Most viral diseases of plants are transmitted by insects. Insects such as aphids, leafhoppers, and whiteflies can feed on infected plants and then transmit the virus to healthy plants by injecting saliva or piercing the plant's tissue with their mouthparts. This process is known as vector transmission.
When the insect feeds on a virus-infected plant, it ingests some of the virus particles along with the plant sap. The virus particles then replicate in the insect's gut and salivary glands, where they can be transmitted to other plants when the insect feeds again. Insects can transmit the virus in a persistent or non-persistent manner depending on the virus type and insect species.
Viral diseases can also be transmitted through infected seeds, plant debris, or by humans who handle infected plants. However, vector transmission by insects is the most common and efficient way that viruses spread between plants. This is why it is important for farmers to control insect populations and to use virus-free planting materials to prevent the spread of viral diseases in their crops.