Silver leaf fungus

Stereum purpureum, commonly known as the silver leaf fungus, is a basidiomycete pathogen belonging to the Stereaceae family. It is a wood-rotting fungus that causes a significant disease known as silver leaf in a wide range of woody plants, including various fruit trees and ornamental species. The fungus acts as a wound parasite, typically infecting the heartwood of trees through open cuts or broken branches.

The host range of the fungus is extensive, primarily targeting Rosaceae family members like apples, plums, cherries, and pears, as well as deciduous trees such as poplars and willows. The disease is characterized by a distinctive metallic or silvery sheen on the leaves of the affected tree, caused by the separation of the leaf epidermis from the mesophyll due to toxic fungal metabolites circulating within the vascular system.

The fungal life cycle involves the production of basidiospores on the fruiting bodies (basidiocarps) found on dead wood or infected trunks. These fruiting bodies are tough, leathery, and display purple or mauve hues. Spores are released into the air, particularly during moist weather, and enter the tree through fresh wounds. Once inside, the fungus colonizes the wood, causing a characteristic dark staining and progressive structural decay.

Environmental conditions play a critical role in the spread of the pathogen, with high humidity and moderate temperatures favoring spore dispersal and infection. Wounds caused by pruning, winter frost, or wind damage serve as the primary entry points. If left untreated, the fungal mycelium continues to spread throughout the conductive tissues, weakening the tree's vascular system and eventually causing total tree dieback.

Effective control strategies focus on prevention and sanitation. Pruning should be performed during dry weather, and all cuts should be treated immediately with protective sealants or fungicides to prevent colonization. It is essential to promptly remove and destroy all infected branches or entire trees showing significant symptoms of the disease. Maintaining tree vigor through balanced fertilization and stress reduction is also crucial for long-term orchard health.