With the recent expansion of blueberry production in Alabama, we are observing the emergence of new economically important blueberry diseases. The bacterial leaf scorch of blueberry caused by the bacterium Xyllela fastidiosa was documented for the first time in the state in 2011. During the next growing season, a new fungal disease, Exobasidium leaf spot has emerged in the southern regions of the state.
This disease affects cultivars of rabbiteye blueberry as well as southern highbush blueberry. Symptoms appear as small, pale yellow spots on the upper leaf surface of young expanding leaves. On the lower leaf surface, the spots are initially light green to white. Later the spots lighten and become white. Spots gradually become brown and necrotic with a reddish-brown margin (Figure 1). Blueberry bushes with high disease severity can start losing leaves and defoliate. Typical Exobasidium spots on the fruit can be easily recognized on ripe fruit and it is also suggested that severe infection may cause premature fruit drop.
Exobasidium spots can also form on young, actively growing shoots early in the season. This leads to progressive tissue necrosis and often results in girdling of the infected shoot. Later shoots become brown to black and necrotic and form a canker.
Studies have established that infection of new growth has been closely associated with extended periods of rain and/or cloudy weather, conditions typical for the current season in Alabama. Evidently, only young leaves, prior to hardening off, and young, green fruit are susceptible to Exobasidium infection. It is also assumed the stage of susceptibility for shoot infection is specific to young, unlignified shoots.
Removing trees on field perimeters and increasing drainage to reduce standing water in problematic locations can reduce the disease. Practices that increase air flow and help speed up the drying of stems, leaves and fruit can result in less infection. Thus, summer pruning and dormant blueberry pruning are essential. Avoid using an overhead irrigation to reduce the risk of splash infection.
Please refer to the Southeast Regional Blueberry Integrated Management Guide for most current disease management options:
Russell Ingram, Jonathan Oliver, Phillip Brannen and Renee Allen. Exobasidium leaf and Fruit Spot of Blueberry. University of Georgia Extension: https://secure.caes.uga.edu/extension/publications/files/pdf/C%201142_1.PDF