Leucostoma canker

Leucostoma persoonii (teleomorph Valsa leucostoma) is a significant fungal pathogen classified within the Ascomycota division. It is the primary causal agent of Leucostoma canker, also known as Cytospora canker, which affects a wide range of fruit trees, particularly those in the Prunus genus.

The disease primarily impacts stone fruit species such as peaches, apricots, plums, and cherries. Symptoms manifest as sunken areas on the bark, distinct cankers that often bleed sap (gummosis), and the eventual necrosis of branches. In advanced stages, the fungus produces small, pimple-like fruiting bodies (pycnidia) that erupt through the bark surface.

The biology of Leucostoma persoonii is opportunistic. The fungus typically gains entry through wound sites caused by winter injury, sunburn, hail damage, or poor pruning practices. Once established, the mycelium invades the cortical tissues and the cambium, effectively girdling the branch and cutting off the supply of water and nutrients, which results in dieback.

Development and spread are highly favored by cool, moist conditions. During wet weather, spores (conidia) are exuded from pycnidia and are disseminated by splashing rain, irrigation, or wind to other vulnerable tissues. The pathogen can survive as a saprophyte on dead wood, making orchards with significant amounts of debris a constant source of inoculum for healthy trees.

Managing this pathogen requires a preventative approach, as chemical treatments after deep colonization are rarely curative. Effective control strategies include:

• Prompt removal and destruction of infected limbs to reduce inoculum pressure;
• Proper orchard sanitation and sterilization of pruning tools with disinfectants;
• Maintaining tree vigor through balanced fertilization and irrigation to improve natural defense;
• Protecting trunks with white latex paint to minimize temperature fluctuations and sunscald.