Recognizing tomato problems
Septoria leaf spot, caused by the fungus Septoria lycopersici, is the most common foliar disease of tomatoes in Iowa. It first appears as small, water-soaked spots that soon become circular spots about 1/8 inch in diameter. The lesions gradually develop grayish white centers with dark edges. The light-colored centers of these spots are the most distinctive symptom of Septoria leaf spot. Spores are spread to new leaves by splashing rain. Heavily infected leaves turn yellow, wither and eventually fall off. Lower leaves are infected first. Defoliation can be severe after periods of prolonged warm wet weather. Infection can occur at any stage of plant development but appears most frequently after plants have begun to set fruit. The fungus survives the winter in tomato debris.
To control Septoria leaf spot, a combination of cultural practices is often needed:
• Plant disease-free transplants far enough apart that the plants will not be crowded after they are full grown. This will allow air to circulate and the foliage to dry rapidly.
• Water at the base of the plants, and in the morning rather than the evening, to minimize the amount of time that the leaves are wet.
• Remove as much plant debris as possible in the fall and promptly plow under or bury the remaining residue.
• Avoid working with plants when leaves are wet.
• Rotate crops so that tomatoes are not grown in the same area of the garden every year.
Early blight, caused by the fungus Alternaria solani, is common in Iowa tomato plantings. Premature loss of lower leaves is the most obvious symptom of the disease. Brown to black spots, 1/4 to 1/2 inch in diameter with dark edges, appear on lower leaves. Spots frequently merge, forming irregular blotches. Dark, concentric rings often appear in leaf spots, resulting in the “target” appearance. Warm, wet weather favors rapid spread of early blight.
Cultural and chemical controls for early blight are the same as for Septoria leaf spot.
Anthracnose, caused by the fungus Colletotrichum coccodes, is probably the most common fruit-attacking disease of tomato in Iowa. Symptoms first become visible on ripe or ripening fruit as small, circular, indented spots in the skin. As these spots expand, they develop dark centers or concentric rings of dark specks, which are the spore-producing bodies of the fungus. In moist weather, these bodies exude large numbers of spores, giving diseased areas a cream to salmon-pink color. By this stage, decay has penetrated deeply into the tomato flesh. Anthracnose appears most commonly on overripe fruit.
The fungus survives the winter on diseased vines, in the soil and in the seeds. Weeks before the fruit ripens, anthracnose can become established on the leaf spots caused by other fungi or by insect-feeding injuries. Spores are spread by rain splash. Warm wet weather causes the disease to spread and the symptoms to develop.
Control measures for anthracnose are the same as for Septoria leaf spot. In addition, harvest at frequent intervals and pick all ripe fruit at each harvest.
Other problems are not caused by infectious microorganisms but rather by environmental stresses on the plant. Some of the physiological problems are blossom end rot and fruit cracking.
Why are my tomatoes cracking?
Fruit cracking is a common problem on tomatoes. Cracks usually appear at the top or stem end of the fruit. Cracks radiate out from the stem or circle the fruit in concentric rings. Fruit cracking is associated with wide fluctuations in soil moisture levels. A heavy rain or deep watering after a long, dry period results in rapid water uptake by the plant. The sudden uptake of water results in cracking of ripening fruit. Generally, fruit cracking is most common on the large, beefsteak-type tomatoes. 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. Mulching and avoiding heavy applications of nitrogen fertilizer should help reduce fruit cracking.
Why do my tomatoes have a spot of rot?
Blossom end rot is a very common problem of 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. Although blossom end rot itself causes only local injury, secondary organisms frequently invade the lesion and cause complete rotting of the fruit. It often occurs in rapidly developing fruit during hot, dry weather. To avoid blossom rot, water consistently and mulch around the plants to conserve soil moisture. Remove the affected fruit so that later-maturing fruit will develop normally.
Sharie Leazer is a Master Gardener Intern.