Field and greenhouse studies have shown that arbuscular mycorrhizal fungi (AMF) can improve plant growth in environments with restricted water availability. The benefits of AMF symbiosis vary among plant species, but the extent to which AMF-mediated drought tolerance varies among subspecific taxa remains poorly understood. In this study, we examine differences in AMF response among three recently diverged, ecologically heterogeneous plant taxa (morphotypes) within the Piriqueta cistoides spp. caroliniana complex. We performed a greenhouse experiment using cuttings of each morphotype inoculated in field-collected soil to test for inoculum source effects of AMF on plant growth under drought. Correlation between AMF colonization and plant performance under drought was significant for all three morphotypes but was strongest for viridis morphotype; this group is associated with mesic, low-phosphorous soils of south Florida slash pine flatwoods. Compared to inocula obtained from other morphotypes’ regions, the AMF obtained from one of the most arid habitats (caroliniana) promoted an equal or greater amount of growth in host plants despite relatively low levels of root colonization. These findings suggest that both genetic divergence among morphotypes and the source of AMF inoculum affect plant growth under drought in P. c. ssp. caroliniana complex.