Searching the Pinus taeda Foliar Mycobiome for Emerging Pathogens Among Brown-Spot Needle Blight and Needlecast Outbreaks in the Southeast United States

Abstract

Needle pathogens cause the discoloration, death, or premature abscission of conifer foliage, reducing growth and vigor, and repeated defoliation may eventually result in tree mortality. Since 2016, forest managers in the southeast United States have reported the increasing scale, frequency, and severity of needle disease outbreaks on the region's principal timber species, loblolly pine (Pinus taeda L.). These recent outbreaks are raising concern throughout the region, as needle diseases are not traditionally considered a threat to P. taeda. Lecanosticta acicola (Thum.) Syd., the native causal agent of brown-spot needle blight, has been recovered from some outbreaks. However, the full array of fungi associated with symptoms has not been explored. In this research, P. taeda foliage was collected from affected stands throughout the region and analyzed to identify fungi associated with needle disease symptoms. We employed targeted molecular diagnostics to confirm the presence or absence of L. acicola and DNA metabarcoding to characterize the foliar mycobiome and screen for other potential pathogens. L. acicola was detected among symptomatic needles from multiple states, particularly in western portions of the P. taeda range, but rarely from stands in eastern states. Metabarcoding revealed pathogens in needles and identified associations among pathogenic fungi, differing symptoms (including needle discoloration and necrosis), and signs of fungal fruiting bodies. Additionally, the fungal community of needles varied with patterns of symptom presentation. This study is the first regionwide assessment of fungi associated with recent large-scale needle disease outbreaks on P. taeda and identifies multiple pathogens that warrant further study.

Keywords

• metagenomics
• microbiome
• mycology
• natural habitats
• plant pathology