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Some plant species within the Convolvulaceae (morning glory family) from South America, Africa, and Australia cause a neurologic disease in grazing livestock caused by swainsonine. These convolvulaceous species including Ipomoea carnea contain the indolizidine alkaloid swainsonine, an inhibitor of α-mannosidase and mannosidase II, and polyhydroxy nortropane alkaloids, the calystegines which are glycosidase inhibitors. Swainsonine has been shown to be produced by a fungal endosymbiont in legumes of the Astragalus and Oxytropis genera, where it causes a similar neurologic disease in grazing livestock called locoism. Here we demonstrate that I. carnea plants are infected with a fungal endosymbiont that was cultured from its seeds and which produced swainsonine in pure culture but not the calystegines. The same fungal endosymbiont was detected by PCR and by culturing in I. carnea plants containing swainsonine. The fungal endosymbiont belongs to the Ascomycete order Chaetothyriales. Plants derived from fungicide-treated seeds lacked swainsonine, but calystegine concentrations were unaltered.