The effect of heat and UVC treatments on seed germination and seed-borne fungi of Pinus sylvestris, Pinus contorta and Pseudotsuga menziesii used in Swedish nurseries

Abstract: Pathogen free seeds are critical to produce healthy plants for forest regeneration. However, recent studies have shown that tree seed carry a high diversity of fungi including known and unknown pathogens. There is a need to control seed-borne fungi to minimize the introduction of fungal pathogens, during plant propagation in reforestation nurseries. Thermotherapy treatments, such as heat and UVC light, are recognised as environmentally safe methods for controlling fungal infestation, but effects of such methods on tree seed fungi are poorly understood. Moreover, inadequate dose and duration of these methods can also weaken the viability of the seed by negatively affecting germination. This study assessed the infection levels and diversity of all and potentially pathogenic fungi (with a focus on known seed-borne pathogen Sphaeropsis sapinea) in five seed lots of Lodgepole pine (Pinus contorta; PC14 and PC15), Scots pine (Pinus sylvestris; PS18 and PS20) and Douglas-fir (Pseudotsuga menziesii; PM14) before and after the heat treatment at 55 °C for 8 h and treatments with the UVC light one or three times. Fungi were cultured from 300 seeds per seed lot on nutrient media and identified using Sanger sequencing. Seed germination was also assessed from approximately 300 seeds per seed lot, before and after the treatments were used. Almost all tested seeds yielded fungi. The high infection level and diversity of fungi, including plant pathogens, was showed for control seed lots of Lodgepole pine and Scots pine. Control Douglas-fir seed lot were highly infected, but fungal diversity was low. High infection level by S. sapinea was detected in the control PS18 seed lot. The fungus was also detected in the control PC14 seed lot, but in much lower frequency. The heat treatment reduced fungal infection levels and diversity to varying degrees in Lodgepole pine and Scots pine seeds. This was the case for all fungi, potentially pathogenic fungi and S. sapinea. However, the reduction did not occur in Douglas-fir seed lot. Unlike the heat treatment, UVC treatments did not reduce the fungal infection of any tree species. Considering only potentially pathogenic fungi, UVC reduced the infection in PS18 and PM14 seed lots, however the infection levels were increased in PC15 and PS20 seed lots. Occurrence of S. sapinea in PS18 seed lot, as well as diversity of all and potentially pathogenic fungi in all seed lots, compare to control treatment, was lower after the UVC treatments. Finally, none of the treatments had a significant negative effect on the germination of the tested seeds. The study demonstrates that tested thermotherapy treatments can be used to reduce or eliminate fungi in conifer seeds without adversely effecting germination, and that the heat treatment at 55 °C for 8 h gives better results than UVC treatments. However, more research is needed to determine the most appropriate dose and duration, depending on the tree species.