Leaf Spot Disease of Chromolaena odorata Caused by Septoria sp. in Guam. V. M. Russo, University of Guam, CALS/AES, UOG Station, Mangilao, Guam 96912. Plant Disease 69:1101, 1985. Accepted for publication 30 August 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-1101c.
The noxious weed Chromolaena odorata (L.) Kr. occurs in many parts of the world and is spreading rapidly in the Pacific Basin. In Mangilao, Guam, necrotic spots with red margins were observed on leaves of flowering plants. Over time, the spots coalesced to involve the entire leaf surface. Septoria sp. was isolated from the spots. There is no report of a Septoria sp. causing disease in Chromolaena. On one stem each of five plants, leaves, from youngest to oldest, were inoculated with spores (approximately 10,000/ml) and mycelial fragments from cultures of the isolated Septoria sp. Each branch was covered with a plastic bag. Uninoculated leaves on another stem of each of the same plants were treated with water only and similarly covered. Necrotic areas similar to those originally observed developed on the oldest leaves 5–8 days after inoculation, and the Septoria sp. was reisolated. Because only the oldest leaves were affected, the fungus apparently attacks senescing tissue and probably is not promising as a mycoherbicide. Naturally infected plants usually are not killed, and resprouting occurs. Repeated applications of the fungus might provide sufficient pressure to damage the plant, but this remains to be demonstrated.