1. Abiotic diseases: Induced by some hereditary or environmental factor such as excess or lack of water, nutrient deficiencies, extreme cold or heat, toxic chemicals (air pollutants, weed killers, or too much fertilizer), mechanical injury. These diseases cannot be transmitted to healthy plants and their control depends solely on correcting the condition causing the disease.
2. Parasitic diseases: Caused by living organisms which derive their food by growing as parasites upon other plants. The most common disease-causing organisms are divided into four main groups:
    - Viruses,
    - Bacteria
    - Fungi
    - Nematodes.
   Pathogen organism (especially viruses) are often spread to healthy plants by insects or on one's hands during normal gardening practices. For example, mealy bugs may carry fungal spores which causes blackspot disease in Asclepiadaceae.

Since killing the pathogens is difficult or impossible, "prevention is better than cure". By observing good hygiene when propagating and growing your plants, you can prevent a lot of diseases from taking hold. A good home gardener recognizes symptoms of plant diseases quickly and takes steps to prevent or control them.  A great percentage of disease is due to poor horticultural choices, such as the wrong plant for the location (e.g. acidophilus plant in a high-pH soil, leading to iron chlorosis), lack of soil improvement before planting (A tight soil, predisposing to Phytophthora root rot), poor planning (five plants in the pot space needed for one mature plant, setting up a decline syndrome), or similar situations. The life cycles of all disease organisms are greatly influenced by environmental conditions. Plant diseases are worst in humid condition and when temperatures are mild. During these times, watch your plant closely for signs of disease.

Tips on How to control or reduce Plant disease incidence.
Once a pathogen infects a plant it is necessary to deal with it to restore plants health. But most disease problems are best controlled with preventive measures. Chemical rescue treatments (placing a protective chemical over the surface of the plant) may act as temporary solutions but are usually not the answer for long-term disease control.
the use of disease-resistant plants on which the disease organism cannot grow. It may be to create environmental conditions that are unfavorable for disease development:
Here some  strategies for disease control:

1. Fertilize and water plants properly: This keep them strong. Healthy plants do not get diseases as easily as weak ones.
2. Watering precaution: It is best to water on the soil between the plants. Do not sprinkle leaves; this only encourages more disease problems. If you must sprinkle plants, do so before 10 a.m. Do not overwater wet sensitive plants. Keep cactus and succulent dry during the winter rest.
3. Hygiene: Remove all dead and seriously cankered parts like flowers rest, dead leafs, and other debris; This will help prepare plants for winter discourage and prevent winter injury and damage from fungal moulds. Diseased stem and root tissues should be burned or buried, not included in a compost !
4. Overwintering protection: Provide suggested winter protection. Winter injury causes wounds that become infected with secondary canker fungi. Plants that have been located out of their natural range are often weakened in this way and predisposed to cankers and insect feeding.
5. Pruning: Prune cactus & succulents (if necessary) in the active growing season, let dry and callusing the wound surfaces allowing a good air flow and removing detaced branches.
6. Tools cleaning: Disinfect saws, scissors and knives used for cutting out diseased branches with methylated spirits or a flame (a cigarette lighter comes in handy for this). This also helps when taking cuttings.
7. Growing containers: Only use new, sterilized or well-washed containers when growing cuttings and sowing seeds.
8. Choosing disease-resistant plant:  Look over a variety of plants  hybrids, varieties and species and nursery catalogs. Select resistant varieties (if they are otherwise horticulturally acceptable) and plant them where you've had problems in the past but have no options this is usually the least expensive and best long-term method of disease control.
9. Improve soil drainage:  This reduce soilborne pathogens that cause Rhizoctonia and Fusarium  root rots.
10. Divide tightly clumped perennial plants: (where appropriate), remove rotted or diseased parts and replant in a new location or pot. Let the cut edges dry before replanting to avoid soft rot bacteria and other soilborne root rots.
11. Outdoor cultivation: For out door cultivation it is best to plant on a raised bed. This will allow excess water to move out of the root area and prevent many root diseases.
12. Support stems: When possible train plants to grow upright using cages or trellises. This will keep them from contacting the soil and reduce stem rots.
13. Use of pesticides: In some cases, plant care products may be required to control plant diseases during the year. These products should be used with caution and only when needed. Read and follow the label carefully. However, they are not always the most effective. Before using any plant care product, make sure the vegetables that you are spraying are listed on the label.

Of course not all plant diseases can be prevented by good hygiene as some are transmitted by insects and others are wind-borne. Aphids , mealy bugs and other sap-sucking insects are the main vectors of viruses.
The spores of fungal diseases are carried in the air, and in rain drops and splashes.

But in some cases diseased plants are voluntarily propagated because the infection produce more attractive features. This is what happens with Aucuba japonica f. variegata where a viral infection produces the mottled leaves or with some viruses induced 'Broken' and unusually coloured varieties or of tulips.