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Cytology and Histology

Histopathology of Pinus radiata Seedlings Infected by Colletotrichum acutatum f. sp. pinea. J. Nair, Former graduate student, Department of Botany, University of Auckland, Private Bag, Auckland, New Zealand; J. B. Corbin, senior lecturer, Department of Botany, University of Auckland, Private Bag, Auckland, New Zealand. Phytopathology 71:777-783. Accepted for publication 7 November 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-777.

Conidia of Colletotrichum acutatum f. sp. pinea germinated on young primary needles of Pinus radiata seedlings within 6 hr at 25 C and 17 klux. Appressoria were formed by the germinating conidia 24 hr after inoculation under a diurnal regime of 14 hr light at 25 C and 10 hr dark at 15 C. Infection pegs from appressoria pierced the cuticle and enlarged into infection hyphae. These spread to all tissues in the needle, leading to collapse of cells. Mycelium extended through the needle into the stem to the vicinity of the vascular tissue. Thereafter fungal growth ceased. Symptoms of infection were: terminal crook, terminal blight, stiffening and thickening of stem, a period of dormancy and stunting. All were not necessarily exhibited in any one diseased seedling. Terminal crook resulted from continued growth of young stem tissue on the side opposite to lateral necrosis near the apex. Terminal blight, consisting of death of complete stem apex, is reported here for the first time. Seedlings 6 mo old and younger were the most susceptible. Infected P. radiata stems showed premature secondary thickening associated with increased xylem tissue.