Rowan rust

The causative agent of the disease is the rust fungus Ochropsora ariae. It is a heteroecious fungus, meaning it requires two unrelated hosts to complete its life cycle. The primary host is the rowan tree (Sorbus aucuparia), while the alternate host is typically the anemone species, which serves as a reservoir for the pathogen.

The disease primarily affects the leaves of various Sorbus species. The initial symptoms appear as bright orange or yellow spots on the underside of the leaves. These spots develop into small, raised pustules containing spores. As the infection progresses, the leaves often turn yellow, curl, and drop prematurely, which is detrimental to the tree's overall health.

High humidity and moderate temperatures during the spring and early summer provide ideal conditions for the development and spread of Ochropsora ariae. Spores are disseminated primarily by wind and rain splashing. The fungus can overwinter in plant tissues or as dormant spores in leaf litter, allowing the cycle to repeat annually.

The economic and aesthetic impact of rowan rust is significant due to early defoliation. This premature loss of foliage reduces the photosynthetic capacity of the tree, weakening it over time. Affected trees become less vigorous, exhibit poor winter hardiness, and may show stunted growth, making them more susceptible to environmental stress and secondary pests.

Integrated pest management strategies are recommended to mitigate the impact of this disease. Cultural practices such as the thorough removal and destruction of fallen leaves in autumn are essential to reduce the primary inoculum load. If an outbreak occurs, chemical control using fungicides containing copper or systemic triazole compounds can be effective when applied during the early stages of the growing season.