Background: This blackberry sample from a commercial nursery in Central Florida was showing signs of brown, angular spots with reddish-purple borders. The spots were limited to the lower portion of the plant.
Diagnosis: After culturing a sample of the plant tissue, we determined that the root cause of the illness was Cercospora leaf spot (Mycosphaerella confusa). This is a common disease of blackberry in the southeast. The spots begin as small and red/purple in color and, as the expand, the tissue inside the circle becomes necrotic and eventually falls out, causing holes throughout the leaf surface. In extreme circumstances, this can lead to leaf drop. The fungus overwinters in fallen plant material and can be harbored by some weed species. The issue typically first shows up in spring as new leaves bloom and secondary infections can occur throughout the warm season.
Treatment: There are chemical controls for Cercospora leaf spot, and the recommendations for this time of year from the 2021 Southeast Regional Caneberry Integrated Management Guide are included below.
There are good cultural controls as well that should be implemented to best control for this and other pathogens. First, be sure blackberry plants are properly spaced and pruned to ensure good airflow. It is quite possible that this is why this particular infection started in the lower portion of the plant where airflow is at its worst. Second, keep a clean field free of weeds. Overgrown weeds not only contribute to increased moisture near the lower parts of the crop but weed species may also be hosts to pathogens.
“Just Rolled into the Clinic” is an ongoing series of interesting issues from the commercial plant clinic at the UF/IFAS Mid-Florida Research and Education Center.