Alder yellows

Alder yellows is a serious phytoplasma-induced disease primarily affecting various species of the Alnus genus. The causative agent is the phytoplasma 'Candidatus Phytoplasma alni', which colonizes the sieve elements of the tree's phloem. Because this pathogen lives strictly within the plant's vascular system and the bodies of its insect vectors, it represents a significant challenge for forest pathologists and plant health authorities worldwide.

The primary hosts of this disease are common alder species, such as Alnus glutinosa (black alder) and Alnus incana (grey alder). While it is most frequently reported in native forest populations, it can also impact alder trees used in landscaping and riparian restoration projects. The disease is transmitted by insect vectors, specifically leafhoppers of the Macropsis genus, which acquire the phytoplasma while feeding on the phloem sap of infected trees.

The disease manifests through a range of distinct physiological symptoms. Typical signs include the formation of witches' brooms (prolific growth of side shoots), leaf chlorosis (yellowing), and microphylla (smaller-than-normal leaves). As the infection progresses, affected trees exhibit bark necrosis, twig dieback, and a general decline in canopy vigor, often leading to mortality after several seasons of persistent stress.

The development and spread of Alder yellows are highly dependent on environmental conditions that favor the insect vectors. Warm and humid springs provide ideal conditions for the colonization and movement of leafhoppers between trees. Once an area becomes infested, the transmission cycle is difficult to break, as the phytoplasma persists within the vector population throughout the growing season, allowing for efficient spread within forest stands.

Current management strategies focus on prevention and sanitation, as there are no chemical treatments available to cure systemic phytoplasma infections in forest settings. Key management practices include:

• Rigorous monitoring and removal of symptomatic trees (sanitation felling).
• Minimizing the movement of potentially infected nursery stock into healthy areas.
• Maintaining healthy stand diversity to reduce the impact of local pest outbreaks.
• Promoting research into resistant alder genotypes to aid long-term reforestation efforts.