Diagnosing and Treating Three Common Tomato Fungal Diseases

In addition to being vulnerable to insect pests, tomatoes are susceptible to a number of fungal diseases that create similar-looking discolored spots on leaves, stems, and fruit. Fungal diseases of tomatoes are often dependent on the weather; wet, humid conditions are when these diseases are at their worst.

While there might not be much you can do to prevent some tomato fungal diseases, early identification enables you to provide a treatment to minimize the damage and prevent further spread through your entire crop.

Many hybrid tomatoes have been bred to resist some types of fungal diseases. But, being disease resistant does not mean complete immunity. Your tomato plants might still become infected, but the built-in resistance means they might not succumb to the disease as easily as a variety that does not have any resistance at all.

Three Common Fungal Diseases

Symptoms for the three most common fungal diseases of tomatoes are quite similar at first glance, but careful inspection should enable you identify the fungi responsible for the disease.

Early Blight

Early blight is one of the most common of all tomato diseases, appearing nearly every season and affecting the leaves, stems, and fruit of affected tomato plants. Severe cases can nearly defoliate a plant, resulting in sun-scald to the fruit. There are two different, but closely related, fungi that cause early blight: Alternaria tomatophila and Alternaria solani. 

Alternaria tomatophila is the more virulent fungi, so in regions where it is prevalent, it is usually the cause of early blight. If A. tomatophila is not found in the region, early blight is usually caused by A. solani. Both these fungi can also affect other members of the nightshade crop family, including potatoes, peppers, and eggplant.

Early blight first appears as small dark spots on older foliage located close to ground level. As the spots grow, they develop target-like rings. The fungus can also affect stems, gradually girdling the plant until it kills the plant. When affecting fruit, the spots are leathery lesions with raised concentric rings. Eventually, the fruit will drop from the vines.

Early blight is most likely to occur during warm weather (82 to 86 degrees Fahrenheit) that is also humid or rainy. A variety of fungicides will slow the spread of the disease; experts recommend rotating chemical fungicides to prevent the disease from developing resistance. A number of cultivars that are resistant to early blight are available, including 'Mountain Supreme', 'Mountain Magic', 'Defiant PhR', 'Jasper', 'Juliet', and 'Verona'.

Late Blight

Late blight is a very serious disease of tomatoes and potatoes—it was the disease responsible for the Irish potato famine of the 1840s. While it is often considered a fungal disease, late blight is actually caused by the oomycete Phytophthora infestans, an organism that is not a true fungi although it exhibits similar traits.

When late blight affects tomato leaves, large brown patches develop. Stem infections show up as hard, dark-brown sections with rounded edges. When the organism affects fruit, dark brown hard spots develop that eventually turn mushy as secondary bacterial infections set it.

Unlike early blight, late blight is more likely to occur during cool damp weather. Various fungicides can sometimes prevent the disease if plants are treated early enough, but once the disease sets in, affected plants must be removed and destroyed to prevent the fungus from ravaging all nearby tomato, pepper, and potato plants.

There are a number of cultivars bred to be resistant to late blight, including 'Mountain Magic' (F1), 'Plum Regal' (F1), 'Defiant PhR' (F1), 'Mountain Merit' (F1), Iron 'Lady' (F1), 'Jasper' (F1), 'Red Pearl' (F1), 'Legend', 'Matt's Wild Cherry', 'Wapsipinicon Peach', 'Lemon Drop', and 'Pruden's Purple'.

Septoria Leaf Spot

Septoria leaf spot It is caused by a fungi (Septoria lycopersici). It can affect tomatoes and other plants in the nightshade (Solanaceae) family, including peppers, potatoes and eggplant,  Like early blight, septoria leaf spot is most likely to occur during warm, wet weather.

When it infects leaves, septoria leaf spots begin as small spots on the undersides of leaves near the bottom of the plant. The spots have dark brown margins with light gray or tan centers and sometimes yellow halos around the spots. These spots are quite distinctive to the disease, and under a magnifying glass you might even be able to see the fungal bodies. Stems can also be affected, but septoria rarely affects the fruit.

Several organic and chemical fungicides can control septoria if applied early. Some tomato varieties like 'Stellar' and 'Brandywise" are known to have resistance to septoria leaf spot.

Identifying Tomato Diseases

If you think your plants might be infected with any of these three tomato diseases, the following table can help you pinpoint the actual culprit The symptoms of early blight, late blight, and septoria leaf spot can be very similar, but if you know what to look for, you shouldn't have much trouble identifying the precise pathogen. Included in the table are some well-known organic treatment methods for each disease.

As you treat your tomatoes, make sure to disinfect any gloves and tools you are using, as they can spread the fungus to other healthy plants. Be sure to clean and sanitize your tools and gloves after touching your plants. Avoid touching plants when they are wet because that too can spread disease.

If this chart is not helpful in determining the cause of your tomato plant disease, contact your local county extension office and provide them with close-up photos or plant cuttings (in a sealed plastic bag) to help them identify the disease for you.