This blog is for Polk County Florida small farms and pesticide applicators as a method of communication and sharing information. This blog is administered by the University of Florida IFAS Extension Polk County Extension Agent, Mary Beth Henry.
Postbloom Fruit Drop (PFD) has been a topic of interest.
What causes PFD? It is a fungal disease Colletotrichum acutatum, that infects citrus flowers during bloom.
What are the symptoms? Petals of citrus flowers develop water-soaked, peach to orange/brown colored spots. Fruitlet drop follows, with the persistent calyx or "buttons" left behind where the fruitlet had been. The causal fungus can be found in the petals, leaves, twigs, and the buttons, serving as a source of infection for other flowers in current and later years. Extended, multiple, and off-season blooms allow inoculum to spread.
What conditions favor the disease? Wet leaves and flowers for extended periods of time favor the development of the disease. Wet El Niño years have been the worst for this disease in Florida. (We are in a predicted Neutral phase for the next three months). Infection occurs within 24 hours of a weather event and symptoms will be seen four to five days later. The disease is spread from splashing and wind-blown rain as well as on humans and equipment in the field.
Which cultivars are affected? Cultivars differ in susceptibility/severity, but all citrus species and cultivars can be affected by PFD. The most commonly affected have been Valencia and navel oranges. Hamlin oranges, grapefruit, and tangerines have been less susceptible.
Control options? The main control products are strobilurin fungicides Abound, Gem, and Headline, and ferbam. There are label limits on these chemicals. Be sure to follow all label instructions. To have a longer residual effect, combine one of the strobilurins with ferbam. Factsheet for full recommendation details. Newer fungicides that have not been fully evaluated may be available. Application timing is key. A model is available online to assist you with this decision. Once you have infected buttons or many symptomatic flowers it is too late for effective control. Scouting your most susceptible blocks several times a week during bloom will help you to stay on top of the problem. Off season bloom should be inspected for symptoms all year. The amount of rainfall in the past five days, the hours of leaf wetness, and how many rainfall events occurred in the last five days are used in the model as well as the history of the disease in the block and the last fungicide application.