Histopathology ofPinus radiataSeedlings Infected byColletotrichum acutatumf. sp.pinea

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Abstract
Conidia of C. acutatum f.sp. pinea germinated on young primary needles of P. radiata seedlings within 6 h at 25.degree. C and 17 klx. Appressoria were formed by the germinating conidia 24 h after inoculation under a diurnal regime of 14 h light at 25.degree. C and 10 h dark at 15.degree. 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 for the 1st time. Seedlings .ltoreq. 6 mo. old were the most susceptible. Infected P. radiata stems showed premature secondary thickening associated with increased xylem tissue.

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