Abstract
Thin section of leaves of six Dioscorea species were examined in an electron microscope. Bundles of filamentous virus-like particle were found in all plants showing severe symptoms but not in those showing mild or no symptoms. Pinwheel cytoplasmic inclusions were found in plants showing both severe and mild symptom and in normal looking plants. Pinwheels were also found in tubers of D. alata cv. ''White Lisbon' from severely infected plants. No pinwheels were present in D. cayenensis and one D. alata cultivar. Masses of bacilliform virus-like particles were detected in the cytoplasm of leaf cells of plants from tubers of D. alata cv. ''White Lisbon' which were affected by the internal brown spot disease. They were found in plants from spot-free tubers. These particles were also present in D. cayenensis in which internal brown spot disease has not been reported.
