California grape phylloxera more variable than expected

Abstract

Many strains of grape phylloxera now have been identified in Cali-fornia vineyards. This variability may be the result of multiple introductions of this pest or of evolution of new strains on susceptible or weakly resistant rootstocks. Thus own-rooted vines, weakly resistant rootstocks and those with V. vinifera parentage should not be used in phylloxerated areas. In addition, because of the observed variability, quarantines are ineffective in preventing the occurrence of biotype B phylloxera, as it appears to evolve independently in different areas. Grape phylloxera, Daktulosphaira vitifoliae (Fitch), is one of the worlds most important viticultural pests. This minute aphidlike insect lives on grape roots and foliage, and is most damaging to grape species that are susceptible to its root feeding. This feeding causes a swelling of the root that develops into a gall and nourishes the insect. As the galling develops it expands and cracks, wounding the root and potentially allowing the entry of rot organisms. The combination of phylloxera's limiting effect on the root system's ability to take up water and nutrients and the effect of rot organisms eventually kills infested vines. Phylloxera can devastate a susceptible vineyard in just a few years. Grape phylloxera have been destroying vineyards around the world for the past 140 years. Fortunately, phylloxera-resistant rootstocks have been available and have effectively defended vineyards against this pest for about 110 years. Despite this long history , many questions remain about phylloxera's biology, its life cycle and how grape species and rootstocks resist its feeding. This paper reviews our recent discoveries that the DNA and feeding behavior of different collections of California phylloxera are relatively diverse. The discovery of diversity in biological populations is not novel, but the amount we found in California was unexpected. Because phylloxera were supposedly introduced into Cali-fornia a relatively short time ago, and they only reproduce asexually on the roots, we expected a relatively low level of diversity. ity to feed on different grape species and cultivars is of central importance to the use of rootstocks. Vitis species and the rootstocks bred from them are not uniformly resistant to grape phylloxera , nor are grape phylloxera types uniformly aggressive on a given species or rootstock. As is common throughout biology, there is variability in both the plant and the insect. Many questions have emerged: What generated the diversity? Are other root-stocks in imminent danger from a damaging biotype? How does this diversity affect the breeding and selection of new rootstocks? To begin addressing these questions , we describe experiments to evaluate the variability of grape phylloxera in California and elsewhere. We also review what is known about phylloxera aggressiveness from other viticultural regions.