A few crops are not grown from seeds, but from parts of the cultivated plants. This is called vegetative multiplication. For example, the potato is grown by planting tubers and sugarcane is cultivated by planting parts of the stem.
With this type of reproduction, certain pests and diseases can develop during one season and be transmitted to the following season.
For example, if for reproduction potato tubers are used from plants that are infested with a virus, the new plants will also be infested.
Also other pests, such as scale insects, can pass to the next crop when using infested planting materials.
To avoid this type of transmission it is recommended to always use clean planting material. Farmers should regularly inspect their crop and mark those plants that are not infested by pests or diseases. This can be done for example by placing a stick near these plants or by attaching a small ribbon or plastic string. At the end of the growing season, these marked clean plants can then be used for the reproduction of the following crop.
If all plants were infested, they should not be used for reproduction purposes. In that case, clean material should be obtained from another area or certified planting material should be purchased.