A reduction of arbuscules in roots of grapevines (Vitis vinifera) observed when ring nematodes were added to field microplots led to the hypothesis that nematode feeding suppresses AMF by competing for root carbohydrates. This hypothesis was tested by growing ‘Pinot noir’ grapevines in a factorial experiment with 3 levels of initial nematode densities (0, 0.1, 1.0 nematodes/g soil), 2 levels of light (full sun, 50% sun), and 2 levels of AMF (+/-). Effects on plant growth were primarily driven by a light and AMF treatment interaction, such that low light increased cane dry matter accumulation at the expense of roots in +AMF vines only. Nematodes had a minor influence on plant growth (leaf mass reduced at the highest nematode density). Because non-AMF vines grew so poorly, the impact of light and nematode treatments were further investigated in +AMF plants only. Nematode populations, AMF colonization, and root carbohydrates were influenced by light or initial nematode density. Root mass, and root sugar and starch concentrations were reduced by low light, but final nematode populations and arbuscule frequency in roots were unaffected by light. Nematodes reduced arbuscule frequency in roots and root starch concentrations. Arbuscule frequency was negatively correlated (p<0.05) to nematode populations and root sucrose, but positively correlated (p<0.05) to root starch. These findings support the hypothesis that ring nematodes suppress arbuscules in roots via competition for carbohydrates. However, the lack of a treatment interaction between light and nematodes in our study further suggests that nematode-AMF interactions are controlled by more than just competition for photosynthate. According to our hypothesis, arbuscular colonization should have been more strongly reduced by nematodes under low light than under high light conditions, which did not occur. First year results of a follow-up study will also be discussed.