Different types of plant diseases and disorders include:
Fungi constitute the largest number of plant pathogens and are responsible for a range of serious plant diseases. Most vegetable diseases are caused by fungi. They damage plants by killing cells and/or causing plant stress. Sources of fungal infections are infected seed, soil, crop debris, nearby crops and weeds. Fungi are spread by wind and water splash, and through the movement of contaminated soil, animals, workers, machinery, tools, seedlings and other plant material. They enter plants through natural openings such as stomata and through wounds caused by pruning, harvesting, hail, insects, other diseases, and mechanical damage.
Pathogenic bacteria cause many serious diseases of vegetables. They do not penetrate directly into plant tissue but need to enter through wounds or natural plant openings. Wounds can result from damage by insects, other pathogens, and tools during operations such as pruning and picking. Bacteria only become active and cause problems when factors are conducive for them to multiply. They are able to multiply quickly. Some factors conducive to infection include: high humidity; crowding; poor air circulation; plant stress caused by over-watering, under-watering, or irregular watering; poor soil health; and deficient or excess nutrients.
Viruses cause major damage to many Indian vegetable crops. They are immobile and are usually transmitted from one plant to another by a living organism called a vector or carrier. The most significant vectors of plant viruses include aphids, whiteflies, thrips, and leafhoppers, which have piercing sucking mouthparts that allow the insects to access and feed on the contents of the plant cells. Viruses can also be transmitted by other insects, mites, nematodes, fungi, infected pollen or vegetative propagating material, contact between plants, and infected or contaminated seeds.
Nutrient disorders are caused by a lack of plant nutrients, or the presence of nutrients at levels toxic to the plant. They affect the functioning of the plant system. When suffering from nutrient or physiological disorders, the plant exhibits disease-like symptoms; therefore nutrient disorders are sometimes mistaken for a disease. Nutrient disorders may result in a reduction in yield.