Fitness responses to environment can shape species distributions, though opposing eco-evolutionary processes can obscure environmental effects. For example, host specificity influences parasite dynamics, but is unclear how specialization of individual parasites or populations scales up to continental distributions. Here, we develop a macroecological framework to determine how host community structure affects continent-scale specialization in Striga hermonthica, an African parasitic plant of cereal crops. We find regional abundance of hosts in cultivated cereal communities is associated with parasite specialization observed in experiments. Moreover, abiotic environment at location of origin predicts parasite performance on pearl millet and sorghum but not maize, possibly due to the shorter coevolutionary history for maize and Striga. Our study demonstrates that patterns of parasite local adaptation to host communities can emerge at continental scales and that differential environmental tolerances of hosts indirectly shape the distribution of specialist and generalist parasites.