How could we know if a plant is diseased, or not?

Just like humans, plants will express symptoms of illness (or disease). Similar to our doctors, one would investigate the symptoms, and other underlying factors, to diagnose the problem. A vast number of symptom types exist, amongst others, stunting, discolouration of leaves, early maturation, displacement of plant structures or tissue with fungal tissue and lodging (in field crops).

Let me show you some examples of the symptoms of various wheat diseases:

Loose smut of wheat stunts growth of the plants (red arrow) and displaces plant structures with fungal tissue

Leaf blotch of wheat discolors leaves

Take-all causes early maturation of wheat ears

Eyespot, or straw breaker of wheat causes lodging of the wheat plant 

I have much more to tell you on plant diseases and their control, and will see you again soon.