By Megan M. Dewdney
Postbloom fruit drop (PFD) is a flower disease mainly caused by the fungus Colletotrichum acutatum. The fungus is present on citrus trees throughout the year but survives by producing resting structures on leaves and stems. The fungus becomes stimulated to produce spores by substances from the early flowers. These early spores infect the initial flowers and form many more spores that cause greater infection.
However, the fungus requires specific weather conditions to be able to infect citrus flowers. Moderately warm temperatures, in the range of 72 to 79 degrees, and long leaf wetness periods of more than 16 hours (usually associated with rainfall) are perfect for the fungus. As the temperatures get cooler or warmer, the wetting period needs to be longer for a successful infection event.
When infection occurs on successive waves of early flowers on weakened trees before the major bloom period, the number of spores greatly increases the possibility of a PFD outbreak if the weather is right during the major bloom. That is why it is important to scout the early bloom for PFD. Knowing whether you have active inoculum in your groves will allow preparation if the weather is suitable for an outbreak while trees are flowering. If symptoms are present, the Citrus Advisory System will identify periods of increased PFD risk in your area when the main bloom comes. If symptoms are not present, you can decide whether PFD is a worry for you based on your recent history with the disease.
Outbreaks of PFD have been limited since 2016, so typical PFD symptoms might be fading into the past. Popcorn and opened flowers are the most susceptible to the disease, so concentrate scouting for symptoms on these stages. PFD lesions are peach to pinkish brown on flower petals (Figure 1). Whole flower clusters show symptoms when conditions are perfect for PFD.
If temperatures are optimal, flowers and symptoms may develop so quickly they are missed when scouting. That is why it is also crucial to know how to identify PFD symptoms on fruitlets and calyces once the disease has passed. Fruitlets that develop from PFD-affected flowers turn chlorotic and fall off trees. Fallen fruitlets leave behind persistent calyces, also known as PFD buttons (Figure 2).
New buttons were observed in the field in 2022 before the cold weather. This is likely the result of favorable conditions in late 2021, meaning inoculum is building in groves. PFD buttons can be differentiated from calyces caused by natural fruit fall when removing them from trees. While calyces that originated from natural fruit fall are easily removed, PFD buttons are not easily taken off trees. Identifying PFD on early blooms is crucial for successful PFD management programs when the main bloom comes.
Megan M. Dewdney is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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