1. Microbial diseases of plants and plant pathogens
Microbial diseases of plants whether caused by viruses, bacteria, fungi, or protozoa, are of ecologic and economic importance.
The development of plant diseases due to microbial pathogens  normally follows a pattern of initial contact of the microorganism with the plant, entry of the pathogen into the plant , growth of the infecting microorganism, and development of plant disease symptoms. Pathogenic micoorganisms may contact the plant in the rhizoplane or the phylloplane. Because most fungal plant pathogens are dispersed through the air as spores, they often contact the leaves and stems of plants.

2. Most vital plant diseases are transmitted by insect vectors, most of these pathogens contact the plant at the phylloplane. Some bacterial and fungal pathogens are also carried by insect vectors.
Plant pathogens may enter plants through wounds or natural openings such as stomata.
Many viruses enter plants through wounds caused by insect vectors carrying the virus, although other enters through the roots with the water being taken up by the plant. Some plant pathogens are able to penetrate the cuticle of the plant directly. Plant cuticles and tissues are often subjected to enzymatic attack by the pathogen that softens them at the site of penetration. Powdery mildews (Erysiphe species), the soft rot of pathogen Botrytis cinerea and Fusarium solani are examples.

3. Microbial pathogens that have successfully entered the plant disrupt normal plant function by producing degradative enzymes, toxins and growth regulators. Soil pathogens produces pectinases, cellulases that results in degenration of the plant structure producing soft rots, and other lesions. Destruction of plant growth regulators by pathogens results in dawrfism whereas the production of IAA, gibberlins and cytokines by some plant pathoges results in gall formation and excessive elongation of plant stems.

5. Toxins produced or induced by the pathogenic organisms in plant interfer with the normal metabolic activities of the plants.
Invasion by plant pathogens may disrupt cell permeability, causing imbalaces in water reactions that lead to the leakage and death to the plant cells.
Diseased plant sometimes show changes in respiratory activity, which may be caused by electron transport uncoupling or changes in the glycolytic pathways of carbohydrate metabolism. Plant pathogen may interfere with carbon dioxide fixation. Foliar pathogens sometimes produce chlorosis, which prevents plant from carrying out oxidative photophosphorylation and producing ATP needed for carbon dioxide fixation. Plant pathogens may cause changes in protein synthesis.

6. Plant under attack by microbial pathogens may react by synthesizing substances called phytoalexins. The phytoklexins help to curb the original infection and protect the plant aginst subsequent microbial challanges. This phenomenon is called systemic acquired resistance

7. Viral Diseases of Plants
Many plant pathogenic viruses are classified according to their ability to cause particular disease. Vectors are important in the transport of plant pathogenic viruses that occur in the soil or in diseased plant tissues of susceptible host plants. Various insects, including aphids, leafhoppers, ants and nematodes can work as vectors for the viral diseases of plants.
Plant pathogenic viruses may also be transmitted within or on infected plant structures. Viruses are also transmitted on plant seeds. The spread of viruses on plant structures involved in the reproductive activity of the plant such as pollen and seeds ensures that viruses are maintained withing the suceptible host plant population.

10. Bacterial diseases of plants
Plant pathhogenic bacterial species occur in the genera Mycoplasma, Spiroplasma, Corynebacterium, Agrobacterium, Pseudomonas, Xanthomonas, Streptomyces and Erwinia.
Because most plant pahtogenic bacteria do not form resting stages, they must remain in intimate contact with plant tissues during all stages of their life history.

11. Some bacterial diseases are caused by pathogens that have a permanent soil phase. For example some flourescent Pseudomonas species cause soft rots in plants. These organisms, which are abundant and occur as sarophytes in the rhizosphere, infect plants through roots. Many plant diseases caused by bacteria are seed borne. These pathogenic bacteria survive on seed survive on seed for a transient period in the soil. Pseudomonas phasiolicola for example is carried in the micropyle and causes halo blight of beans.

12. Crown gall is an extremely interesting plant disease that occur after viable Agrobacterium tumefaciens cells enter either through wounded surfaces of susceptible dicotyledonous plants, usually at the soil-plants stem interface. Occuring in fruit trees, sugar beets and other broad leaved plants the disease is manifested by the formation of tumor growth, the crwon gall.

15. The tumour maintenance principle has been identified as a fragment of a large tmour inducing plasmid called the Ti-plasmid. A fragment of this bacterial plasmid is transferred to the plant where it is maintained in the tumour tissue. Strains of Agrobacterim lacking this plasmid fail to induce plant galls.

16. Fungal diseases of Plants
Most plant diseases are caused by pathogenic fungi. Perhaps the most the important economic fungal diseases of plants are caused by the rust and smuts. The fungi are Basiodiomycota and have a complex life cycle. There are more than 20,000 species of rust fungi and more than 1000 species of smut fungi. Examples include Smut of corn and wheat by Ustilago, Loose smut of sorgum by Sphaelotheca, rust of cereals by Puccinia

17. Survival and infectivity of most plant pahtogenic fungi depend on prevailing conditions of temperature and moisture. Germination of mycelia generally occur only within -5 ̊C and +45 ̊C with adequate supply of moisture, Spores, though, can retain vability for long periods under enviromental conditions that do not allow forgermination.

18. Changes in environmental conditions may affect the pathogen, the host, or both. Most plant pathogens develop best and cause the most severe disease of plants during the warmer months of the year; during winter many are inactive. Some fungi, however, such as Typhula and Fusarium, which cause snow mold of cereals and turf grasses, respectively, thrive only in cool seasons of cool regions. In some cases the optimal temparature of either pathogen or the host. For example, the fungus Thielaviopsis basicola, which causes black root rot of tobacco, has an optimal growth temperature that is higher than optimum temperature for the disease.

19. Life cycle of Rhizoctonia solani causing various diseases in plants
Life cycle of Rhizoctonia solani causing various diseases in plants. Invasion of plant occurs through roots