Bonfire is a variety of tuberous begonia grown for its masses of orange flowers and long blooming period. When grown in U.S. Department of Agriculture plant hardiness zones 10 and above, it’s maintenance-free. High humidity and overly moist conditions, however, can lead to rot.
The first place you’ll notice symptoms of rot is in the stems–they’ll change color and may collapse. From there, the disease will move to the roots and tubers.
The fungus that causes rot resides in the soil. Its spores are commonly spread when water from the soil is splashed onto the plant.
The ideal method of control is a fungicide soil drench. Integrated pest management specialists with the University of California, Davis, suggest mefenoxam, applied according to the manufacturer’s instructions.