Tomato Diseases and Disorders in the Home Garden

As vegetable gardeners, we look forward to harvesting armfuls of produce throughout the growing season. Of these, tomatoes win the prize as the most popular vegetable grown in backyard gardens across America. Whether tomatoes are grown in pots, raised beds, or in a truck patch, homeowners need to be aware of tomato diseases.

Disease can strike quickly and be difficult to diagnose. Tomatoes (Solanum lycopersicum) can be grown in almost any moderately well-drained soil. A good amount of organic matter will increase yield and reduce production problems. Tomatoes and related vegetables, such as potatoes, peppers, and eggplant, should not be planted in the same area more than once in three years. Ideally, any cover crop or crop preceding tomatoes should be members of the grass family. Corn, an excellent rotation crop with tomatoes, supplies large amounts of organic matter and does not promote the growth of disease organisms that attack tomatoes.

Early Blight

Early Blight is caused by the fungus Alternaria solani, which is present worldwide wherever tomatoes are grown. Prevalent throughout the United States, the fungus survives on infected debris in the soil, on seed and volunteer tomato plants, and other solanaceous hosts, such as Irish potato, eggplant, and black nightshade. Spores can spread to tomatoes in spring via wind or splashing rain. The fungus needs a wet surface to germinate and grow. Low leaves that drip with morning dew provide perfect conditions for early blight.

The fungus is first observed on the plants as small, brown lesions, mostly on the older foliage. These spots enlarge and concentric rings in a bull's-eye pattern are visible in the center of the diseased area. As the spots expand, tissue surrounding the spots may turn yellow. In most cases, early blight damage will be limited to the lower third of the tomato plant. If warm temperatures and humidity occur at this time, much of the foliage is killed. Lesions on the stems are similar to those on leaves and sometimes girdle the plant if they occur near the soil line (collar rot). On the fruits, lesions reach considerable size, usually involving nearly the entire fruit. Concentric rings are also present on the fruit. Infected fruit frequently drops.

To prevent early blight, plant disease-resistant tomato cultivars. Eradicate weeds and volunteer tomato plants, rotate crops, space plants appropriately so they do not touch each other, mulch and fertilize properly, and avoid wetting foliage with irrigation water. Remove and properly dispose of infected lower branches and leaves--do not compost. At the first sign of early blight, prune off all leaves within 12 inches of the ground. Disinfect clippers in a solution of one part Clorox bleach to nine parts water.  Do not remove more than 20% of the plant's total leaf mass.

Septoria Leaf Spot

Septoria Leaf Spot is a destructive disease of tomato foliage, infecting the petioles and stems by the fungus Septoria lycopersici; the fruit is not affected. Infection usually occurs on the lower leaves after plants begin to set fruit. Early symptoms are usually dark spots, eventually, the center of the spot becomes whitish to tan, sometimes with tiny, dark specks (spore-producing bodies). Like early blight, Septoria leaf spot symptoms usually start on the oldest leaves. Spotted leaves die prematurely, resulting in early defoliation, fruit sunscald, and poor fruit flavor and color. Severely spotted leaves turn yellow, die, and fall off the plant. The fungus is most active when temperatures range from 68 to 77°F, the humidity is high, and rainfall or overhead irrigation wets the foliage. The fungus is not soil-borne, but can overwinter on residue from previous crops, decaying vegetation, and some wild hosts related to tomato.

Bacterial Spot

Bacterial Spot is becoming an increasingly devastating disease of tomato. Not only can the pathogen directly damage the fruit, severe foliar infection can lead to defoliation, reducing both the quality and quantity of the fruit. This may be due in part to the increasing frequency of severe weather events that favor disease development. Young bacterial spot foliar lesions can be difficult to distinguish from those of early blight. When managing bacterial spot as well as other bacterial diseases, it is important to keep in mind that bacteria readily multiply and spread under warm, wet conditions.

Late Blight

Late Blight is a potentially serious disease of potato and tomato caused by the fungus Phytophthora infestans. Late blight is especially damaging during cool, wet weather. The fungus can affect all plant parts. Young leaf lesions are small and appear as dark, water-soaked spots. These leaf spots quickly enlarge, and a white mold will appear along the margins of the affected area on the lower surface of leaves. Complete defoliation (browning and shriveling of leaves and stems) can occur within 14 days of the first symptoms. But, if you don't recognize the initial symptoms, it can seem as though the plant dies in a few short days. Late blight destroys the leaves of the plants, thus depriving plants of photosynthetic energy supply.  Infected tomato fruits develop shiny, dark, or olive-colored lesions, which may cover large areas.  Fungal spores are spread between plants and gardens by rain and wind. A combination of daytime temperatures in the upper 70s with high humidity is ideal for infection. Infections have become increasingly common in the Northeast. Never accept tomato seedlings that show suspicious leaf spots of any kind.

The following problems are caused by environmental stresses on the plant caused by too much or too little nutrients, moisture, or light. Plants also experience stress from chemicals, air pollution, and competition with other plants.

Blossom End Rot

Blossom end rot is a very common problem on green and ripe tomatoes. It first appears as a sunken, brownish black spot 1⁄2 to 1 inch in diameter on the blossom end of the fruit. Blossom end rot is caused by a calcium deficiency that is related to low levels of calcium in the soil or wide fluctuations in available moisture. To prevent blossom end rot, maintain a steady rate of plant growth without stress. A consistent and ample supply of moisture can reduce the problem by maintaining a steady flow of calcium from the soil to the fruit. Mulching also helps conserve soil moisture. Blossom end rot is more serious when an excess of nitrogen fertilizer has been applied. If blossom end rot occurs, remove the affected fruit so that later-maturing fruit will develop normally.

Fruit Cracking 

Fruit cracking is associated with rapid fruit development and wide fluctuations in water availability to the plant. Two types of cracks may develop on tomato fruit. Radial growth cracks extend from the stem area, and concentric cracks encircle the fruit, usually on the shoulders.

Fruit that has reached the ripening stage during dry weather may show considerable cracking if the dry period is followed by heavy rains and high temperatures. Tomato varieties differ considerably in the amount and severity of cracking under climatic conditions. The severity of cracking is determined by rainfall and irrigation amounts, variety, and stage of maturity. As the fruit ripens, the strength of bonding between cells progressively decreases, resulting in more severe cracking.

There are steps you can take to help prevent or minimize diseases. Purchase certified seed. When buying plants, look for cultivars resistant to common diseases.  Maintain good air circulation around plants; don't crowd them. Follow proper fertilization and irrigation methods. Rotate crops; never plant tomatoes in the same spot and try a three-year rotation if possible. Remove diseased plant material and dispose of it.  Do not compost it.