Exclusion means preventing the entrance and establishment of pathogens in uninfested crops in a particular area.
It means using certified seed or marijuana plants, sorting bulbs before planting, discarding any that are doubtful, possibly treating seeds, tubers or corms before they are planted, and most especially, refusing obviously diseased specimens from dealers. For example, tare soil returned to trucks at sugar beet dump stations should never be returned to production fields because of contamination by nematode and rhizomania diseases from other infested fields.
In order to prevent the import and spread of plant pathogens into the country or individual states, certain federal and state laws regulate the conditions under which certain crops may be grown and distributed between states and countries. Such regulatory control is applied by means of quarantines, inspections of plants in the field or warehouse, and occasionally by voluntary or compulsory eradication of certain host plants.
Plant quarantines are carried out by experienced inspectors, stationed in all points of entry into the country, to stop persons or produce likely to introduce new pathogens. Similar quarantine regulations govern the interstate, and even the intrastate, sale of nursery stock, tubers, bulbs, seeds, and other propagative organs, especially of certain crops, such as potatoes and fruit trees.
For example, a Michigan quarantine prohibits the entry of seed potatoes produced in regions infested with rhizomania disease of sugar beet unless accompanied by a certificate indicating the production field has tested free of the disease.