Dirina stenhammarii

Dirina stenhammarii is a species of crustose lichen that often settles on the bark of deciduous trees. In agronomy, it is frequently misidentified as a parasitic disease; however, it functions as an epiphyte, meaning it uses the tree trunk as a physical support rather than consuming living plant tissues or causing direct rot.

The lichen consists of a symbiotic relationship between a fungus and algae. It thrives in humid environments and is typically found on mature trees where the bark remains undisturbed for many years. It does not actively infect the tree, but its presence is a clear indicator of the specific microclimatic conditions surrounding the tree, such as stable humidity levels.

The visible signs of this lichen are flat, greyish-white or crusty patches on the stems and larger branches. While it does not kill the tree directly, excessive coverage can potentially block lenticels, which are the tree's breathing pores. Furthermore, a thick layer of lichen can create a micro-habitat for various bark-dwelling pests, which may indirectly affect the health of the host tree.

Development and spread are facilitated by moist, stagnant air and lack of sunlight reaching the lower trunk. During wet periods, the spores disperse and colonize new surfaces. Managing its presence is essentially a matter of orchard hygiene and ensuring proper air circulation within the tree canopy through structural pruning and maintenance.

Control strategies should focus on prevention rather than chemical eradication. Regular maintenance, such as scraping the bark with specialized brushes, helps to remove the physical crust. Whitewashing the trunks with lime-based solutions is also a highly effective method to alter the surface pH, making the environment hostile for the lichen to re-establish itself on the bark.