Dealing with Diseases and Disorders of Tomatoes

Each year I receive numerous calls and emails from home owners regarding sick tomato plants. Most of the time it is Early Blight or Septoria Leaf Spot that is affecting the plants. With all the rain we have had the last four years, it’s no wonder these diseases are so prevalent. Here are some of the most common diseases and disorders I see on tomato plants and how to treat them.

early blight



Early Blight is caused by the fungus Alternaria solani. It survives in infected leaf or stem tissues on or in the soil. The first symptoms usually appear on the older, lower leaves of the plant and consist of small, irregular, dark brown to black, dead spots ranging in size from a pinpoint to 1/2 inch in diameter.  As the spots enlarge, concentric rings may form as a result of irregular growth patterns by the organism in the leaf tissue. This gives the lesion a characteristic "target-spot" or "bull's eye" appearance. There is often a narrow, yellow halo around each spot and lesions are usually bordered by veins. When spots are numerous, they may grow together, causing infected leaves to turn yellow and die. If not controlled, the leaves dry up and drop from the plant as the disease progresses up the main stem. Recommended practices include mulching plants with straw, rotation, removing lower leaves that touch or are near the ground, and the use of fungicides. Daconil, Maneb, and Mancozeb are recommended for control, or any product containing Chlorothalonil. Always read the label directions before applying any pesticide.


septoria leaf spot

Septoria Leaf Spot: Numerous, small, watersoaked spots, which are the first noticeable characteristic of Septoria leaf spot, appear on the lower leaves after fruit set. Spots enlarge to a uniform size of  approximately 1/16 to 1/4 inch in diameter. They have dark brown borders and tan or light colored centers. 

Yellow haloes often surround the spots. Severely infected leaves die and drop off. Septoria leaf spot is easily distinguished from early blight by the uniform, small size of the spots and the lack of concentric rings in the spots; however, Septoria leaf spot is sometimes confused with bacterial spot of tomato.  The presence of fruiting bodies of the fungus, visible as tiny black specks in the centers of the spots, confirms Septoria leaf spot. Control weeds in and around the edge of the garden. Preventative practices include mulching plants with straw, rotation, removing lower leaves that touch or are near the ground. Fungicides containing chlorothalonil such as Daconil and Maneb are recommended for the homeowner.

bacterial spot

Bacterial Speck/Spot are bacterial diseases of tomato that can cause localized epidemics during warm (spot) or cool (speck), moist conditions. Bacterial spot can cause moderate to severe defoliation, blossom blight, and lesions on developing fruit. Foliar symptoms of bacterial spot and speck are identical. Small, water-soaked, greasy spots about 1/8 inch in diameter appear on infected leaflets. After a few days, these lesions are often surrounded by yellow halos and the centers dry out and frequently tear. Lesions may coalesce to form large, irregular dead spots. In mature plants, leaflet infection is most concentrated on fully expanded and older leaves and some defoliation may occur. Spots may also appear on seedling stems and fruit pedicels. Unlike Early Blight that occurs on the lower leaves, Bacterial spot and speck will occur all over the plant. Applications of mancozeb plus copper soon after transplanting may help retard development and spread of bacterial spot and speck. Rotate tomatoes with non-solanaceous crops with at least 2 to 3 years between tomato crops. Avoid rotation with peppers, which are also susceptible to bacterial spot. Plant only seed from disease free plants or seed treated to reduce any bacterial population.

cracking

Cracking is a physiological disorder caused by wide fluctuations in soil moisture. Tomatoes often start to crack during warm, rainy periods, especially if this weather comes after a dry spell. The tomatoes expand too fast and are most likely to crack when they have reached full size and are beginning to turn color. Some resistant varieties include Early Girl and Jet Star. Be sure to apply adequate moisture throughout the growing season to avoid the problem.

catfacing

 Catfacing is another physiological disorder of tomatoes. Tomatoes develop unusual swelling and streaks of scar tissue. It is caused by abnormal development of the tomato flower at blossom time. Cold weather at the time of blossom set intensifies the deformities. Catfacing is not a disease. It is most common in the large-fruited beefsteak type tomatoes.




blossom end rot



Blossom-end rot is a disorder that occurs on the bottom or blossom end of the fruit. It appears as a sunken, water soaked spot. The spot turns brown or black, and dry and leathery as it grows larger. It is not an infectious disease. It affects both green and ripe tomatoes and is caused by a calcium deficiency, which is usually the result of wide fluctuations in soil moisture. Keep tomato plants well watered so they can take up the calcium from the soil.



sun scald



Sunscald develops when high temperatures retard the development of good color. Tomato fruits exposed directly to the hot sun may scald. Sunscald is localized damage to the tissue often accompanied by discoloration. Good foliage cover is helpful in preventing scalding.

Heat Stress: Yellow Shoulder is caused from high temperatures and causes the shoulder or top of the tomato to turn yellow. Hard white cores in tomatoes are also a heat stress disorder. Keep plants well watered and consider erecting a shade cloth over your plants.

Many of the disorders are quite common. They are not caused by insects or disease and are not infectious. Little can be done for most of them, but the fruit may be eaten if the affected portions are removed.
(by Jennifer Schutter, Horticulture Specialist, Adair County, University of Missouri)
(photos are from various locations)