Biological constraints affecting root damage on grapevines infested with grape phylloxera (Daktulosphaira vitifoliae (Fitch) [Hemiptera: Phylloxeridae])

Grape phylloxera, Daktulosphaira vitifoliae (Fitch), is a globally distributed grapevine pest. Root-feeding phylloxera cause galls on mature roots (tuberosities) and young feeder roots (nodosities). Virulent soil fungi enter root tissues through tuberosities; the resulting infections cause tissue necrosis and root death leading to decline and death of the vines. Understanding factors influencing incidence and severity of phylloxera-related vine damage could provide insights into the mechanisms of damage and suggest management strategies.; Rootstocks which limit the establishment of phylloxera populations on mature roots reduce phylloxera-related vine damage; resistance to fungal root infections is not a criterion for rootstock selection. A field trial exposed an array of root types to virulent F. oxysporum isolates. Results showed that strongly resistant rootstocks were damaged by the fungi. This suggests that phylloxera resistance does not necessarily confer Fusarium resistance to rootstocks.; Vine stresses are used to increase wine grape quality. Such stresses predispose other woody plants to fungal infections. Mature grapevines were stressed with deficit irrigation, canopy pruning, and cluster management. Root damage caused by F. oxysporum infections was not increased by stress, suggesting that such management would not increase phylloxera damage.; Phylloxera populations on nodosities are causing vine damage of strong rootstocks. By exposing rootstock tissues to a virulent Fusarium isolate and an avirulent phylloxera biotype, significant fungal damage occurred through probing activity without established feeding sites. This suggests a new mechanism for vine damage.; Effective sampling can establish the presence of phylloxera populations. An emergence trap modified from an Australian design was effective at detecting phylloxera populations in the field. Site-specific factors influenced trap counts and could contribute to variation in vine damage seen in the test sites.; This work suggests that the lack of a relationship between phylloxera and Fusarium resistance puts the durability of resistant rootstocks into question when the existence of phylloxera probing behavior is considered. Vine-stressing practices can be used in this scenario without aggravating damage. Additional research into Fusarium variability and phylloxera population dynamics will help clarify the potential for future rootstock failures.