Grape phylloxera (Daktulosphaira vitifoliae (Fitch)) establish populations and feed on mature roots of susceptible grapevine cultivars, allowing the entry of fungal pathogens which cause vine damage. Grape phylloxera on resistant rootstocks do not establish populations on mature roots, feeding only on immature rootlets and causing little or no vine damage. We have observed damage on mature, unpopulated roots in close proximity to heavily populated rootlets on Concord (Vitis labruscana) grapevines in upstate New York. We hypothesize that phylloxera exhibit Homopteran probing behavior on mature roots, causing small wounds which become infected with Fusarium and other soil-borne secondary fungal pathogens. Laboratory and field experiments are currently underway which will evaluate the ability of phylloxera to allow fungal damage on mature roots of several resistant rootstocks.