INITIAL INTRODUCTION AND SPREAD OF INFESTATION

The most striking feature of nematode distribution and damage within a field is the irregularity of infested areas. Damaged crops will appear as irregular patches or streaks that may vary in size, shape, and number. These variations usually reflect the compounding of nematode stress on a plant by such other factors as physical soil differences and irrigation and drainage patterns. Previous patterns of cropping, initial introduction, and soil movement through cultivations are also important factors. Consider a field of uniform soil texture, cropping history, and management practices, such as irrigation and drainage facilities, that is under a regular rotation sequence that includes sugarbeets. At a single site within an otherwise uninfested field, sugarbeet cyst nematode is introduced. Cysts containing viable eggs are introduced by contaminated harvesting or cultivation equipment, tractor or truck wheels, human feet, or animal hooves; by irrigation water; or by wind blown in from an adjacent infested area. Following introduction, at a single site or at separate sites within the field, the nematode becomes established during host crop development and, over a period of years, increases population levels. As the population increases, the sites gradually enlarge in area and, when the nematode population reaches a damaging level, the sites show as unthrifty or damaged plant areas within the field. In later years, under short rotations, these areas enlarge further and coalesce until large areas of the field become uniformly infested. The spread of nematodes within the field from these original sites of introduction is mediated largely through the movement of cysts floating on water used in flood or furrow irrigations. Similarly, cattle grazing on beet tops at harvest can distribute infestation by their hooves. In this example, processes related to initial introduction and subsequent spread of infestation are the major determinants of fieldwide distribution.

From: McKenry, M.V. and P. A. Roberts. 1985. Phytonematology Study Guide. University of California, Division of Agriculture and Natural Resources.

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