Bacterial spot of Tomatoes

Xanthomonas campestris pv. vesicatoria

Hosts. Classification of the Xanthomonas species that cause bacterial leaf spot is currently in flux. Up until a few years ago, bacterial leaf spot causing Xanthomonads were divided into four groups, A, B, C, and D, based on pathogenicity and physiological characteristics. As of 2004 though, a new classification system was proposed that would change the name of X. campestris pv vesicatoria to X. euvesicatoria (previously group A), and recognized the species X. vesicatoria (group B), X. perforans (group C), and X. gardneri (group D). These changes have yet to be fully recognized, so this article will include information on all groups (A, B, C, and D) of bacterial leaf spot causing Xanthomonads.
The principal hosts of BLS causing Xanthomonads are tomatoes and peppers, though other incidental hosts have been recorded, mainly weeds. Group A contains most of the pepper infecting strains, though some strains from groups B and D have also been reported to cause symptoms on pepper plants. Strains from all four groups have been isolated from infected tomato plants. Some strains are able to infect both pepper and tomato plants, while some can only infect one of these plants.
Symptoms:
Bacterial leaf spot affects all above ground parts of the plant:
Fruit. Pepper fruit do not frequently show symptoms, largely due to the fact that developing fruit is often dropped if the pepper plant is infected. Fruit that isn’t dropped can also be damaged and misshapen due to increased sun exposure after plants drop infected leaves. When symptoms on the fruit do occur, they start as pale-green, water-soaked areas and eventually become raised, brown, and rough.
Leaves and Stems. Bacterial colonization of intracellular spaces induces the macroscopically visible symptoms including water-soaked lesions on the leaves that later become necrotic. Though these spots start out at about .24 inches in diameter, they increase in size and number, eventually causing the leaves to drop off. Plants can drop 50-100% of their foliage. BLS can also affect the stems of plants, leading to elongated, raised, light-brown cankers, less than .25 inch long. Defoliation occurs more commonly in pepper plants than tomatoes, so tomato plants with bacterial leaf spot often have a scorched appearance due to their diseased leaves.
Disease Cycle. Xanthamonas campestris pv. Vesicatoria survives on tomato and pepper plants, seeds, and debris from infected plants as it cannot live in the soil for more than a few weeks. The bacterium can also be found in association with wheat roots and some weed species which are both considered sources of inoculum as well as diseased tomato and pepper plants. In cold climates, Xanthamonas campestris pv. Vesicatoria infection is mostly caused by contaminated seed material, both on and inside of seeds. If it survives on seeds, it will infect the cotyledons of the growing plant as it emerges from the seed coat. Internally infected seeds will produce diseased plants from the point of germination. Systemic symptoms such as wilting, yellowing, and dwarfing are not typical of plants infected at the point of germination. However, foliage loss can happen when localized symptoms on leaves become severe. If the bacterium survives on debris, it may infect healthy plants through stomata as well as wounds on leaves and fruit. It is spread by direct contact of plants with debris, human movement of the bacteria from debris to plants, and can easily travel from debris to healthy plants through saturated soils via water movement. Once infected, plants begin to develop lesions on the leaves as well as fruit, becoming inoculum sources for further infection.

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