The causal agent is the fungus Gymnosporangium sabinae (Dicks.) G. Winter. The development cycle of the pathogen requires an alternation of hosts: various species of juniper serve as the primary host, where the fungus overwinters, while the common pear serves as the intermediate host, on which the aecial stage develops.
Initial symptoms on pear leaves are detected in late April to early May as small yellow-green spots, which eventually grow up to 1.5 cm, acquire an orange or reddish-brown color, and become covered with black dots of spermogonia. On the underside of the leaf, aecia develop in the form of cone-shaped protrusions.
The infection affects not only the leaves but also petioles, young shoots, and fruits. Infected shoots thicken and show stunted growth, and fruits become deformed. Mass development of the disease leads to impaired photosynthesis, reduced winter hardiness, and significant crop losses.
Currently, specific physiological races of the pathogen have not been identified, and data on the identification of specific resistance genes in Pyrus communis L. cultivars are absent. All studied cultivars and species of the genus Pyrus are characterized by varying degrees of susceptibility, with resistance assessed primarily through field performance.
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