Influence of Leptographium terebrantis S.J. Barras and T.J. Perry on Pinus taeda L. physiology, growth and productivity

Abstract

Loblolly pine (Pinus taeda L.) is the predominant tree species in forest plantations across the southeastern U.S., but over the past several decades, cases of declining loblolly pine have been reported in localized areas in central parts of Alabama and Georgia. Bark beetle vectored fungus Leptographium terebrantis is frequently isolated from roots of the declining loblolly pine trees and has been implicated as one of the agents contributing to the decline and or mortality. Although the fungus has been shown to be pathogenic to seedlings, saplings and roots of mature loblolly pine trees, its ability to contribute to growth decline is unknown. This study examined the effect of L. terebrantis infection on loblolly pine growth and hypothesized that L. terebrantis infestation will induce sapwood occlusions, affect physiological functions, reduce leaf area and cause growth decline in loblolly pine trees. Using L. terebrantis colonized sterile toothpicks at differential inoculum densities; the sterilized toothpicks served as useful substrates for the production and transfer of L. terebrantis propagules into living tissues of loblolly pine. The pathogen compromised xylem function by causing phloem lesions, sapwood occlusions and significantly reduced specific hydraulic conductivity. These modifications, however, did not impose moisture stress on the foliage of young loblolly pine trees and the trees survived by producing new growth devoid of the pathogen infestation. In mature loblolly pine trees, the pathogen infestation affected moisture content and foliar mineral nutrition. Nitrogen concentration decreased below the sufficiency level and was most severe at high inoculum density relative to the control trees. In contrast, manganese concentration increased and was most elevated in the high inoculum treatment trees coupled with elevated calcium. L. terebrantis III infestation limited carbon fixation caused a reduction in leaf area and the ratio of leaf area to sapwood area. Collectively, the physiological and morphological changes coupled with moderate drought caused growth decline which was more pronounced at high inoculum density resulting in loblolly pine mortality. At lower inoculum densities the loblolly pine trees completely tolerated the pathogen by producing new sapwood to sustain the trees physiological processes. The results of the study demonstrate that L. terebrantis is not passively associated with declining loblolly pine trees but can negatively influence loblolly pine growth.