1. Microbiology, Virology, and Immunology Department
Phytopathogenic microorganisms

2. History of plant pathology Causes of plant disease Bacteria Fungi
Viruses and viroids
Disease cycles
Control
Like human beings and other animals, plants are subject to diseases. 
History of plant pathology
While early civilizations were well aware that plants were attacked by diseases, it was not until the invention of the first microscope that people began to understand the real causes of these diseases. There are references in the Bible to blights, blasts, and mildews. Aristotle wrote about plant diseases in 350 BC and Theophrastus ( BC) theorized about cereal and other plant diseases. During the Middle Ages in Europe, ergot fungus infected grain and Shakespeare mentions wheat mildew in one of his plays.
After Anton von Leeuwenhoek constructed a microscope in 1683, he was able to view organisms, including protozoa and bacteria, not visible to the naked eye. In the eighteenth century, Duhumel de Monceau described a fungus disease and demonstrated that it could be passed from plant to plant, but his discovery was largely ignored. About this same time, nematodes were described by several English scientists and by 1755 the treatment of seeds to prevent a wheat disease was known.
In the nineteenth century, Ireland suffered a devastating potato famine due to a fungus that caused late blight of potatoes. At this time, scientists began to take a closer look at plant diseases. Heinrich Anton DeBary, known as the father of modern plant pathology, published a book identifying fungi as the cause of a variety of plant diseases. Until this time, it was commonly believed that plant diseases arose spontaneously from decay and that the fungi were caused by this spontaneously generated disease. DeBary supplanted this theory of spontaneously generated diseases with the germ theory of disease. Throughout the rest of the nineteenth century scientists working in many different countries, including Julian Gotthelf Kühn, Oscar Brefeld, Robert Hartig, Thomas J. Burrill, Robert Koch, Louis Pasteur, R. J. Petri, Pierre Millardet, Erwin F. Smith, Adolph Mayer, Dimitri Ivanovski, Martinus Beijerinck, and Hatsuzo Hashimoto, made important discoveries about specific diseases that attacked targeted crops.
During the twentieth century advances were made in the study of nematodes. In 1935 W. M. Stanley was awarded a Nobel Prize for his work with the tobacco mosaic virus. By 1939, virus particles could be seen under the new electron microscope. In the 1940s fungicides were developed and in the 1950s nematicides were produced. In the 1960s Japanese scientist Y. Doi discovered mycoplasmas, organisms that resemble bacteria but lack a rigid cell wall, and in 1971, T. O. Diener discovered viroids, organisms smaller than viruses.

3. Epiphytes – microbes which are on all plant surfaces.
Plant associated bacteria may be beneficial or detrimental.
Epiphytes – microbes which are on all plant surfaces.
Endophytes – microbes live inside plants.
Phytopathogens  – organisms that cause infectious disease 
Populations of 106 CFU (colony-forming units/milliliter) or higher are normally required for bacteria to function as biological control agents or cause infectious disease.

4. Plant pathology (also phytopathology)
is the scientific study of plant diseases caused by pathogens (infectious organisms) and environmental conditions (physiological factors).
Organisms that cause infectious disease include fungy, bacteria, viruses, viroids, phytoplasmas, protozoa, parasitic plants
None of these pathogens are visible to the naked eye, but the diseases they cause can be detected by the symptoms of wilting, yellowing, stunting, and abnormal growth patterns.
Once inside the plant tissue, bacterial plant pathogens usually grow intercellularly, that is, they grow between cells.

5. of phytopathogenic bacteria
Physiology
of phytopathogenic bacteria
Most phytopathogenic bacteria are aerobic (live in the presence of oxygen) and some are facultative anaerobes which can grow with or without oxygen. 

6. Plant pathogenic bacterial genera
Erwinia Pseudomonas Ralstonia Xanthomonas Agrobacterium (Rhizobium)
Clavibacter
Bacillus
Streptomyces
Xylella
Phytoplasma

7. by wind, water, and soil movements; by insect vectors;
There are several ways by which bacterial plant pathogens may be spread:
by wind, water, and soil movements;
by insect vectors;
by infected seeds;
by contaminated tools.
There are many different ways or modes of dissemination of disease causing bacteria. The use of planting material obtained from infected crops/plants helps in spreading diseases from one place to other. Tools and implements used for various agricultural works carry the disease causing bacteria from one field to another. Rain also plays an important role in the dissemination of pathogenic bacteria.
The bacteria enter the plant
through natural openings, like the stomata of the leaves,
or through wounds in the plant tissue. 

8. Methods of penetration and invasion
by bacteria
Host-Pathogen Interactions
Infection of plants by bacteria can occur in multiple ways. Infection is generally considered to be passive, i.e. accidental, although a few cases of plant chemoattractants have been reported. Bacteria can be sucked into a plant through natural plant openings such as stomata, hydathodes or lenticels. They can enter through abrasions or wounds on leaves, stems or roots or through placement by specific feeding insects. 
Infection is generally considered to be passive, i.e. accidental, although a few cases of plant chemoattractants have been reported.
Artificially, bacteria are most commonly introduced into plants by wounding.

9. Types of bacterial plant diseases
Foliar diseases (e.g., spot, speck, streak, blight)
Vascular diseases (e.g., wilt, blight)
Soft rots
Developmental diseases (e.g., canker, scab, gall)
Symptomatology of bacterial diseases is extremely varied, but usually characteristic for a particular pathogen. Symptoms can range from mosaics, resembling viral infections, to large plant abnormalities, such as galls or distorted plant parts. 

10. GALLS are characterized by abnormal outgrowths of plant tissue.
Some of the abnormal outgrowths of plant tissue produced in galls are large and swollen, while others are small and divide rapidly.
Infected plants will develop smooth, light-colored galls on its roots and stems. As the galls age, they develop into hardened, discolored galls that eventually slough off to make room for new, secondary galls. These formations inhibit the plant's ability to transport nutrients and water throughout the plant. This lack of transport results in the plant's loss of vigor which is also accompanied by growth stunt and branch and twig dieback.
As galls develop, they may stop the passage of nutrients and water in the plant, causing death.

11. This disease is soil-borne.
Xanthomonas campestris
Gall tissue is composed of disorganized, randomly proliferating cells that multiply in the intercellular (between the cells)spacesin the vicinity ofthe wound. In the presence of the pathogen rapid and continuous cell division  (hyperplasia and hypertrophy) of the plant tissue persists. Gall damage can be benign to deadly.Crown gallfirst appears assmall, whitish,soft round overgrowths typically  on the plants crown or at the main root. The color of galls (tumors) caused by A. tumefaciens can be orange-brown and Gummosis due to bacterial infection – Pseudomona ssp.
 Crown gall, in particular, may develop in sugar beets, fruit trees, and other plants with broad leaves and stems that come out of the ground.
This disease is soil-borne.

12. Bacterial galls can be produced by the genus Agrobacterium and certain species of Arthrobacter, Pseudomonas, Rhizobacter and Rhodococcus. 

13. Bacterial wilts
Slime-producing bacteria are the causative agents of bacterial wilts.
These bacteria plug up the plant’s vascular system (the vessels that carry water and nutrients) and cause the plant to wilt. 
Infected plants wilt rapidly and often die.
bacteriua grow in the xylum tissue of plants and affect the water transport system. The mechanical blockage of the system which transports water in plants, lead to wilting. The wilting of leaves is one of the early symptoms of this disease. Later on, the disease spreads to other parts of the plant; finally, the whole plant wilts and dies. It is not possible to treat the plants affected by bacterial wilt. 
Read more at Buzzle: 

14. Bacterial wilts
There are four main wilt-causing bacterial plant pathogens : Corynebacterium insidiosum causes wilts in alfalfa; Erwinia tracheiphila causes wilts in cucumbers; Erwinia stewartii produces wilts in sweet corn; and Pseudomonas solanacearum causes wilts in tobacco.

15. Leaf spots - the most common symptom of bacterial disease
Spot-causing bacterial plant pathogens: Pseudomonas striafaciens, Xanthomonas pruni.
 Bacterialspots will appear as streaks orstripes on monocotyledonous plants. Almost all bacterial leafspots and blights are caused by the genera Pseudomonas and Xanthomonas.
The bacterial plant pathogens that produce leaf spots may also infect the stems and fruits of the diseased plant. The spot-causing bacterial plant pathogens are: Pseudomonas spp., Xanthomonas spp.

17. Blights or deadening of tissue on leaves, stems or tree trunks, and rots

18. Fire blight - Erwinia amylovora

19. Fungal infections
Fungi can grow on living or dead plant tissue and can survive in a dormant stage until conditions become favorable for their proliferation. They can penetrate plant tissue or grow on the plant’s surface. Fungal spores, which act like seeds, are spread by wind, water, soil, and animals to other plants.
Warm, humid conditions promote fungal growth.
About 80% of plant diseases can be traced to fungi, which have a great capacity to reproduce themselves both sexually and asexually. Fungi can grow on living or dead plant tissue and can survive in a dormant stage until conditions become favorable for their proliferation. They can penetrate plant tissue or grow on the plant’s surface. Fungal spores, which act like seeds, are spread by wind, water, soil, and animals to other plants. Warm, humid conditions promote fungal growth. While many fungi play useful roles in plant growth, especially by forming mycorrhizal associations with the plant’s roots, others cause such common plant diseases as anthracnose, late blight, apple scab, club root, black spot, damping off, and powdery mildew. Many fungi can attack are variety of plants, but some are specific to particular plants.

20. Methods of penetration and invasion by fungi

21. Fungal infections
While many fungi play useful roles in plant growth, especially by forming mycorrhizal associations with the plant’s roots, others cause such common plant diseases as anthracnose, late blight, apple scab, club root, black spot, damping off, and powdery mildew.
Many fungi can attack are variety of plants, but some are specific to particular plants.
About 80% of plant diseases can be traced to fungi, which have a great capacity to reproduce themselves both sexually and asexually. Fungi can grow on living or dead plant tissue and can survive in a dormant stage until conditions become favorable for their proliferation. They can penetrate plant tissue or grow on the plant’s surface. Fungal spores, which act like seeds, are spread by wind, water, soil, and animals to other plants. Warm, humid conditions promote fungal growth. While many fungi play useful roles in plant growth, especially by forming mycorrhizal associations with the plant’s roots, others cause such common plant diseases as anthracnose, late blight, apple scab, club root, black spot, damping off, and powdery mildew. Many fungi can attack are variety of plants, but some are specific to particular plants.

22. Viruses and viroids
While more than 300 plant viruses have been identified.
The symptoms of viral infection include yellowing, stunted growth in some part of the plant, and plant malformations like leaf rolls and uncharacteristically narrow leaf growth. The mosaic viruses can infect many plants. Plants infected with this virus have mottled or streaked leaves; infected fruit trees produce poor fruit and a small yield.
The viruses and viroids that attack plants are the hardest pathogens to control.

23. Effective treatment of bacterial infections in plants may involve radical treatment or removal of the infected plant parts or removal of the diseased plant altogether. A more drastic measure may mean the removal of entire communities of susceptible or diseased plants.

24. Mechanical inoculation and early stages in the systemic distribution of viruses in plants

25. Forms and locations of survival of fungi and bacteria between crops

26. Control control of plant disease begins with good soil management
selection of disease-resistant plants 
disposal of infected plants 
plan crop rotation to eliminate over-wintering
disinfect pruning tools
prevent surface wounds on plants
expose plant material you wish to compost to dry air, heat and sunlight to kill any disease-causing bacteria
Crop rotation is an important part of reducing plant diseases. Pathogens that favor a specific crop 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.
Control Measures for Bacterial Diseases The various bacterial diseases are treated using an integrated management approach. It is difficult to completely cure or reverse back the spread of bacterial diseases mentioned above. Right thing to do is therefore, preventing the disease from spreading further; one can think of preventing bacterial diseases by the use of resistant plant varieties. Following the quarantine specifications or guidelines also is one of the ways to prevent the spread of bacterial diseases. Use of chemical pesticides is amongst the commonly used measures for treatment of bacterial diseases. Antibiotics like streptomycin and oxytetracycline are useful in controlling the growth of pathogenic bacteria. Bordeaux mixture is an age-old chemical pesticide used in the treatment of bacterial diseases of plants. There are number of bacterial diseases caused by plants. These diseases are quite harmless in their initial stages, however, they do have the potential to cause death of the affected plant; it is therefore, necessary to take timely action for controlling the disease. The different diseases caused by bacteria in plants can be controlled with proper management.

27. Control control insects that might vector bacterial diseases
if the disease is systemic, affecting the stem as well as leaves, the plant cannot recover. Destroy it to prevent the spread of the disease to other plants.
Make sure the soil is healthy with plenty of organic nutrients and beneficial microorganisms.