In addition to being prone to attack from insect pests, tomatoes are susceptible to a number of fungal diseases that create similar discolored spotting on leaves, stems, and fruit. Fungal diseases of tomatoes are often dependent on the weather; wet, humid weather is when these diseases are at their worst.
While there may not be much you can do to prevent some tomato fungal diseases, early identification may let you provide a treatment to minimize the damage and prevent further spread through your entire crop.
Three Common Fungal Diseases
Symptoms for the three most common fungal diseases of tomatoes are quite similar at first glance, but careful inspection should let you identify the precise fungus responsible.
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 family, including potatoes 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 Iron Lady, Mountain Supreme, Mountain Magic, Defiant PhR, Jasper, Juliet, and Verona.
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 is caused by the oomycete Phytophthora infestans, an organism that is not a true fungus though 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 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 fungus (Septoria lycopersici). It can affect tomatoes and other plants in the Solanaceae family, including potatoes and eggplant, Like early blight, septoria is most likely to occur during warm, wet weather.
When it infects leave, 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 septoria, and under a magnifying glass you may 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 varieties known to have resistance to septoria leaf spot include Heather, Summer Sweetheart, Green Zebra, Juliet F1, Brandywise, Iron Lady F1, and Stellar F1.
Identifying Tomato Diseases
If you think you are witnessing one of these three tomato diseases, this table can help you pinpoint the right one. 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, take caution with any gloves and tools you are using, as they can spread the fungus to other healthy plants. Be sure to clean your tools and gloves after gardening to prevent the spread of the fungus.
|Disease||Early Blight||Late Blight||Septoria Leaf Spot|
|Responsible Pathogen||Alternaria solani||Phytophthora infestans||Septoria lycopersici|
|Leaf Damage||Spots 1/4–1/2 inch in diameter, with tan centers, concentric rings, and yellow halos.||Pale green spots near tips, turning turning brown to purplish-black. When humid, fuzzy mold appears on leaf undersides.||Numerous brown spots 1/16–1/8 inch in diameter with black specks in center; no yellow halo.|
|Fruit Damage||Dark, sunken spots appear on the stem end of fruits.||Brown, leathery spots on top and sides of green fruit. When humid, white mold also forms.||Fruit not affected, though sunscald can be a problem due to foliage loss.|
|Stem Damage||Dark, sunken cankers at or above soil line.||Black and brown spots gradually spread. Entire vine killed quickly when humidity is high.||No stem damage.|
|Optimal Conditions||High humidity, temps 82–86 degrees F.||High humidity, temperatures 60– 80 degrees F.||High humidity, temperatures 60–and 80 degrees F.|
|Organic Treatment||Remove lower leaves after first fruit sets; remove affected leaves; plant in a different area next year.||Pull and destroy plants; select resistant varieties; plant tomatoes in a different area of the garden.||Remove infected foliage as it appears; clean tools before moving to another plant; plant in a different area next year.|
If this chart proves unhelpful in determining the cause, take photos or actual leaves (in a sealed plastic bag) to a garden store. A knowledgeable employee should be able to help identify the disease.