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Ecology and Epidemiology

Effects of Leaf Age, Host Growth Stage, Leaf Injury, and Pollen on the Infection of Sunflower by Alternaria helianthi. S. J. Allen, Junior research fellow, Department of Botany, University of New England, Armidale, N.S.W. 2351, Australia; J. F. Brown(2), and J. K. Kochman(3). (2)Associate professor, Department of Botany, University of New England, Armidale, N.S.W. 2351, Australia; (3)Senior plant pathologist, Queensland Department of Primary Industries, Toowoomba, Queensland, 4350, Australia. Phytopathology 73:896-898. Accepted for publication 29 November 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-896.

Laboratory and glasshouse experiments were conducted to investigate the effects of various host factors on the growth and development of Alternaria helianthi on sunflower (Helianthus annuus). Lesions caused by A. helianthi developed most rapidly on leaves of plants that were at the anthesis or seed-filling stages of growth and on the older leaves of plants at the vegetative or budding stages of growth. Chlorotic halos were most prominent surrounding lesions on younger (vegetative or budding stage of growth) sunflower plants and were almost absent from lesions on older plants. The presence of artificially induced injuries of the leaf surface increased the number of lesions per square centimeter caused by A. helianthi relative to the number of lesions on uninjured leaves. The germination of conidia and the branching of germ tubes was increased when sunflower pollen was mixed with conidia of A. helianthi. The data indicate that the susceptibility of sunflower plants is greatest at the anthesis or seed-filling stages of growth. Consequently, it is suggested that sunflower be planted so that the crop matures when environmental conditions (temperature, leaf wetness, etc) are least suitable for disease development.