GRAPE ROOTSTOCKS IN CALIFORNIA VINEYARDS: THE CHANGING PICTURE

The insect was spread rapidly through the established grape districts of California causing widespread destruction of vineyards, however, it was noticed, even at these early times, that the insect's destructive behavior was influenced by the soil environment in which the vines were growing. Soil texture, moisture content, and surface temperature all had strong effects on the insect's ability to reproduce on vine roots and to eventually destroy the vineyard (1, 4).

Unlike the situation which developed in Europe, where the winged, sexual form of phylloxera allowed the insect to spread rapidly and uniformly through large vineyard districts, the movement in California was slow and sporadic. This was eventually attributed to the fact that California phylloxera had a simplified life cycle, being restricted to an asexual, root-infesting form. Spread of insects from a center of infestation was by a slow, seasonal movement of wingless nymphs or by distribution of infested planting stock.

This difference in biology of phylloxera between Europe and California was attributed to the much lower relative humidity found in California vineyard districts during summertime.

Therefore, even today, 120 years after the first detection of phylloxera in California, only some 30% of the vineyards of the state are infested to the degree that they require the use of phylloxera resistant rootstocks. This is due in part to the lack of a winged form and to the predominance of sandy soils in the Central Valley where 75% of the grape acreage exists.

This fact presents a complexity to the grape rootstock decisions for California vineyardists in the Central Valley — that is, not only which variety of rootstock would be most suited for their immediate needs, but whether or not it is necessary to employ a phylloxera stock in the first place. The interior valleys of California are widely infested with parasitic nematodes which thrive in the warm, irrigated sandy soils of these valleys. Growers in the past have relied mostly on soil fumigants and nematicides to protect their vineyards. However, recent legal regulations in California have either banned or severely limited the use of these chemicals. As a result, today there is a strong grower interest in the testing and selection of suitable nematode resistant stocks for these areas in California.

Soon after the discover of phylloxera in Europe, the possibilities of using the resistance to the insect found in native American species of Vitis was exploited by European investigators. Between 1885 and 1900 a tremendous effort to develop grafting stock was put forth and the reconstruction of the phylloxerated vineyards of the world was under way.

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