Basal injury syndrome of Pinus needles

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Abstract
Two types of macroscopic lesions, basal blister and basal necrosis, disrupting the surface tissues of Pinus needles under the fascicular sheath, are described in detail for the first time. In basal blister, hypertrophy and hyperplasia of mesophyll parenchyma cause an oval to elliptical bulge on the needle surface with a longitudinal split of the epidermal and hypodermal layers. In basal necrosis, the epidermal and hypodermal cells become necrotic, while the underlying mesophyll cells undergo structural disorganization. The causal agents are not known but in this study scale insects and deposition of airborne pollutants were associated with basal injury syndrome. The occurrence of basal injury symptoms increased with the age of the needles and the ambient concentration of acid rain precursors. The sulfur and fluoride concentration of the fascicular sheath also increased with the age of the foliage and the ambient pollutant concentrations. Basal injury on sites remote from strong sources of air pollution was significantly correlated with fascicular sheath sulfur concentrations within age-classes of older foliage. These remote site correlations occurred at basal injury rates and fascicular sheath sulfur concentrations lower than those typical of acutely impacted sites near strong sources of air pollution, and thus may represent a chronic impact resulting from acid deposition over wide geographic regions. A soft-bodied scale insect, Matsucoccus secretus Morrison, was often found under the fascicula sheath of Pinus ponderosa Dougl. ex Laws. but did not appear to be a primary cause of basal injury.