The climatic variations within California - hot interior valleys, cool high mountain regions, and cool coastal and northern regions - are diverse enough to influence the pattern of nematode distribution. Some nematodes may have adapted to strict climatic requirements; others may be broadly adapted to survival and proliferation within any of these climatic extremes. Within a geographic area, distribution often largely depends on farming factors, but also may be determined by additional environmental requirements, such as availability of host plants (whether agricultural crops or natural weed species), particular soil texture, and chemical and moisture characteristics.
Stubby root nematodes in California are adapted to a wide temperature range. The most common species, Paratrichodorus minor, has extensive dsitribution in agricultural regions throughout California, including the Imperial Valley, Central Valley, and coastal regions. However, stubby root species predominate in soils with a high sand content (usually 70 percent or more sand) and, therefore, there are limits based on soil textural preferences within any one geographic area. Because stubby root nematodes have characteristically wide host ranges, the influence of crop type is not important in determining distribution of the various species.
At the generic level, Pratylenchus is widely distributed throughout California, and does not appear to be restricted by climate, soil characteristics, or host crop preferences. Thus, we may expect to find root lesion nematodes in any soil sample. However, various species of Pratylenchus show preferences to a variety of characters that influence their geographic distributions. For example, both P. scribneri and P. thornei are found throughout all geographic and climatic agricultural regions of the state, and have wide host ranges on annual field and vegetable crops. However, P. scribneri shows a textural preference for sandy loam and coarser textured soils, whereas P. thornei shows a preference for the clay and loam soils such as those in the Imperial Valley, Sacramento Valley, or eastern slopes of the San Joaquin Valley.
Pratylenchus minyus is the most widely distributed root lesion nematode. It is commonly associated with annual crops in California, in particular with grain and grass crops. It is also commonly found associated with alfalfa and other forage crops that may support grassy weeds. Pratylenchus brachyurus is found predominantly in the interior valleys of California including the Imperial, Coachella, and southern San Joaquin; it appears to be adapted to the warmer soil temperatures found within California and shows some crop preferences, for example for cotton and beans. This species has not been found in the cooler coastal and northern California regions.
Pratylenchus vulnus is the common root lesion nematode of perennial crops, although other species such as P. minyus can be found among grassy orchard and vineyard floors. They do not appear restricted by soil types. Pratylenchus penetrans is associated with cool regions, especially where cherries or apples are grown. A significant host preference between the two species that attack perennials is that P. vulnus is a severe pathogen of figs, walnuts, citrus, and grapes, whereas P. penetrans is not considered an important pathogen on these crops and is seldom associated with them. The differences in host preferences and pathogenicity among species of root lesion nematodes demonstrates the critical importance of determining which lesion nematode species is present within a given field.
Stem and bulb nematode (Ditylenchus dipsaci) has been recorded from all the major agricultural regions in California. This wide distribution probably reflects its spread on such infested planting material as garlic cloves. This is a good example of where agricultural and cultural practice is a primary determinant of nematode distribution.
The two foliar nematode species (Aphelenchoides fragariae and A. ritzemabosi) also have a distribution that reflects the movement of infested propagative plant material. Both species are most commonly found where their ornamental hosts and strawberry hosts are grown. The infection process depends on moist conditions. These species are prominent in, and mostly limited to, the coastal county areas of Orange and Los Angeles, the southern San Francisco Bay Area counties, and the northern coastal counties of Humboldt and Del Norte. Aphelenchoides fragariae, the more common of the two species, has also been found occasionally, in some interior counties, particularly Stanislaus, San Joaquin, and Shasta. Although foliar nematodes are generally more common in temperate zones, common infestations of subtropical plants grown in greenhouses indicate a wider distribution.
Citrus nematode (Tylenchulus semipenetrans) is a warm temperate and subtropical species with a wide distribution in California, and is particularly prevalent in the oldest citrus growing areas. Host requirement appears to be the most important distribution determinant; host range is confined to citrus species, olives, persimmons, grapes and several ornamental crops. Its wide distribution may reflect movement of infested planting stocks during the earlier years of the citrus industry worldwide, although its spread by this means has greatly decreased in recent years. It appears in all soil textures in which citrus is planted, from gravelly sand and sandy soils to clay loams and clays. Severe infestations have been observed on sour and sweet orange rootstocks on all soils in California and Arizona. Ecological conditions in medium textured soils favor citrus nematode.
The distribution of sugarbeet cyst nematode (Heterodera schachtii) is primarily related to sugarbeet production: sugarbeets are grown in every climatic region and sugarbeet cyst nematode is distributed within most sugarbeet areas. The Imperial, Salinas, Sacramento, and northern San Joaquin valleys and central coastal California are major areas of infestation. This nematode is not restricted by soil temperature, persisting equally well in the hot Imperial Valley and the cooler coastal beet growing areas. It is also adapted to the complete range of soil types, from the sandiest soils to the heavy clay soils to peat and muck soils. Poor sanitation practices, and the movement of contaminated harvesting and cultivation equipment between sugarbeet growing regions have no doubt played a large part in the spread of this nematode.
Root-knot nematodes (Meloidogyne sp.) are distributed throughout California's agricultural areas. The different species show some regional preferences, however. Meloiodgyne incognita, M. arenaria, and M. javanica have similar distributions and are concentrated in the warm interior valleys of Imperial, Caochella, and San Joaquin. In these areas, M. incognita appears to dominate over M. javanica. Apart from Ventura County, coastal areas are mostly free from infestation of M. jananica. Meloidogyne incognita has been found in the coastal monterey, Santa Barbara, and Santa Cruz counties, and is reported from Sacramento, Butte, and Tehama counties at its northern distribution limit. These species have wide host ranges that generally do not constrain distribution. However, a few host preference differences can be important. For example, M. incognita is the only root knot species that parasitizes commercial cotton and, therefore, it may be more common on land regularly cropped to cotton.
Meloidogyne hapla is widely distributed in California although infestations tend to be localized. This species has temperature optima in the different phases of the life cycle, which are about 4 to 5C lower than for M. incognita and M. javanica, as shown by the wider distribution of M. hapla in the cooler areas. These three species show some adaptation to local climatic conditions, however; equally important may be the host preferences of each. Meloidogyne hapla development is strongly favored by alfalfa. Meloidogyne arenaria has a scattered but localized distribution in California. It has been found in Coachella, southern San Joaquin, and Sacramento valleys, and in Monterey and Contra Costa counties, althoug it is less commonly encountered. In northern California, particularly in the Tulelake Basin area, M. naasi parasitizes barley, wheat, and grasses.
The extensive host ranges on both annnual and perennial crops in California of the most common root knot nematodes and the periodic movement of infested rootstocks and planting material have no doubt expedited spread of these important pathogens. Meloidogyne species occur in a wide range of soil textures, but they appear to predominate in coarse textured sandy and sandy loam soils. Plant damage is often accentuated in sandy fields or in sandy patches or streaks within a field. Thus, from a pest management standpoint, soil texture is critically important to root knot nematode distribution and potential pathogenicity.
From these detailed examples, we can see that climate or temperature preference, soil textural preference, host crop preference, area of initial introduction, and movement of contaminated plant or agricultural equipment each can significantly influence nematode distribution, depending on the environmental adaptation and parasitic habit of a particular nematode species.
From: McKenry, M.V. and P. A. Roberts. 1985. Phytonematology Study Guide. University of California, Division of Agriculture and Natural Resources.