Viruses
Viruses are smaller than bacteria and can be seen only with a powerful electron microscope.
They exist in living cells.
Viral diseases may take a long time to recognize because the only effect on the crop is the gradual loss of vigor.
Plants are small and yields drop or cease.
Sometimes the signs of the infection are obvious: red or yellow streaks appear on grasses and blotchy s 'mosaic' markings appear on the leaves of other plants, but it is often difficult to distinguish virai disease from mineral deficiency.
Viruses spread when infected stem, tuber or root sections are planted.
They multiply in the new plants.
Sucking insects (such as aphids) can transfer viruses from plant to plant.
Nematodes can carry them between roots. Some viruses can survive drying and heat.