Alder rust

The causative agent of alder rust is the specialized fungus Melampsoridium hiratsukanum, which belongs to the class of Basidiomycetes. It is a highly specialized parasite that primarily affects various species of alder, causing premature wilting and leaf shedding. The disease follows a distinct seasonal developmental cycle, typical of rust fungi, characterized by the formation of urediniospores and teliospores on the leaves of the host plant.

The main crops at risk are grey alder (Alnus incana) and black alder (Alnus glutinosa). Under conditions of high humidity and favorable temperatures, the pathogen can cover vast areas of forest stands, park zones, and riparian strips where these tree species grow. Young seedlings and trees in dense stands are most severely affected by the infection.

Symptoms appear in mid-to-late summer as small yellow-orange or rust-brown pustules located on the underside of the leaf blade. As the disease progresses, the leaf loses chlorophyll, turns yellow, and becomes spotted, leading to premature leaf fall. In severe cases, trees can lose up to 80-90% of their canopy before autumn sets in.

High air humidity, frequent fogs, and moderately warm weather facilitate the development of the infection. Fungal spores are easily carried by the wind over long distances, ensuring rapid epiphytotic spread within forest ecosystems. The pathogen overwinters as teliospores on fallen infected leaves, which serve as the primary source of inoculum at the beginning of the next growing season.

The harmfulness of alder rust lies in the significant weakening of the physiological state of the trees. Due to premature leaf loss, photosynthetic processes are disrupted, and the tree depletes its nutrient reserves, which lowers its winter hardiness and resistance to other pests. Repeated infections over several years can lead to dieback and the gradual death of weakened specimens.

• Collection and destruction of fallen leaves in autumn to reduce the infection reservoir.
• Thinning of dense stands to improve crown aeration.
• Use of resistant alder species and cultivars during reforestation.
• Application of fungicides in nurseries and ornamental stands at the first signs of disease.