At the very least, one must know what nematodes are present in a field in order to chose a nonhost crop. This requires sampling the field and obtaining an analysis of the nematodes present. If root-knot or lesion nematode are present, an identification to species is needed. If dagger nematode is present, it should be determined whether or not the species is Xiphinema index.
Once the nematodes present have been determined, one can consult a host lists or database and determine if profitable nonhost crops, or suitable resistant varieties are available. At the same time, one might also be able to ascertain from the databases if local weed species are hosts for the nematodes present in the field.
Utilizing available knowledge of damage thresholds, rates of population increase and decline, and the effect of time of planting and harvesting on a rotation program are somewhat more complex and will be covered in subsequent sections.
Crop rotation is most frequently considered for annual crops or perennials such as alfalfa which can be profitably grown for several years. Growers of longer term perennials such as grapes and stonefruits and nuts should also be thinking of longer term plans. If nematodes are present in a perennial situation, it is likely that these crops will need to be replanted sooner than those grown in noninfested areas. An interim period of several years in nonhost annual crops may help to avoid the need for chemical control and may become more necessary as the availability of fumigant nematicides becomes limiting. Annual crops, for example, will allow time for woody roots to decay which will hasten the decline of nematodes.
Of the various factors known to be important, temperature has been looked at in the greatest depth. Some examples of the length of time for a generation under optimum conditions are given in the table.
In a particular cropping system, there is often a single crop for which the grower receives the highest rate of return. The prime strategy then of the crop rotation program is to grow that crop as often as possible. In some areas of California, this would be cole crops, in others potatoes, tomatoes, or perhaps carrots, all of which can suffer serious nematode damage.
In a crop rotation program, it is advantageous to be able to plan several years in advance. If the expected rate of population increase is known, one can forecast what the population will be at harvest. This combined with knowing the expected rate of decline will allow forecasting when the most profitable crop can be planted once again and so forecast if the rotation chosen will be economical. One could also consider other possible options such as harvesting the crop early when nematode numbers might be lower which might then permit a shorter rotation period.