Cranberry Sclerotinia

Systematic position and pathogen type. Sclerotinia oxycocci is a fungal pathogen belonging to the Kingdom Fungi, Phylum Ascomycota. It is a highly specialized parasite that exclusively targets Vaccinium species, with a primary focus on cranberries. It is the causative agent of the economically damaging "cottonball" disease, which affects the fruit set and causes significant yield losses in commercial cranberry production.

Diseases and host plants. The primary disease caused by this pathogen is known as cottonball or hard rot. The infection specifically targets cranberry plants (Vaccinium macrocarpon). The disease manifests during the early stages of fruit development, transforming healthy berries into infected, mummified structures filled with white fungal mycelium, which is the hallmark of this pathogen.

Biology and life cycle. The life cycle is synchronized with the phenology of the cranberry plant. In early spring, sclerotia (hardened fungal masses) that have overwintered in the soil produce apothecia. These structures release ascospores, which are dispersed by wind to the cranberry flowers. The spores germinate on the flower parts, colonizing the developing ovary and replacing internal berry tissues with fungal growth.

Conditions for development and harmfulness. High humidity and cool, wet weather during the blooming period are critical for the spread of Sclerotinia oxycocci. These conditions favor the release and germination of ascospores. The economic impact is severe: infected berries remain light and rigid, making them unsuitable for fresh market or processing, and in severe cases, the disease can devastate a large percentage of the harvest.

Protection and control measures. Management of this disease requires an integrated approach to minimize the primary inoculum and protect the crop during the most vulnerable stages. Control strategies focus on breaking the infection cycle. Recommended actions include:

• Applying a layer of sand to cover the soil, effectively burying the sclerotia and preventing apothecia formation.
• Implementing precise fungicide application schedules timed with the bloom period.
• Improving field drainage to reduce excessive moisture in the canopy.
• Sanitation practices, such as removing debris from the field to reduce overwintering sites for the fungus.