Introduced to Europe after the discovery of the New World, tomatoes were once thought by Europeans to be poisonous. The rumor is that European nobles ate the fruit on lead plates, and given that tomatoes are highly acidic, the chemical reaction between tomato and lead contaminated the food and poisoned nobles. Stories like these are likely myth, and luckily, our views of tomatoes have changed. Tomatoes are now considered the pride and joy of many gardeners’ summer harvests. They yield a tremendous amount of fruit, grow big, and are generally easy to grow!

However, there are still numerous obstacles one can encounter when growing tomatoes.

Whether your tomato plants are thriving or need mega life-support, below are a few of the common diseases that can affect tomatoes, how to diagnose your plants, and tips for prevention and remedies.

1.)    Blossom Drop

Symptom: The yellow flower dies and falls off the plant, producing no fruit.

Cause: Temperature

Extremely high or low temperatures interfere with a tomato plant’s ability to self-pollinate. If temperatures are too high, the flower pollen becomes tacky and non-viable for pollination. In low temperatures, the female part of the flower (pistil) can become inert, unable to be fertilized. When either case happens, the flowers remain unfertilized, and die as a result.

Tomatoes grow best between 70°F/21C° (nighttime) and 85F°/29C° (daytime), with humidity levels between 40-70%. Tomato plants can survive a day or two with extreme temperatures, but if you suspect an extended heat wave, consider shading them during the middle of the day. Also during hot weather, help your plants pollinate by shaking the leaves to help them pollinate themselves – bees tend not to pollinate gardens on exceptionally hot days. If you live in a very humid environment, some good varieties to grow are Eva Purple Ball, Flora-Dade, Grosse Lisse, Jubilee, Moneymaker, Sun Gold, Taxi, and Yellow Pear. If you live in a very hot environment, consider Florasette, Heat Wave, Solar Set, Sunchaser, Sunmaster, Sunpride, and Surfire varieties.

Other possible causes: too much or too little nitrogen, lack of water, stress from insects or fungus.

2.)    Blossom End Rot

Symptom: Rot at the bottom of the fruit (dark brown/green or black)

Cause: Calcium deficiency

Blossom end rot is a nutritional deficiency and not directly a result of any pest or fungus. More specifically, blossom end rot is a calcium deficiency in the fruit. Plant cell walls rely on calcium ions to maintain their strength and structure. A deficiency in calcium will cause those cell walls to weaken, producing cell decay and ultimately rot.

Soil conditions directly affect nutrient uptake in plants. Blossom end rot can also be the result of a lack of moisture in the soil, which inhibits the plant’s ability to absorb nutrients from its roots, up the stem to the fruit. If your moisture levels are good, then your soil might be calcium deficient. If you add calcium to your soil, be aware you could also raise your soil’s pH overtime. A good pH for tomato plants is around 6-6.8.

Pesticides and fungicides are useless against this kind of problem.

3.)    Yellowing Leaves

Symptom: Leaves turn yellowish or pale-green and lose their vibrant green color.

Cause: nutrition and/or fungi

Not to be confused with Early Blight (see below), yellowing is commonly the result of chlorosis. Chlorosis is a plant condition where leaves lack chlorophyll, the photosynthetic pigment that allows for sun absorption, causing them to turn yellow. If new leaves are yellow while older ones are green, the problem is usually a deficiency in manganese, zinc or nitrogen. If old leaves are yellow, but new leaves are green, then you have an iron deficiency (more common). Alkaline soil inhibits iron uptake -- anything above a pH of 7 is considered “alkaline”. Get a test kit to test your soil. Try to achieve pH of 6-6.8. Anything higher than 7 makes iron insoluble, preventing iron uptake for your plants.

If only a small portion of lower leaves are yellowing, it is not cause for worry. Plant sunburn can also cause some yellowing (see below).

4.)    Fusarium and/or Verticullum Wilt

Symptom: The plants leaves and stems are wilting, turning yellow or dark in color, and are decaying.

Cause: Fungal infection

If you see more intense discoloration beyond the pale yellowing of leaves, you could have a fungal infection caused by two fungi: Fusarium or Verticullum. You’ll see a narrow column of brown stains in the vascular tissue on the edge or middle of the stem. These diseases are soil-borne and can survive in the soil without a plant host for extended periods of time. Though the disease won’t kill your plants directly, it will compromise the plant’s water uptake by blocking the plant vessels from carrying nutrients and water throughout the plant, stunting growth and causing eventual decay. In addition, the toxins expelled by the virus are transmitted throughout the plant, infecting nearby leaves.

If you’re certain your plants are getting enough water, yet you see these symptoms, it’s likely a fungus problem. Unfortunately, garden-safe fungicides do poorly against these two fungi. The best methods to deal with this problem is through prevention. If you bring in outside plants from a nursery be sure to quarantine them for a couple of weeks before introducing them to your garden bed. Be sure to buy plants that carry a “V” on the plant tags. This means they have resistance or immunity to the fungi. If your plants show signs of the two fungi, dispose of them immediately. If not disposed of, they fungi may affect your garden for years.

5.)    Fruit Cracks

Symptom: Concentric or radial cracks near the stem of the fruit, usually on beef-steak varieties.

Cause: Wide fluctuations in watering

Fruit cracking happens when the skin on a fruit grows slower than the amount of water the fruit absorbs. It’s like a balloon being filled up with water until it eventually splits open. If a plant has a long dry spell with little to no water, a sudden surge of water can create an imbalance in water absorption by the plant, causing splitting and cracking.

Cracked fruit can rot easily, so if you see it, harvest immediately. Cracked fruit is ugly but totally safe to eat so long as it is not rotten! This problem is more common in large beef-steak varieties of tomatoes. Be sure always to give your plants a deep soaking once a week during hot weather to create more regularity in the soil moisture. Consider also using mulch to regulate weeds and slow moisture evaporation. Tomatoes with good crack resistance are: Cherry’s Delight, Creole FA, Mountain Pride VFF, Marglobe, Peron, JetStar and Red Calabash. 

6.)    Sun Scalding

Symptom: Pale, papery-like white blotches on fruit.

Cause: Sudden exposure to direct sunlight and high temperature.

Sun scalding happens when fruit is not protected by shade and is exposed to direct sunlight with high temperatures. Some varieties are more resistant to it than others. Usually it occurs on developing green fruit that was once protected by leafy shade but had that shade removed. The cell tissue becomes bleached by the sun, weakening the cell structure on the skin and giving it a paper-like consistency. Though there is nothing wrong with eating sun scalded fruit, such fruit can become infected by fungus or rot if left on the vine. If infected or rotted, don’t eat.

Gardeners might experience this problem when they have leaf disease on their plant. Pruning can expose previously shaded fruit to direct sunlight, creating the conditions for sun scalding. Be careful to not over prune your plants; tomato fruit needs shade.

Warning for hydroponic growers: Sun scalding is common when transplanting indoor plants to outdoor environments. Indoor lighting is less intense than the sun, so making a sudden move outdoors will shock your plants and cause sun scalding on their leaves. If moving outdoors, your plants need to be acclimated to the intense sun slowly (few hours a day for at least 2-3 days). Don’t abruptly put them under a hot summer sun with 14 hours of sunlight.

7.)    Early Blight

Symptom: Small dark spots on older leaves near the ground that gradually grow to about ½ inch in diameter infecting nearby parts of the plant. Infected stems turn brown, wilt, and rot. Infected fruit becomes leathery and black. Overall, it will look like your plant is decaying.

Cause: Fungus infection

There are two fungal pathogens that cause this problem: Alternaria tomatophila and Alternaria solani. Usually the fungus originates in the soil, which is why Early Blight affects the bottom portion of the plants that have closest contact with the soil. Tiny fungal spores splash up from the soil, contaminating the plant. The spores can spread by wind, human contact, or garden equipment, which can cause re-infection in future seasons. Like most fungi, it spreads most rapidly in highly humid environments (over 90%) and spores can germinate in a wide range of temperature: between 47°F/8°C and 90°F/32°C.

Some gardeners will see Early Blight and begin removing the affected leaves, attempting to prevent the fungus from spreading to other parts of the plant. However, there is an incubation period of these pathogens, and symptoms of the disease usually mean the infection began a week or so earlier. If you see signs of Early Blight, they do make some organic fungicides. Look for products with active ingredients “Penthiopyrad” and “Boscalid.” Unless you’re confident that the infection is small, I’d suggest removing the plant to avoid infecting nearby plants.

For prevention, be sure to remove all plant debris after each harvest and thoroughly clean your soil of all weeds. Don’t water the tops of your plants; water at the base to avoid exposing leaves to moisture. To avoid fungal transmission, don’t harvest fruit or work the garden when the leaves are wet. Mulch can act as a barrier between soil and leaves, which can help prevent fungal contamination. Tomato varieties resistant to these fungal infections are Early Cascade, Floramerica, Jetstar, Manlucie, Supersonic, and Surecrop.

8.)    Powdery Mildew

Symptom: White, powdery-looking spots on the leaves, eventually causing leaf decay.

Cause: Fungal infection

There are variety of types of fungus that cause this, depending on where you live. For example, powdery mildew in California, New York, and Florida are caused by different fungal strains. Powdery mildew can wreak havoc on your crops. The fungal spores are airborne, so one infected plant can infect the whole lot. Greenhouses are more susceptible to macro-infection due to the lack of open air and close proximity between plants (greenhouse plants aren’t good at social distancing). These fungi are “obligate” parasites, which means they can only survive on a living host – unlike Early Blight. They harm plants by invading the host cells and extracting nutrients, causing the plant leaves to wither and die. These fungi do not need moisture to spread, making them particularly problematic to control. The fungi do not spread to the fruit, though fruit production will be drastically disrupted or eliminated by the infection.

Because plants can’t take medicine or antibiotics like humans, most plant infection needs to be addressed topically. You can spray preventative chemicals on the non-affected plants and remove the affected plants. Unfortunately, the disease spread extremely rapidly, so if you see powdery mildew, it may be too late. There are various DIY treatments on the internet, including baking soda, soap, and water, though I haven’t tried these.

9.)    Leaf Curl or Leaf Roll

Symptom: Edges of leaves roll up or twist towards their stems.

Causes: There are probably 6-7 different causes of leaf roll, ranging from physiological issues, environmental problems, to disease and insects. Unfortunately, there is no unanimous agreement about which one is most prevalent. The causes include: over-fertilization, under-watering, over-watering, excessive top growth, bad root growth, root damage, high winds, improper soil fertility (pH), and broad mites. Vine tomatoes (indeterminate) tend to suffer from leaf roll more than bush tomatoes (determinate).

The good news is that leaf roll usually doesn’t significantly affect crop yield, and it is not a direct sign of imminent health decay. If you see leaf roll, do some basic checks on your plants and see if there are other more significant problems present. If not, then just keep an eye on your plants. If those leaves begin to turn yellow, then you could have an iron deficiency problem (see “Yellow Leaves” above).

10.)    Black Spots (Septoria Leaf Spot)

Symptom: Brown to black round spots about the size of a blunt pencil tip. If you see larger black spots on fruit the size of a pencil eraser, this is a sign of a bacterial infection. Do not eat tomatoes with bacterial infection.

Cause: Fungal infection

Septoria leaf spot is a fungal infection that affects the plant leaves but not the fruit. The Septoria pathogen usually spreads from water splashing on the underside of the plant, infecting the lower, inner leaves first. The pathogens then can travel up the plant, showing up as dark spots speckled throughout the leaves. Eventually, the leaves turn yellow, then brown, and die off. This fungus is annoying as you could be playing cat-and-mouse with i all season.

Once evident, the Septoria leaf spot cannot be cured, only managed. You can remove some leaves affected by the fungus, but don’t become overzealous. Like with any disease, there is usually an incubation period, so symptoms of a disease won’t show up until it’s already been infected. If you do snip off affected leaves, never snip off more than ¼ of the leaves. The more leaves you snip off, the more of the pathogens you remove, but, by removing leaves, you’re also removing the plant’s energy source, which compromises its vitality. Use caution. When clipping off infected leaves, dispose of them far away from any garden or compost to limit re-infection. As a general rule, do not compost any diseased vegetation, including leaves with Septoria pathogens.