Red leaf spot

Red leaf spot is a fungal disease caused by the pathogen Exobasidium rostrupii. This fungus primarily acts as an obligate parasite, impacting various plant species, with significant economic implications for the cultivation of American cranberry (Vaccinium macrocarpon) and other related Ericaceae family members.

The disease is characterized by the appearance of distinct red or reddish-brown spots on the leaves. As the infection progresses, the affected leaf tissue may thicken or become distorted. Under humid conditions, a whitish, velvet-like layer of mycelium and spores often emerges on the underside of the leaf, indicating active sporulation.

The development and spread of Exobasidium rostrupii are heavily dependent on environmental conditions. High humidity, frequent rainfall, and stagnant air within plant canopies create an ideal environment for spore germination and infection. Dense plantings, which prevent the foliage from drying quickly, are particularly susceptible to rapid disease outbreaks.

The detrimental impact of red leaf spot includes severe leaf drop and diminished plant vigor. By hindering the photosynthetic efficiency of the plant, the infection negatively affects fruit bud formation and overall crop yield. In severe cases, repeated annual infections can lead to the decline of the entire plantation if not managed properly.

Management and prevention strategies focus on integrated pest management practices:

• Maintaining proper plant spacing to facilitate air circulation and canopy drying.
• Removing and destroying symptomatic plant tissue to reduce the primary inoculum load.
• Applying recommended fungicides preventatively during periods of high humidity.
• Improving site drainage to reduce microclimatic humidity within the growing area.