Emerging Threats in Southern U.S. Pine Plantations: Temporal Dynamics of Fungal Communities and the Impact of Lecanosticta acicola.

Globally, pine forest ecosystems are under increased threat of foliar fungal pathogens. This includes brown spot needle blight (BSNB), caused by Lecanosticta acicola. High disease severity of BSNB has been observed in loblolly pine plantations across the Southeastern U.S., causing substantial declines in productivity. Because foliar disease outcomes depend on phyllosphere community interactions, shifts in community composition under climate variation may influence outbreak potential of L. acicola. To investigate these interactions, fungal communities in first- and second-year symptomatic and asymptomatic needle tissue were examined over two years across six loblolly pine plantations in central Louisiana. L. acicola was consistently enriched in symptomatic needles and emerged as a strong indicator of disease, including increasing crown dieback, particularly in first-year needles. Disease progression was associated with reduced fungal diversity and pronounced shifts in community composition, consistent with microbiome dysbiosis. Additional fungi, including Lophodermium and Soleella, were enriched in symptomatic needles, likely representing opportunistic associates with a potential role in disease. There were distinct differences in the relationship with climate variables for symptomatic and asymptomatic communities. Symptomatic communities were associated with higher humidity, higher minimum temperatures, and reduced solar radiation, whereas asymptomatic communities were correlated with warmer, drier conditions. Our findings demonstrate that BSNB severity reflects both L. acicola infection and broader needle fungal community disruption, with first-year needles being especially vulnerable. These results underscore the need to integrate microbial community dynamics and climate into disease monitoring and management, as increasing humidity, warmer nights, and more variable precipitation likely elevate fungal pathogen risk.