Many fields do not have nematode problems. Growers with medium textured soils, adequate rooting depth, proper irrigation management, and reasonable rotation practices are least likely to experience nematode problems. Growers who fumigate before each high value crop are also unlikely to experience nematode problems; such practices, however, may not always be profitable.
Nematodes are especially problematic when soils, irrigation practices, or management are marginal. Marginal soils may include the sand or clay soils, shallow soils, or highly stratified soils. Marginal irrigation practices are those that fail to provide water at the needed frequency or quantity. Marginal management involves such decisions as crop rotation or cropping sequences. An example of an improper rotation would be: broccoli, sugarbeets, brussels sprouts, broccoli, and sugarbeets (all hosts) in rotation where sugarbeet cyst nematode is present. A second example would be the removal of a perennial crop and the replanting of a similar perennial crop the following spring. These situations precede or aggravate nematode problems.
Nematode problem areas are usually pocketed or irregular spots of poor growth. In Califonia's alluvial valley soils, damaged plants are frequently in sandy streaks across the field. Nematode incidence in a particular field may or may not be restricted to areas of poor growth. For several reasons, it is also helpful to sample the healthy appearing areas adjacent to poor areas.
In some regions of California, nematodes present an indigenous problem. For example, the Imperial Valley has a vast acreage contaminated with sugarbeet cyst nematodes. The grape-growing areas of Lodi, and the Napa and Livermore valleys are currently the dominant habitat of Xiphinema index. Susceptible plants grown in sandy soils of the San Joaquin Valley are most likely to experience damage from root knot nematodes. Walnut orchards throughout the state are most likely inhabited by the root lesion nematode, Pratylenchyus vulnus. Older citrus growing areas are a common habitat for the citrus nematode Tylenchulus semipenetrans.
Nematode sampling history provides a helpful diagnostic tool. Once a nematode population has been found on a farm, chances of completely removing it are remote. However, some nematode species are more easily controlled than others.
History of nematicide usage is also a helpful tool. The response of plants to a nematicide has provided a major way to identify nematode presence. This method does have several limitations. One must be cautious when interpreting results from multispectrum pesticides. Additionally, there are field conditions that are suboptimal for pesticide use and may serve to confuse interpretation.
There are certain crops that are highly susceptible to specific nematodes. For example, tomatoes, kiwi, French Colombard grapes, and Lovell peaches have root systems that, in coarse textured soils, are highly susceptible to damage by root knot nematodes. On the other hand, certain cotton, grape, peach, and tomato varieties do have levels of tolerance or resistance to these same nematodes.
Symptoms of nematode damage in established perennial plantings are generally subtle or nonspecific. The most common symptoms include stunting and chlorosis of affected plants. Among annual crops, the plants may show wilting. In gross terms, the plant exhibits a restricted root system that does not make sufficient water and nutrients available to the plant.
Soilborne nematode problems are generally most apparent in pocketed or irregular areas within a field. Irrigation problems, wilting, chlorosis, foliar mite problems, or nutritional problems may be associated.
Making accurate quantitative asssays is very difficult. Nematode numbers generally provide minimal value for predicting potential crop loss unless numerous soil, plant, management, and ecological factors are also considered. Two examples of damage threshold values for California nematodes can be cited. (1) The presence of more than one sugarbeet cyst nematode egg per gram of soil can indicate future damage to sugarbeets grown in the warm, clay loam soils of the Imperial Valley. (2) The presence of one root knot nematode, one Pratylenchus vulnus, one citrus nematode, or one Xiphinema index in a field to be planted to grapes should cause serious concern about nematode abatement.
From: McKenry, M.V. and P. A. Roberts. 1985. Phytonematology Study Guide. University of California, Division of Agriculture and Natural Resources. Publication 4045.