From Cryptic Infection to Forest-Scale Risk Mapping: A Critical Review of Remote Sensing Technologies, Indicators, and Research Priorities for Heterobasidion and Armillaria Root Rot

Heterobasidion and Armillaria may infect tree roots and cause internal decay without visible changes in the crown. Decay is therefore often discovered only after windthrow or harvesting. Conventional remote-sensing observations do not identify the causal fungus directly. They record changes in trees or stand conditions that may be associated with root rot. We reviewed these indicators and the technologies used in studies of Heterobasidion and Armillaria, with particular attention to their possible use in forest management. Three semantic searches in OpenAlex and Semantic Scholar identified 1500 records on disease indicators, sensing technologies and research gaps. Screening retained 106 records, representing 84 unique publications. Information on disease stage, sensing method, reference data and validation was compiled. Site, stand and management data were mainly used to estimate the probability of root rot occurrence. LiDAR and aerial images were effective when damage was already visible as crown decline, mortality or canopy gaps. RGB images were used to identify decay exposed after harvesting. Detecting internal decay in standing trees was more difficult. Hyperspectral studies reported moderate and mainly site-specific results. Small affected classes, inconsistent reference standards and limited independent validation made it difficult to determine whether the results would apply in other forests. Remote sensing could not reliably distinguish root rot from other causes of tree stress from the evaluation data and is currently better suited to locating areas for field inspection than to confirming the causal pathogen.