Bacterial speck is caused by Pseudomonas syringae pv. tomato, and was first reported in 1929 in Taiwan. It has been known in the United States since 1933, and has been widespread since the late 1970s. There has been an increase in reports of this disease in recent years. It primarily affects tomato and a number of weeds. The disease is favored by low temperatures (64o to 75o F) and high moisture.
Bacterial spot is caused by Xanthomonas campestris pv. vesicatora. The disease was first seen in 1912 in Texas, and is now widespread. In North America, it is found in the east from Florida to Quebec. It is also found in South America, Europe, Asia, and Africa. It causes spots on the leaves and fruit of tomato, pepper, black nightshade and ground cherry. This disease is favored by warmer temperatures (75o to 86o F) and high moisture.
Symptoms. Small, superficial black specks appear on fruit infected with bacterial speck. These specks are less than 1/16" in diameter, slightly raised and have a distinct margin. As the fruit grows, the specks often will become sunken into pits, since the healthy tissue grows faster than the specks. The tissue around the specks often remains green longer than the rest of the ripening fruit. The largest specks often appear on the smallest fruits, since immature green fruit is more susceptible.
Leaf spots are round and dark brown to black. A light-colored ring may appear around the spots when they are older. The spot goes all the way through the leaf, and is most visible on the bottom of the leaf. Often, several nearby spots will grow together. When this happens on young leaves, the tissue between the spots tears as the leaf grows. Spots on the petioles and stems are dark brown to black and oval to elongated. Spots can appear on all aboveground parts of the plant, except the petals.
Bacterial spot symptoms on leaves are greatly affected by moisture levels. Under moderate moisture, leaves have small less than 1/8"), black spots that are round yet somewhat angular. The upper surface may be somewhat greasy and translucent, with a black margin. The center, translucent tissue will soon dry and will appear very thin. It may crack. The spots will often be more numerous on younger leaves and may occur on only one or two leaflets on any given leaf. Under conditions of heavy moisture, the leaves will look blighted rather than spotted. The margins and the areas between the veins will yellow, and will later become water-soaked and greasy looking. Many small spots are usually present, and these may grow together. The leaf will eventually die. If seedlings are severely spotted, the leaves may turn yellow and drop.
Spots on fruit begin as dark raised spots surrounded by a water-soaked border. As the fruit expands, the spots may appear to be sunken in pits, as the healthy tissue will grow faster than the spotted tissue. Older spots may reach a diameter of 1/3" and are black, slightly raised and have lobed borders. They do not penetrate the seed cavity of the fruit, but are superficial. As they age, the centers of the spots disintegrate and sink, and eventually form a crater. The skin of the fruit is often cracked at the edge of older spots. The water-soaked border is absent for older spots.
Disease Differences. The two diseases can be differentiated in several ways. Bacterial spot causes larger fruit spots. The leaf spots for bacterial spot are angular, while those for bacterial speck are round. For a definite diagnosis, place a piece of diseased tissue in a drop of water under a microscope. The speck bacteria ooze out of the tissue in a semi-gelatinous mass, the spot bacteria ooze out freely.
Similar Diseases. Bacterial speck and spot might be confused with other common diseases. The symptoms of other diseases are somewhat different, though. Early blight leaf spots are generally larger (1/2"), have a target appearance, and may have a light-colored ring around each spot. Early blight causes large target-like spots near the stem of green or ripe fruit. Septoria leaf spots are circular, brown to greyish, and often have tiny black dots in them. Septoria causes no fruit spots.
Disease prevention. For both diseases, use disease-free seeds and transplants. Hot water treatment of seeds can be very helpful, as well. Avoid overhead irrigation. When irrigating, do it early in the day to allow the plants to dry before night. Avoid working the fields when wet. Control weeds and tomato volunteers. See current recommendations for chemical control measures.
For bacterial speck, use a crop rotation of at least one year. Keep cull piles away from production areas. Avoid clipping plants when transplanting. Plant resistant cultivars if they are available.
For bacterial spot, use a 2 to 3 year crop rotation without peppers or wheat. Control weeds. Plow under debris or remove from field. Research in Florida has indicated that high levels of nitrogen resulted in lower levels of this disease than did lower nitrogen levels. Higher calcium levels resulted in higher disease levels than did lower calcium levels. This knowledge may help in avoiding bacterial spot. No resistant varieties are available.
Pam Mercure, IPM Program Assistant, University of Connecticut
Sherf, A.F. and A. A. MacNab. 1986.Vegetable Diseases and Their Control. John Wiley and Sons, New York.
Jones, J.B. Bacterial Spot in Compendium of Tomato Diseases. 1991. APS Press. St. Paul, MN. p27. J.B. Jones, J. P. Jones, R.E. Stall, T. A. Zitter, eds.
Jones, J.B. Bacterial Speck in Compendium of Tomato Diseases. 1991. APS Press. St. Paul, MN. pp. 26- 27. J.B. Jones, J. P. Jones, R.E. Stall, T. A. Zitter, eds.
Jones, J.B., C.D. Stanley, A.A. Csizinszky, S.P. Kovach, and R.G. McGuire. 1988. Pottassium and Nitrogen Fertilization Rates Influence Susceptibility of Trickle Irrigated Tomato Plants to Bacterial Spot. HortScience. 23:1013-1015.
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