Black rot

Black rot, caused by the fungal pathogen Physalospora obtusa (syn. Botryosphaeria obtusa), is a destructive plant disease affecting various pome fruits, primarily apple and pear trees. This ascomycete fungus causes necrosis in bark tissue, leading to cankers, and triggers severe fruit decay in orchards and during post-harvest storage.

The host range is broad, but the economic impact is most significant in commercial apple production. The pathogen attacks all woody parts of the tree, potentially girdling limbs and trunks. Infected fruits initially develop small, circular purple spots that expand into characteristic brown or black rot zones, eventually becoming mummified.

The life cycle of Physalospora obtusa involves the survival of the fungus in pycnidia and pseudothecia found on dead bark and mummified fruits remaining on the tree or the ground. During rainy spring periods, spores are discharged and disseminated by splashing water, wind, and insects to fresh tissue, particularly where the bark is compromised.

Disease progression is highly dependent on environmental conditions, with warm temperatures and high humidity being the primary drivers of infection. Trees under abiotic stress, such as drought, winter injury, or poor pruning practices, are significantly more susceptible to colonization by this opportunistic fungus.

Integrated pest management (IPM) is essential for effective control. Key practices include rigorous sanitation, such as removing cankered branches and mummified fruit, and applying appropriate fungicidal spray programs. Additionally, maintaining tree vigor through balanced fertilization and proper irrigation helps to enhance the tree's natural resistance to the pathogen.