Grape Phylloxera: Biology and Genetics of Rootstock Resistance and Cause of

Our understanding of phylloxera biology, damage and variation has changed appreciably over the past few years due to research partially funded by the American Vineyard Foundation. Biology: Pylloxera populations do not accumulate from year to year. Populations reach a peak midsummer and then decline until harvest after which they rise again for a second peak, falling again as the soil temperature drops below 18°C, their threshold. Populations stay low over the winter until temperature again exceeds the threshold in spring. The fall of populations midsummer begins at veraison and reaches its nadir at harvest. We hypothesized that this fall is due to competition between phylloxera and the susceptible vine for sucrose released from starch stored in the phloem parenchyma at the feeding site. This hypothesis has been supported by 1) a series of month-long population studies on roots in the vineyards, and 2) experiments in which we measured sucrose released from starch and the disappearance of starch in excised roots. In addition, our study of variability of insects in the native range indicates that there are sexual forms of phylloxera in the leaf galls of desert phylloxera on wild vines. Damage: Our population studies (above) demonstrate that the weight of all phylloxera on vine roots is very small and cannot account for the damage seen in the vineyards. However, over the last four years we have demonstrated initially in greenhouse experiments and then in the field that soilborne phytopathogenic fungi that enter feeding wounds of phylloxera can cause sufficient necrosis to account for the vineyard damage. Many fungal species are involved; the primary ones are Fusarium and Pythium. Root necrosis changes over the season and can cause as much as 25%root loss in one summer. The rot is minimal at the end of summer and highest in early spring. Roots of all sizes appear to be equally affected. Soil conditions influence the prevalence of the rot. Survey data suggest that the organically managed soils have less root necrosis than conventionally managed soils, though populations of phylloxera in both soils may be similar. Variation: Phylloxera populations vary genetically and in their abilities to use particular root types as hosts. Our work with biotype B and AXR#1 demonstrated this variability many years ago. Recently, we have been looking at phylloxera colonies originating on various rootstocks for evidence of increased virulence. We found populations in California, Germany and Hungary which have increased virulence on 5C and S04 when compared with phylloxera that did not come from the roots other than V. vinifera or AXR#1. The increased virulence is primarily on immature roots though there is some ability to utilize mature roots as well. Reports of damage in the field in Germany suggest that damage is due to feeding on immature roots rather than mature roots (as occurs with V. vinifera or AXR#1 in California). The virulent colonies from California and Europe appear to be genetically related. These results need further research to determine relevance with regard to stability of rootstocks.