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BACTERIAL DISEASES OF PLANTS AND THEIR CONTROL

Aleksa Obradović

University of Belgrade, Faculty of Agriculture, Institute for Phytomedicine, Belgrade, Serbia

E-mail: aleksao@agrif.bg.ac.rs

Prokaryotes associated with plants

Soon after Robert Koch’s (1843–1910) discovery in 1876, that anthrax, a disease of animals, including humans, was caused by a bacterium Bacillus anthracis, T. J. Burrill in Illinois showed that the fire blight disease of pear and apple was also of bacterial etiology (1878). In the early 1890s, Erwin Smith pioneered research in phytobacteriology and showed that several other plant diseases were caused by bacteria. He reported that crown gall disease, which he considered similar to cancerous tumors of humans and animals, was caused by bacteria. A century later, studies of this tumor causing bacterium Agrobacterium

tumefaciens led to the discovery that during the process of infection it transfers part of its DNA to the DNA of a plant cell, causing expression of specific genes consequently creating favorable conditions for the bacterial growth and deformation of the transformed plant tissue. The discovery that the bacterium acts as a natural genetic engineer of plants led to the use of this bacterium as a vector of small DNA segments coding for desirable characteristics, to a plant cell subjected to the genetic transformation, which formed the basis of biotechnology, especially of plants (Agrios, 2005).

Early discoveries of the pioneers in plant bacteriology were soon followed by reports of other bacteria associated with plant diseases. Later on, a group of bacterial pathogens, difficult or impossible to culture in the laboratory, were described and called fastidious vascular bacteria. They were found in either the xylem or phloem tissues and interfered with the transport of water and nutrients in the plant. Many of them were vectored by sucking insects (leaf-hoppers, plant-hoppers, and psyllids). Also, symptoms of general yellowing or reddening of the plant or of shoots proliferating and forming structures that resembled witches’ broom have been causing pathologists’ attention for many years. These diseases were thought to be caused by viruses, but no viruses could be found in such plants. In 1967, Doi and colleagues in Japan observed wall-less mycoplasma-like bodies, different from true bacteria, in the phloem of plants exhibiting such symptoms. They showed that the mycoplasma-like bodies and the symptoms disappeared temporarily when the plants were treated with tetracycline antibiotics. Since then, mycoplasma-like organisms (MLOs) that infect plants have been reclassified as phytoplasmas, and some of them that have helical bodies are named as spiroplasmas. Both phytoplasmas and spiroplasmas belong to Mollicutes (Agrios, 2005).

The taxonomy of plant pathogenic bacteria is currently in flux based on recent advances in bacterial classification. Most plant pathogenic bacteria belong to the following genera: Erwinia, Pectobacterium,

Pantoea, Agrobacterium, Pseudomonas, Ralstonia, Burkholderia, Acidovorax, Xanthomonas,

Clavibacter, Streptomyces, Xylella, Spiroplasma, and Phytoplasma (Arsenijević, 1997; Agrios, 2005).

Biology and epidemiology

Bacteria can be both beneficial and pathogenic. Beneficial bacteria are involved in such diverse processes as digestion in animals, nitrogen fixation in the roots of certain legumes, the decomposition of animal and

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plant remains, and sewage disposal systems. Pathogenic bacteria, on the other hand, cause severe and often fatal diseases in humans, animals, and plants. Although considered structurally simple, bacteria are extremely diverse from a metabolic standpoint and are found almost everywhere on Earth.

Phytoparasitic prokaryotes are naturally equipped with pathogenicity factors (Goto, 1990), such as:

• Presence of cell wall degrading enzymes like pectinase which can dissolve plant cell wall and release internal cytoplasmic contents.

• Toxin production which cause gummosis, necrosis or chlorosis and damage to plant host tissue. • Production of effector proteins to suppress host defense mechanisms. • Excessive formation of phytohormones like IAA and tumor induction. • Exopolysaccharide and gum production to block xylem vessels.

In addition to these, they possess special survival characteristics, like an ability to persist in water, air, soil, insects and plant debris for a long time. Most of them developed various survival properties such as resistance to desiccation, ability to enter wounds or natural openings of plants, attach, invade and colonize host plant tissue, and multiply within it in order to establish the population sufficient to cause the disease. Phytopathogenic bacteria are easily disseminated by wind, water, insects, soil, humans and farm equipments and practices which facilitate bacterial transfer from inoculum sources to susceptible hosts. They can be splashed around by rains or carried by the wind, birds, or insects. People can unwittingly spread bacteria by contaminated tools or cultural practices such as pruning from infected orchard trees to healthy ones.

Propagation of infected plant material is a major way pathogenic bacteria are moved over great distances. No matter how the bacterial pathogens are disseminated, they require a wound or natural opening, to get inside a susceptible plant. Also, environmental factors play an important role in pathogenesis of plant bacterial diseases. During the season plant pathogenic bacteria may grow harmlessly in epiphytic phase i.e. on plant surfaces and overwinter or survive unfavorable environmental periods or the absence of a susceptible host by either going dormant in infected tissue, infested soil or water, or in an insect vector.

Symptoms / Diseases

All plant types, like cereals, vegetables, fruits, grasses, shrubs, trees and even medicinal plants are prone to bacterial infections. During the host-pathogen interaction, some plant pathogenic bacteria produce toxins or inject special proteins that lead to the host cell death or they produce enzymes that break down key structural components of plant cells and their walls. An example is soft-rotting bacteria producing enzymes that degrade the pectin layer responsible for holding plant cells together. Still others colonize the water-conducting xylem tissue causing wilting of plants. Agrobacterium species even have the ability to genetically modify their hosts and bring about the formation of cancer-like overgrowths called crown gall. However, major bacterial diseases of plants include leaf spots, blights and discoloration, rots, wilts, dwarfing and stunted growth, galls, cankers, fruit deformation, retarded ripening, etc. (Agrios, 2005).

Necrotic spot diseases usually occur on leaves, fruits and stems. Rapid progression of the disease is known as blight. Spots of dicotyledonous plants can vary in shape and appearance, depending of the pathogen and structure of leaves. At the beginning spots are water-soaked, which is of diagnostic importance. In monocotyledons, spots are more like stripes or streaks and lesions turn necrotic quickly. Blight pathogens are disseminated usually by water.

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Wilting indicates bacterial activity in xylem vessels of plant and often is followed by death. It is usually caused by slime or gum producing bacteria which plug xylem passages and interferes with translocation of nutrients and water.

Rots are often caused by opportunistic strains which generally infect wounded tissue. Debris and infected volunteer plants can harbor the inoculum. Humid weather and high population of phytoparasitic nematodes and insects contribute to higher intensity of infection. Pathogen enters via wounds and produce enzymes that hydrolyze middle lamellae. Lamellar cells disintegrate and die forming slimy liquid and soft rotted watery tissue. Infected material is usually invaded by secondary parasites or saprophytes responsible for unpleasant rotten smell.

Canker type of disease affects trunks, stems, branches and twigs. Very often it is followed by gummosis or gum production. Hardening of exuded gum forms characteristic brown or orange brown sunken mass or canker. Some cankers can be soft and moist and produce oozing of bacterial exudates, further facilitating the pathogen dissemination by insects, splashing rain or contaminated equipments.

Gall is cancer like growth in plants formed usually by Agrobacterium strains (A. tumefaciens, rubi and vitis). Gall is made up of disorganized mass of plant cells in rapidly dividing state. Overgrowth of tumor or gall cut off flow of nutrients and water via xylem vessels resulting in death of plants. Galls can be formed on trunk, fruits, stems and underground plant parts.

Control

Bacterial diseases in plants are difficult to control. Usually, emphasis is on the control of sources of infection or inoculums reservoirs, and preventing the spread of bacteria rather than on curing the plant. In order to provide acceptable efficacy and sustainability, a combination of control measures is often required to keep particular disease under control. Therefore, integrated disease management approach is frequently implemented by advanced farmers (Obradović et al., 2008). Control measures for bacterial plant pathogens include:

1. Genetic Host Resistance

• Use of plant cultivars or hybrids resistant to a particular bacterium is the most efficient way to avoid severe infection and heavy production losses (Arsenijević, 1997). Recently, a novel strategy has been introduced based on activation of plant resistance mechanisms against pathogens by application of either synthetic chemicals or non-harmful microorganisms (Obradović et al., 2004, 2005).

2. Cultural Practices

• Bacteria-free seed or propagation material is crucial factor in preventing the pathogen occurrence and distribution. Propagate only bacteria-free nursery stock.

• Sanitation practices, such as disinfestation of pruning tools, cleaning and decontamination of machinery and storage facilities, removing infected plant debris, are very important for reducing the spread of inoculum.

• Crop rotation can be very effective in reducing bacteria over-wintering. However, pathogens that have a limited host range are deprived of their preferred host when crops are rotated. This reduces the virulence of the pathogen and is a natural way to reduce plant disease.

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• Appropriate planting density, soil management, fertilization and watering, aiming at providing optimal conditions for plant growth, reduces crop predisposition to bacterial diseases (Arsenijević, 1997).

3. Chemical Applications

Although it is much less successful than the chemical control of fungal diseases, the use of chemical pesticides to control bacterial diseases of plants has become standard practice among small gardeners and commercial growers. Due to short list of available compounds, this still has to be combined with other preventive measures in order to achieve acceptable results. Relying on chemical crop protection only often ends up with excessive use of pesticides with questionable efficacy and environmental consequences.

• Foliar applications of copper-containing compounds are still the most frequently used treatment in many crops attacked by pathogenic bacteria.

• Antibiotics: streptomycin and/or oxytetracycline may also help kill or suppress plant pathogenic bacteria prior to infection and reduce spread of the disease, but they will not cure plants that are already diseased. However, there are serious concerns about the use of antibiotics in plant protection due to potential development of the pathogen resistance and its transfer to other harmful bacteria as well as other concerns related to the uncontrolled effects of antibiotics on the environment and humans. Use of antibiotics in our plant production is considered illegal (Obradović and Ivanović, 2007).

• Pesticides can be efficiently used for controlling weeds which compete with the crop or harbor the pathogen or its vectors. Selective control of insects known as pathogen vectors or plant feeders that may create the pathogen points of entry will help to eliminate risks of plant infection and disease development.

4. Biological Control

• There are biologically based products (Blight Ban, Agrocin K84, Agriphage, etc.), that has been successfully used for managing bacterial diseases of plants. They are based on natural properties or interaction of microorganisms which antagonize or attack plant pathogenic bacteria (Obradović et al., 2005; Jones et al., 2007; Balogh et al., 2009)

5. Government Regulatory Measures

• The implementation of strict quarantines that exclude or restrict the introduction or movement of plant pathogens or infected plant material represents the first line of defense and therefore is very important in protecting national agriculture and preventing serious losses in susceptible hosts by a newly introduced pathogen.

• Disease forecasting, based on ecological behavior of the pathogen, meteorological data and host plant life cycle, supported by computer-based simulation modeling, has been also in use for predicting some plant epidemics.

Conclusion

The primary goal of research on plant pathogenic bacteria is to develop strategies for disease control. Experience has demonstrated that antibacterial compounds provide temporary control only. Therefore, in-depth knowledge of the pathogen itself is essential. Evolution of agricultural practices, global circulation

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of plant material, changes in climate, and raised environmental concerns require novel strategies for plant protection. These supposed to provide acceptable efficacy, minimize environmental risks, and secure sustainability of agriculture in order to feed the world.

Acknowledgement

The paper is presented as an activity within the project III46008 supported by Ministry of education and science, Republic of Serbia.

Literature

Agrios, G. N. (2005): Plant Pathology. Elsevier Academic Press, p. 922. Arsenijević, M. (1997): Bakterioze biljaka. S Print, Novi Sad, p. 576. Balogh B., Momol M.T., Obradović A., Jones J.B. (2009): Bacteriophages as agents for the control of

plant pathogenic bacteria. In: Disease Control in Crops – Biological and Environmentally Friendly Approaches, Edited by: D. Walters, Scottish Agricultural College, Edinburgh, UK. Blackwell Publishing Ltd., Wiley-Blackwell, pp. 246-256.

Goto, M. (1990): Fundamentals of Bacterial Plant Pathology. Academic Press, Inc., p. 342. Jones J. B., Jackson L. E., Balogh B., Obradović A., Iriarte F. B., Momol M. T. (2007): Bacteriophages

for plant disease control. Annual Review of Phytopathology, 45: 245-262. Obradović A., Jones J.B., Momol M.T., Balogh B., Olson S.M. (2004): Management of Tomato Bacterial

Spot in the Field by Foliar Applications of Bacteriophages and SAR Inducers. Plant Disease, 88: 736-740.

Obradović A., Jones J.B., Momol M.T., Olson S.M., Jackson L.E., Balogh B., Guven K., Iriarte F.B. (2005): Integration of biological control agents and systemic acquired resistance inducers against bacterial spot on tomato. Plant Disease, 89: 712-716.

Obradović A., Ivanović M. (2007): O primeni antibiotika u zaštiti bilja. Biljni lekar, 1: 52-59. Obradović A., Jones J.B., Balogh B., Momol M.T. (2008): Integrated management of tomato bacterial

spot. In: Integrated Management of Plant Diseases Caused by Fungi, Phytoplasma and Bacteria. Edited by A. Ciancio and K. G. Mukerji. Springer Science + Business Media B. V., pp. 211-223.

BAKTERIOZE BILJAKA I NJIHOVA KONTROLA

Aleksa Obradović

Univerzitet u Beogradu, Poljoprivredni fakultet, Institut za fitomedicinu, Beograd, Srbija

E-mail: aleksao@agrif.bg.ac.rs

Izvod

Ubrzo pošto je R. Koh (1843-1910) otkrio da je prouzrokovač antraksa bakterija Bacillus anthracis, T. J. Buril u Ilinoisu je 1878. godine pokazao da je plamenjača jabuke i kruške takođe bakteriozne prirode. Time su započela proučavanja bakterija kao parazita biljaka. U vremenu koje sledi, fitobakteriologija se razvijala kao nauka odmah iza kliničke i veterinarske bakteriologije, da bi poslednjih godina ta istraživanja tekla uporedo zahvaljujući čestom korišćenju biljaka kao model organizama u genetičkim istraživanjima. Sve vreme, osnovni zadatak fitobakteriologa je bio da pronađu efikasne metode zaštite bilja od fitopatogenih bakterija. Sa manje ili više uspeha, zaštita se oslanjala uglavnom na integraciju efekata otpornosti biljaka, agrotehničke mere, hemijsku zaštitu, biološku kontrolu i administrativne mere.

MICROBIOLOGIA BALKANICA 2011

7th BALKAN CONGRESS OF MICROBIOLOGY

8th CONGRESS OF SERBIAN MICROBIOLOGISTS

Serbian Society for Medical Microbiology Serbian Society for Microbiology

October 25-29, 2011 Continental Hotel Belgrade, Serbia

Prof. Aleksa Obradović Poljoprivredni fakultet u Beogradu Institut za fitomedicinu Beograd, 18. jul 2011. Poštovani Prof. Obradoviću, Čast nam je da Vas u ime Naučnog i Organizacionog odbora pozovemo da iz oblasti ekološke mikrobiologije održite uvodno predavanje na naučnom skupu medjunarodnog značaja:

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