Grape rootstocks in California

Chiarappa & Buddenhagen (1994) have reported a false loss of resistance to Phylloxera (now renamed Daktulosphaira vitifoliae, Fitch) in the hybrid grape rootstock "AxR#1" in the Napa and Sonoma Valleys of California. It appears that Lider (1957) had discovered a considerable increase in yield from the use of this rootstock and, as a consequence, some 20,000 hectares were planted with it in the Napa and Sonoma Valleys. Unfortunately, it appears that Lider had not used a susceptible control in his experiments and that Phylloxera was absent from his test plots. The hybrid rootstock was assumed to be resistant when, in fact, it was an escape from infection. It had about half the level of resistance of the genuinely resistant rootstock Rupestris St George. During the course of about a quarter of a century, this large area of vineyards has been showing increasing levels of Phylloxera damage, and it will have to be replanted at an estimated cost of one billion dollars. It should be added that attempts to explain this apparent loss of resistance in terms of new biotypes, and as a breakdown of vertical resistance are incorrect. This is clearly a continuous pathosystem (see 4.12) and vertical resistances will not occur.

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