By the time disease symptoms appear, disease pathogens are inside the plant and usually beyond control. Therefore, it is important to prevent penetration of pathogens.
Insect control can be important for controlling spread of viruses, mycoplasmas, bacteria and fungi.
Mulching can help control diseases by preventing the contact of foliage with soil. Don't bring diseased plants into the garden. Immediate removal of diseased plants or plant parts reduces the chance of the disease spreading. Selective pruning and careful sanitizing of pruning equipment can prevent spread of disease. Rotate crops to avoid soil-borne diseases.