By Andre B. Gama and Megan M. Dewdney
Postbloom fruit drop (PFD) of citrus is a disease caused mostly by the fungus Colletotrichum acutatum. This fungus survives by producing resting structures on leaves and stems. When flowers start to appear in groves, these resting structures produce spores that can cause PFD.
However, the fungus requires specific weather conditions to be able to infect citrus flowers. Moderately warm temperatures, around 78 F, and long leaf wetness periods of more than 16 hours (usually associated with rainfall) are perfect for the fungus. The resting structures make a small number of spores when flowers appear and start the early-flower infections.
Once symptoms develop on infected flowers, the fungus makes tens of thousands of spores in each lesion, increasing the chances for severe PFD outbreaks. When this happens on the successive waves of early flowers on weak 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 if there is inoculum in your groves will allow preparation if the weather is suitable for an outbreak while trees are flowering. If you spot symptoms, the Citrus Advisory System will identify periods of increased risk in your area when the main bloom comes.
Major PFD outbreaks have not occurred in Florida since 2016, so typical disease symptoms might not be fresh in everyone’s memory. Growers can identify PFD by looking for peach to pinkish-brown lesions on flower petals at the popcorn or opened stages (Figure 1). Whole flower clusters show symptoms when conditions are perfect for PFD. Popcorn and opened flowers are the most susceptible to the disease, so concentrate scouting for symptoms on these stages.
If temperatures are optimal, flowers and symptoms may develop too quickly to be seen when scouting. That is why it is also crucial to know how to identify PFD symptoms on fruitlets and calyces later in the season. Fruitlets that develop from PFD-affected flowers may turn chlorotic and fall off trees. Fallen fruitlets leave behind persistent calyces, also known as PFD buttons (Figure 2).
New buttons have been observed in the field in 2021, likely the result of favorable conditions in late 2020. This means inoculum is building in groves. PFD buttons can be distinguished from those caused by natural fruit fall by trying to remove them from trees. While it is easy to remove calyces that originated from natural fruit fall, PFD buttons are not easy to take off trees. Identifying PFD on early blooms is crucial for successful PFD management programs when the main bloom comes.
Andre B. Gama is a University of Florida graduate student pursuing his doctorate degree in plant pathology, and Megan M. Dewdney is an associate professor at the Citrus Research and Education Center in Lake Alfred.