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Histological Evidence that Fusarium lateritium is an Exopathogen on Sweetpotato with Chlorotic Leaf Distortion. C. A. Clark, Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Center, Louisiana Agricultural Experiment Station, Baton Rouge, 70803-1720; Phytopathology 82:656-663. Accepted for publication 25 February 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-656.

Apical meristems, axillary buds, leaves at different stages of development, and nodes from vines of sweetpotato (Ipomoea batatas) with symptoms or in remission from chlorotic leaf distortion (caused by Fusarium lateritium) were examined by light and scanning electron microscopy. Apical meristems and axillary buds from healthy plants and plants affected with chlorotic leaf distortion were covered with numerous pearl glands and a mucilagelike material. On plants affected with chlorotic leaf distortion, hyphae were observed on and within this mucilage on the apical meristem, leaf primordia, and young, folded leaves. A dense layer of hyphae was observed between halves of developing, folded leaves and on the adaxial surface of recently unfolded leaves. On older, expanded leaves, mucilagelike material was observed only rarely around the base of pearl glands, and hyphae were present only as scattered clumps with broken ends. No fungal penetration structures were observed on the surface of the plant, and no fungal structures were observed inside any part of the shoot system. It is concluded that the F. lateritium/sweetpotato chlorotic leaf distortion system represents an exopathogen-suscept relationship. Implications of this relationship are discussed.