A challenging job: Plant pathology in the urban environment

Plants in the city guarantee multiple benefits and satisfactions, but they are exposed to a variety of site conditions, environmental factors, and physical disturbances which influence their survival. Human activity creates urban soils that are distinct from their natural counterpart for physical, chemical and biological features. Surface crusting is water- and gas-repellent, with restricted aeration and water drainage. Vertical and spatial variability is great. Anthropogenic materials and contaminants are present. Soil structure is modified leading to compaction. Nutrient cycling is interrupted and soil organism activity modified. Environmental chemical pollution is a key stress factor for plants in urban areas. The objective of pruning urban trees is to produce safe, strong, healthy, attractive plants, but wounds produced by topping and improper pruning may serve as entry points for decay organisms. There are particular cases where individual trees, because of their condition and location, pose an unacceptable risk to people or property. Trees or tree parts can fail particularly during loading events such as wind and snow storms. Trees fail when the load (weight and motion of the crown) exceeds the mechanical strength of their stems, branches or root systems. This is true for both sound and defective trees, but defective individuals can only withstand a fraction of the load that sound trees can withstand. The failure of limbs or entire trees, however, is often predictable, detectable, and preventable. Tree risk analysis has evolved over time from a qualitative art to a quantitative science. Modern techniques and procedures can be used to minimize the risks associated with tree failure and to identify structural defects. So, the mission of urban plant pathologists is to verify if a tree has what it takes to stand up in a secure way. This is based on the evaluation of the acceptable safety factor (safe shell limits). Trained and able staff must be available for periodical tree inspections. University courses of Agricultural and Forest Sciences should discuss and deepen these crucial issues.