Many tomato diseases can be prevented and some can be cured.
Tue, Jul 27 2010 at 4:16 PM
Tomato diseases seem to come in as many varieties as tomatoes themselves.
Every gardening enthusiast dreams of growing juicy red tomatoes but several factors must be considered in the process of planting and harvesting in order to (hopefully) end up with good quality, disease-free fruit.
The diseases that affect tomatoes range from viruses and pests to harmful chemicals. And although most problems arise from unfavorable weather conditions, the diseases can be remedied easily if plants are carefully supervised.
Here are some common tomato diseases, based on three broad categories, along with the symptoms and preventive measures.
Fusarium wilt is caused by fungi of the Fusarium species known as Fusarium oxysporum. Yellowing begins on the lower tomato leaves, which turn dingy brown and begin to wilt. Soon the tomato shoots become infected and begin wilting as well. Typically, wilting begins on one side of the plant’s leaves or shoots and spreads to the other side as the infection worsens. Once the infection spreads, the plant vessels become damaged and block the water-feeding system.
Fusarium wilt favors temperatures between 70 and 80 F. It especially likes wet weather because it allows the disease to spread quickly through nearby tomato plants. Fusarium wilt is also spread by contaminated seeds, soil, plants and equipment. It often enters a new plant through the roots when transplanting.
Unfortunately, no chemical control is available to treat this infection. The best bet would be to plant where tomato plants have not been planted for a term of two to four years. While planting on infected soil, fumigation of the soil with meta-sodium two to three weeks prior to planting proves effective in removing Fusarium fungus from the soil. Tomato plant varieties resistant to Fusarium wilt are commonly marked with a capitol “F” on the seed packet or transplant.
Verticillium wilt is caused by the fungi Verticillium albo-atrum and V. dahliae. Verticillium wilt is a soil-borne fungal disease that starts as yellow, V-shaped areas, which narrow at the leaf margins. These yellow areas grow over time, turn brown and the leaf dies soon after.
Despite the sickly appearance of infected branches, the upper part of the plant often continues to grow. Tomatoes growing on infected branches drop before reaching maturity or get sunburned due to the lack of shade that the foliage would have provided.
The Verticillium fungus survives up to eight years in the form of tiny black resting structures called microsclerotia. Soil temperatures of 70 to 77 F favor infection and disease development. The disease seems to be more severe in neutral to alkaline (high pH) soils. Avoiding plantation in the same spot for at least three to five years can help prevention of Verticillium wilt. If it is a recurring problem, wilt-resistant varieties could be purchased. These are labeled with a “V” on the plant tag or seed packet.
Fungal leaf and fruit spots or blights
Early blight is caused by Alternaria solani and the disease may cause serious defoliation, resulting in decreased yield and quality. The fungus can affect the foliage, stems and fruit of tomatoes. A. solani survives between crops on infected plant debris and can be carried on tomato seeds and infected tubers. It first appears as small brown to black lesions on older foliage. The surrounding tissues may turn yellow and numerous leaves may be affected. As the wounds enlarge, they often develop concentric rings giving them a ‘bull’s eye’ or ‘target-spot’ appearance. Affected leaves may die prematurely, exposing the fruits to sunscald.
Wet weather and stressed plants increase the likelihood of attack. Removing affected plants and thoroughly cleaning the garden debris is one of the most effective prevention measures. Copper and sulfur sprays can prevent further development of the fungus. Using resistant or tolerant cultivars, carefully handling the tubers to prevent wounding and permitting tubers to mature in the ground before harvesting are also helpful measures.
Late blight, caused by the fungus Phytophthora infestans is a disease that progresses very rapidly and affects both the leaves and the fruit of the tomato plant. Lesions start as irregular, greenish, water-soaked spots on leaves, petioles and stems. Under cool, moist conditions, spots rapidly enlarge to form purplish black lesions that can girdle affected stems, killing the foliage farther out. During periods of high humidity and leaf wetness, a cottony white mold is visible on lower leaf surfaces at the edges of lesions. In dry weather, infected leaf tissues quickly dry up and the white mold disappears. On green fruits, gray-green water-soaked spots form, enlarge, coalesce and darken, resulting in large, firm, brown, leathery-appearing lesions. If conditions remain moist, abundant white mold develops on the lesions and secondary soft-rot bacteria may follow, resulting in a slimy wet rot of the entire fruit. On ripe fruits, lesions have cream-colored concentric zones, which eventually coalesce and affect the entire fruit.
Measures that prevent late blight in tomato plants include checking for dark lesions on leaves or stems, air-drying freshly harvested seeds at least three days, destroying unplanned or 'wild' tomatoes by cultivation or herbicides, avoiding wetting of foliage when irrigating, keeping greenhouse temperatures warm and selecting fields where winds enhance drying of the plants or have shorter dew periods. Also, the bio-control material, Serenade, can be used until harvest time, and Sonata can be applied up to and on the day of harvest. Copper sprays offer some control as well.
Blossom end rot is a very common problem on green and ripe tomatoes. It occurs at the end of the blossom and rots the tomato on the bottom. It is first seen when the fruit is about half its full-grown size. Blossom end rot causes a water soaked spot on the fruit that grows and darkens, and it might actually cover as much as half of the fruit, as it develops further. The spores become leathery black and completely ruin the fruit.
Blossom end rot is caused by a calcium deficiency that is related to wide fluctuations in available moisture. A consistent and ample supply of moisture can reduce the problem by helping to maintain a steady flow of calcium from the soil to the fruit. Mulching the area around tomato plants also helps conserve soil moisture and reduce fruit cracking. Blossom end rot is more serious when nitrogen fertilizers have been applied in excess. Staking and pruning tomato plants may also increase the occurrence of blossom end rot.
Herbicide injury is caused by misapplication or drift of 2,4-D, MCPP and other growth regulator herbicides. Tomato plants are highly sensitive to these chemicals throughout the growing season. First symptoms are seen in the downward curling of leaves in growing plants. Leaves often become narrow and twisted toward the tip, with prominent, light-colored veins. The symptoms are most pronounced on portions of the plant that are actively growing when the exposure occurs. In severe cases, stems and petioles become thick, stiff, and brittle with warty outgrowths. Affected plants usually recover. The fruit, however, may become cat-faced or develop a plum shape and may be hollow and seedless.
Spraying at low pressures, using a coarse-spray nozzle and applying the spray as close to the ground as possible helps prevent herbicide injury. Also avoiding the use of pesticides in sprayers that have previously contained herbicide is a good practice, as traces of herbicide are likely to remain in the sprayer even after thorough rinsing.
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