They can be caused by:
1. Nematodes
2. Viruses
3. Bacteria
4. Parasitic higher plants
5. Fungi

Plant diseases caused by nematodes
Soybean cyst nematodes
Heterodera glycines
Stem and bulb nematodes
Ditylenchus dipsaci
Nematodes are wormlike in appearance but taxonomically different from true worm.
They live freely in fresh or salt waters or in soil and feed on microorganisms, plants and animals.

1.Soybean cyst nematodes
Heterodera glycines
Plant: Soybean
Parasite: Heterodera glycines

Symptoms:
The root system of the infected plants appears smaller with fewer bacterial nodules and clusters of cyst nematodes.
Infected soybean are stunted and their leaves turn yellow and fall off early.

2.Stem and bulb nematodes
Ditylenchus dipsaci
Plant: Stem and bulb of plants
Parasite: Ditylenchus dipsaci

Symptoms:
Infected bulb: Curled green leaves and the scales becomes softened, loose and pale gray in color.
Infected bulb may be split and become malformed, odorless and light in weight.
Infected tuber: lesions appear on the tuber as small brown blemishes. On the cut surface, the underlying tissues appear granular and spongy.
The blemishes grow larger and the final stage is expressed by mummification of the tuber.

Plant disease caused by viruses
1.Diseases caused by
Tobacco mosaic virus
4.Diseases caused by
Lettuce mosaic virus
6.Disease caused by
Papaya ringspot virus
2.Diseases caused by
Tomato mosaic virus
3.Diseases caused by hordeiviruses
5.Disease caused by sugarcane mosaic virus

1.Diseases caused by
Tobacco mosaic virus
Disease name: Mosaic virus
Plant: Tobacco
Pathogen: Tobacco mosaic virus

Symptoms:
Various degrees of chlorosis, curling, distortion, dwarfing of leaves, flowers and entire plants.

2.Diseases caused by
Tomato mosaic virus
Disease name: Mosaic virus
Plant: Tomato
Pathogen: Tomato mosaic virus

Symptoms:
Leaflets may be long and pointed, sometimes shoestring-like.
Internal browning of fruits.

3.Diseases caused by hordeiviruses
Disease name: Barley stripe mosaic virus (BSMV)
Plant: Barley (Hordeum spp.), grain crops and wild grasses
Pathogen: hordeivirus

Symptoms:
Chlorotic spots, yellow-brown strips and sometimes dwarfing and necrosis of plants.

4.Diseases caused by
Lettuce mosaic virus
Disease name: Mosaic virus
Plant: Lettuce
Pathogen: Lettuce mosaic virus

4.Diseases caused by
Lettuce mosaic virus
Disease name: Mosaic virus
Plant: Lettuce
Pathogen: Lettuce mosaic virus

Symptoms:
Mottling (yellowing of leaves), distortion and marginal necrosis of leaves, dwarfing of plant and failure to produce lettuce head.

5.Disease caused by sugarcane mosaic virus
Disease name: Sugarcane mosaic virus
Plant: Sugarcane, other gramineae (corn, sorgum,…)
Pathogen: Sugarcane mosaic virus

Symptoms:
Pale patches and twisted distorted leaves.
Necrotic and stunted stems.

6.Disease caused by
Papaya ringspot virus
Disease name: Papaya ringspot virus
Plant: Papaya
Pathogen:Ringspot virus

Symptoms:
Fruit: Dark green and slightly sunken rings with unpleasant taste.
Yellow mosaic leaves with stunting of the plant.