Abstract/Summary

Worldwide, trees face increased levels of stress both abiotic and biotic, e.g., climate change, insect and disease invasions associated with trade. Impacts are especially prominent on high value cultivated trees, typically already stressed through human oversight. Such stress increases susceptibility to attack by Armillaria fungi; root and stem phytopathogens with a wide host range. Saprobic ability allows their survival following host/tissue mortality; persistence on buried woody inoculum extends into decades. Local inoculum volume is the other key factor in infection risk. A. mellea is a highly aggressive species and is a key focus for control efforts: this study aims to further knowledge that can be used within, or in the development of, its integrated management. Current Armillaria management relies on laborious physical removal. Chemical controls are ineffective or banned due to environmental concerns. Biological controls, e.g., single Trichoderma strains, have shown much potential experimentally but failed to provide a control for widespread use. Endophytes are a potential source of targeted biological control agents, but their development is challenging. The use of well-characterised bacteria and compounds that stimulate host immunity are under-investigated. In vitro, combination of a sub-lethal potassium phosphite dose, 750-1000mg/L agar, and antagonism by Bacillus subtilis QST713 reduced A. mellea growth by around 80%, meriting further in planta testing. Early Armillaria symptoms are ambiguous/cryptic, hindering timely detection and compounding existing management issues. While studies utilize leaf physiology measurements to monitor infection, there has been little investigation of their application for early detection. Stomatal conductance and chlorophyll responded 35-40 days after Armillaria mellea inoculations in Ligustrum vulgare, a sensitive woody host. This occurred prior to conclusive visual symptoms and faster than the duration of typical host infection assays on Fragaria × ananassa; a herbaceous host which demonstrates comparatively rapid onset of symptoms and mortality. Chlorophyll fluorescence measures also reacted. These infection signatures can be masked by drought and other physiological stresses. Combination of measurements can improve classification of infected and uninfected plants. Armillaria research methodologies, i.e. for fungal culture or host inoculation, face little comparison and vary greatly between studies. As inoculum substrates, the large seeds of Aesculus hippocastanum increased the extent and speed of disease symptoms and mortality compared to conventional Corylus avellana billets, on Fragaria and Ligustrum hosts. Quercus robur seeds had the lowest performance. The differences reflect resources provided to the fungus. On agar, subculture by homogenized mycelium halved variability in comparison to colony fragments. These findings can increase research efficiency.