Tomatoes may be the nation’s most popular garden vegetable. Whether gardeners grow them in pots, raised beds or in a truck patch, they need to be aware of diseases that can strike quickly and be difficult to recognize.

Tomatoes (Solanum lycopersicum) can be grown in almost any moderately well-drained soil. A good amount of organic matter can increase yield and reduce production problems.

But, tomatoes and related vegetables, such as potatoes, peppers and eggplants, should not be planted on the same land 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.

Here are some tomato diseases:

• Early blight — Caused by the fungus Alternaria solani, which is present worldwide and survives on infected debris in soil, on seed, on 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 first appears as small, brown lesions, mostly on older foliage. The spots enlarge and concentric rings in a bull’s-eye pattern can be seen in the center of the diseased area. As the spots expand, surrounding tissue may turn yellow.

In most cases, early blight damage will be limited to the lower third of tomato plants. If warm temperature 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, use resistant or tolerant tomato cultivars. Use crop rotation, eradicate weeds and volunteer tomato plants, space plants so they do not touch each other, mulch plants, fertilize properly, don’t wet plants with irrigation water, and keep plants growing vigorously.

Trim off and dispose of infected lower branches and leaves. Do not compost. Clip off all leaves within 12 inches of the ground, but do not remove more than 20% of the plant’s total leaf mass.

Clean the clippers between plants and sterilize them in a solution of 1 part Clorox bleach to 9 parts water.

• Septoria leaf spot — This disease, caused by the fungus Septoria lycopersici, affects tomato foliage (petioles and stems) but not the fruit. Infection usually occurs on the leaves near the ground, after plants begin to set fruit. Early symptoms are usually dark spots on the oldest leaves; eventually, the center of the spot becomes whitish to tan, sometimes with tiny, dark specks, which are spore-producing bodies, evident in the light area.

Young bacterial spot foliar lesions can be difficult to distinguish from those of early blight or Septoria leaf spot; however, as the lesions expand, early blight lesions will develop concentric rings while lesions from Septoria will become tan in the center will small black dots called pycnidia.

Early blight fruit lesions will develop similar concentric rings as on the leaves while Septoria-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 degrees, the humidity is high, and rainfall or overhead irrigation wets the plants. The fungus is not soil-borne but can overwinter on crop residue from previous crops, decaying vegetation and some wild hosts related to tomato.

• Bacterial spot — Not only can this pathogen directly damage the fruit, severe foliar infection can lead to defoliation, reducing both the quality and quantity of the fruit.

• Bacterial wilt — Also called Southern bacterial blight, this serious disease is caused by Ralstonia solanacearum (formerly Pseudomonas solanacearum), a bacterium that survives in the soil for extended periods and enters roots through wounds made by transplanting, cultivation or insects, or where secondary roots emerge.

Disease development is favored by high temperatures and high moisture. Bacteria multiply rapidly inside water-conducting tissues, filling them with slime and resulting in a rapid wilt of the plant, while the leaves stay green. If an infected stem is cut crosswise, it will look brown and tiny drops of yellowish ooze may be visible.

• Fusarium wilt — This warm-weather disease is caused by the fungus Fusarium oxysporum. First, drooping and wilting of lower leaves will occur, with a loss of green color followed by wilting and death of the plant. Often leaves on only one side of the stem turn golden yellow first. The stem of wilted plants shows no soft decay but, when cut lengthwise, the lower stem will have a dark brown discoloration.

The fungus is soil-borne and passes upward from the roots into the stem. Invasion occurs through wounds in roots growing through infected soil. Long-distance spread is through seed and transplants.

To combat, grow plants in pathogen-free soil, using disease-free transplants and growing only cultivars at least resistant to races 1 and 2 of Fusarium wilt. No chemical control is available.

• Late blight — Especially damaging during cool, wet weather, this potentially serious disease of potato and tomato is caused by the fungus Phytophthora infestans, which can affect all plant parts.

Young leaf lesions are small and appear as dark, water-soaked spots. They quickly enlarge, and a white mold will appear at the margins of the affected area on the lower surface of leaves. Complete defoliation can occur within 14 days from the first symptoms. 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.

No single is effective against all important foliar diseases. For example, mancozeb and chlorothalonil give good control, but copper gives only fair control of early blight. Chlorothalonil is one the best products for managing late blight.

Copper-based products are effective against bacterial diseases, but some strains of spot bacteria are resistant to copper. Therefore, it may be necessary to use a combination of products in a spray program to optimize disease management.

One important consideration is that products have different preharvest intervals (PHI). Once you spray, you should wait for the recommended number of days before you harvest.

The following problems are caused by environmental stresses on plants, such as too much or not enough nutrients, moisture and light; exposure to toxic chemicals or air pollution, and competition with other plants:

• Blossom end rot — A very common problem on green and ripe tomatoes, this first appears as a sunken brownish-black spot 1⁄2 to 1 inch in diameter on the blossom end of the fruit. It is caused by a calcium deficiency or wide fluctuations in moisture.

To prevent, maintain a steady rate of plant growth without stress. A consistent and ample supply of moisture can help maintain a steady flow of calcium from the soil to the fruit. Mulching also will help by conserving soil moisture.

Blossom end rot is more serious when an excess of nitrogen fertilizer has been applied.

If blossom end rot occurs, remove affected fruit so that later-maturing fruit will develop normally.

• Fruit cracking — This is associated with rapid fruit development and wide fluctuations in water availability. Radial growth cracks radiate from the stem while 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. As the fruit ripens, the strength of bonding between cells progressively decreases, resulting in more severe cracking.

To prevent or minimize diseases:

• Purchase certified seed.

• Buy 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. Try a three-year rotation, if possible.

• Remove diseased plant material and dispose of it. Do not compost it.

For more help, stop at an Extension office or a garden store and give a detailed description of what is visible on your plants. If you take diseased leaf samples for identification purposes, keep them in a sealed plastic bag so the disease does not spread.

The Penn State Master Gardeners of Columbia County and Montour County are at the Ferry Street Growers’ Market the first Saturday of the month, weather permitting. Penn State MG volunteers help to educate the public on best practices in consumer horticulture and environmental stewardship.

Leighow has been a Penn State Extension Master Gardener of Columbia County since 1995 and gardens in Numidia.

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