Pruning helps keep plants healthy and productive. Give your plants good airflow to help prevent and minimize blights. Prune branches that touch the soil when they’re still small, as well as any “suckers” you see developing. Suckers are branches that grow out of the “V” where a branch meets the stem.
Apply mulch. A thick layer of mulch will prevent soil splash, help suppress weeds, and keep the soil moist. As it breaks down, it’ll add organic matter to the soil. It’s a must for preventing diseases such as early blight and blossom-end rot.

Water Management

Avoid overwatering. Fungal diseases need moisture to thrive, and overwatering can encourage their growth. As I mentioned in the blossom-end rot section on Page 18, overwatering can also inhibit plants from taking up enough nutrients, leaving them more susceptible to disease pressure.
Water the roots. Use drip irrigation or soaker hoses to keep moisture off the foliage and prevent soil from splashing up onto the plants.

Dealing with Diseased Plants

Prune affected foliage. You can manage early blight and late blight by trimming and destroying affected foliage or plants. Pruning lower affected branches can slow the progression of early blight. Often, removing and destroying plants affected with late blight can prevent its spread to other plants.

Burn, bury, or hot-compost diseased foliage and plants. Leaving diseased material in the garden can encourage these fungal diseases to continue to flourish. Remove any diseased plants from your plot. Generally, it’s best to bury them away from the garden or burn them to prevent spores from being transferred back into your garden. You can compost diseased plants as long as your pile reaches a temperature of at least 140 degrees Fahrenheit to kill fungal diseases.

Fungal diseases can put a damper on your tomato harvest. Recognizing signs of them early and taking steps to prevent them can help ensure your success in future seasons.


Jordan Charbonneau is a homesteader from West Virginia, where she lives in an off-grid cabin with her husband, Scott, and way too many animals. She’s a regular contributor to the Southern Exposure Seed Exchange blog, and you can also find her on her website, Rabbit Ridge Farm, at www.RabbitRidgeFarmWV.com.