The best information on the ability of nematodes to be carried in irrigation water comes from work conducted in the state of Washington. These studies demonstrated that irrigation canals transported a dozen different genera of plant parasitic nematodes. They also showed via greenhouse trials that many of these nematodes were viable. Conclusions were that growers applying an acre inch of water were also applying several million parasitic nematodes.
One relatively inexpensive method to combat this problem would be to first pump irrigation water into settling ponds, allow nematodes to settle which should only require a few minutes, and then draw water off the top for irrigation.
Other potential methods for spreading nematodes which have not been intensively studied include wind , birds, and other animals. It has been demonstrated, for example, that cysts of the sugarbeet cyst nematode can pass through the digestive system of cattle and retain viability.
Manure should be properly composted before using so that it will reach temperatures lethal to nematodes. Also keep in mind that the soil beneath a pile of manure may also be infested with nematodes. Don't be greedy, take the manure but leave the soil behind.
The frequency with which growers contact nematologists after planting rootstocks with galls, lesions, or other root abnormalities, indicates that checking for nematodes before rather than after planting needs to be stressed. Typically there is a time factor involved in that planting crews are on site or already in progress when the symptoms are discovered. If planting must proceed, then at least some stock should be retained unplanted and that has not come in contact with the grower's soil if the source of a potential infestation is to be determined.