Identification and Distribution of Fungal Pathogens Associated with Loblolly Pine Defoliation and Tree Mortality in the Southeastern United States

Abstract

Loblolly pine (Pinus taeda L.) is a predominant timber species native to the southeastern United States. Since 2016, the species has been experiencing needle defoliation and tree mortality. Symptoms appear as yellowing to progressively browning needles. However, it is uncertain if this disease results from a single or a combination of several fungal pathogens. This study investigated the potential causal agent(s) of this disease as well as pathogen impacts on loblolly pine foliage and foliar chemistry. Climatic regression models were developed to predict loblolly pine defoliation severity in following years. Based on their colony morphology and molecular ITS-rDNA sequence data, a total of 28 species of fungi representing 17 families were recovered from symptomatic loblolly pine needles from five States. Lecanosticta acicola, was repeatedly recovered and identified as the predominant pathogen in Alabama. No sexual stage was observed for L. acicola and only the single mating type MAT-1-1 was recovered which suggests low genetic variation. Other pathogens recovered in the study were Lophodermium spp., Coleoporium spp., Rhizophaera spp., and Diplodia spp. Needle pathogen, L. acicola had substantial effects on loblolly pine foliage and foliar chemistry. Trees with high incidence produced statistically significantly shorter shoots and needles compared to low incidence trees. Needle nutrient contents of N, S, Na, and B showed a positive correlation with L. acicola severity. High incidence trees had an increased level of total phenolics in their needles that correlated to L. acicola infection and severity. Seventy trees at seven long-term monitoring plots showed progressively chlorotic and defoliated crown in Chatom, Alabama, Washington County from 2019 to 2021. The best 5-factor regression model predicted that the previous year’s February, May and June temperature and July and fall precipitation as the best predictors of loblolly pine defoliation severity in following years in Alabama. Increasing summer months temperature and precipitation and decreasing fall months precipitation are expected to favor loblolly pine defoliation severity and spread in following years. Climatic models were developed to aid private landowners and forest managers to monitor, plan and adjust their management strategies accordingly. Brown spot needle blight is an emerging and potentially devastating disease in loblolly pine plantations in Alabama.