This is based on New Zealand Qualification Authority Unit 8025  entitled "Describe plant microbiology.”
The infection process of plant disease: This process undergoes three different steps which are as follows
Pre-entry: various pathogens uses different ways of entering a plant. For example there are some pathogens like appressoria, which forms special penetrations structures and other pathogens uses pre openings such as pores or wounds for their entry in plants.
Entry: Entry of host by direct penetration is usually done by fungal pathogens. This occurs by the application of pressure and enzymatic degradation of the plants cell wall and cuticle. Many other plant pathogens and virus enters through the other way i.e via pre exiting opening.
Colonisation: If there is parasitic relationship between host and the pathogen then only there could be establishment of successful plant infection. Pathogens can further be divided into two main categories of Biotrophs and Necrotrophs. Mycorrhiza, which literally means fungus-root, colonize the cortical tissue of roots during periods of active plant growth. They live in and around the root zone of plants, extending far out from the plant's roots with their own network of thread-like filaments known as hyphae. This greatly extends the effective surface area of the plants roots.:
Physical changes to the plant Necrotrophs are the one which secrete toxins that kills the cells before colonising. Thus they have very little effect on physiology of plant whereas biotrophs incorporates and thus lead to modify different aspects of host physiology such as Respiratory rate increases and Photosynthetic rate decreases. Due to killing of the root system ability of plant to absorb water decreases and thus wilting and defoliation of plant takes place. Xylem vessels are blocked which effects the vascular system of the plant. The physical changes that can result (an example identified at Genetic Dissection of the Initiation of the Infection Process and Nodule Tissue Development in the Rhizobium–Pea (Pisum sativum L.) Symbiosis
The beneficial effects for plants and micro-organisms from interactions Interaction between plant are microbiology is the part of study of ecology.In these association plats and microorganisms have certain good and benefits,there are few associations and results
Root nodule formation (actinorrhiza) is a small gall like abnormal swelling x found especially in roots of leguminous plants which developed by the attack of bacteria Rhizobium and Bradyrhizobium,it help plant to fix atmospheric nitrogen by converting it to Ammonia and then it converted to nitrates and amino acids. Plants having nodules increase the soil fertility by increasing nitrogen content of the soil. It also appears in certain non leguminous plants like Alnus and bayberry 
Rhizosphere: This narrow region is associated by soil micro-organisms and is influenced by root secretion. Soil microbes are regulated by the nutrients released by roots which also leads to the change in the chemical and physical properties of soil. This further leads to the increase in growth of plant species.
Mycorrhiza: These are the one which usually grows between fungi and plants. The fungi increases the absorption capabilities of water and nutrients by colonising the root system of the host plant and even protects the plant from certain other pathogens. On the other side the plant provides carbohydrates to the fungus gained from the process of photosynthesis. Other benefits of mycorrhizal associations that are of interest to plant pathologists include biological control and the various growth promotion effects that enhance establishment of plants in the field. Mycorrhizal roots are generally more drought tolerant. The general vigour of mycorrhizal plants makes them more tolerant of limited root loss due to diseases.:
Epiphytes: These have no roots and grow upon another plant some of them makes food by photosynthesis using moisture from air or from moisture laden pocket of the host plant, plant take these nutrients and give physical support to them  Sometimes these organisms may damage the host plant as well.
There can even more be beneficial effects for both microorganisms and plants due to there interactions as outlined at Protect and Survive: research on plant micro organism interactions to improve crop health
[Power Point presentation - Plant & Microbe Interactions: plant roots]
Some examples of plant diseases
Rice blasts: Fungus named Magnaporthe grisea gives rise to a plant disease named Rice Blast. This disease occurs by the attack of the fungus to the aerial part of the rice plant. Spores are generated by the fungus which can easily be dispersed by wind and rain. They germinate by rapid growth of hyphal element called germ tube. The germ tube tip gets enlarged and forms an infection structure called appressorium. This leads to the penetration of the plant cuticle. After getting in the hyphae swells. Thus the cells fills up in 24 hrs, in 48 hrs penetration of the epidermal cells occur and colony growth increases. Thus the symptoms of the infection starts showing up in about 5 days after inoculation.
Corn Smut : is caused by the fungus Ustilago maydis. The fungus causes gall formation on all above-ground parts of corn plants. The smut fungus survives from year to year in old, smutty corn stalks. Spores may be blown by wind for considerable distances to new plants. The fungus often enters plants through wounds made by hail, cultivating equipment, or detasseling. Infection may also occur through the silks. The fungus grows down the silks to the kernels and causes galls on the ears. The galls are at first enclosed in a silvery white membrane. As they mature, the membrane breaks and a black, powdery mass of spores is exposed. If plants become severely diseased before they are a foot high, they are usually killed.
Citrus canker is a disease mainly found in citrus species caused by the bacterium Xanthomonas axopodis. canker
Fire blight outlined on this website is what it looks like, how it infects the plants and how to control it.
Phytiphthoram caused by fungi.
Plant defence against pathogens: Plants have got various mechanisms to defend themselves from the pathogens. First one is an active defence. This means whenever a pathogen tries to infect a plant through its leaf surface a plant forms a wall thickening i.e papilla which is directly formed under the attempted point pf penetration. This papilla usually contains chemicals which are not suitable for pathogen . Thus it prevents the pathogen to enter the plant body.
In other type of plant defence, the plant kills the cell which have been penetrated by the pathogen. This leads to the killing of the pathogen due to the accumulation of the toxic substance by the dead plant cell. Cells at the point of infection commit suicide. Cell death results from the combined action of nitrous oxide and reactive oxygen intermediates. Cell walls around the point of infection are also reinforced through the action of reactive oxygen intermediates.
Some plant diseases and their controls:
|Powdery mildews||The white growth on leaves and stems is the powdery mildew fungus growing in teh surface of the plant tissue.||a regular preventative spray, watering only early in the day and pruning out surrounding vegetation to improve sunlight and drying of plant foliage.|
|Rust Diseases||orange to reddish brown spots or lesions will form on leaves, twigs or fruits.||removal of infected leaves or plants, spray with the same solution at one to two week intervalduring periods of wet weather.|
|Leaf Spotting and Blighting||Irregular tan or brown areas on the leaves, especially along major veins of the leaf blade. Deformation and twisting of leaves||collect and destroy all foliage with disease symptoms use of a fungicide|
|Root and Crown Rots Caused by Water Moulds||Poor growth, yellowing, or stunting of the top portion of the plant||Improving the drainage of the soil. The fungi do not survive well in well-drained planting beds.|
The following websites provide more information on various plant diseases with their symptoms, disease cycles and methods of controls.
[Power Point presentation - Control of Disease]