University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) multi-county citrus Extension agent Ajia Paolillo discussed brown rot management in the September Extension newsletter, Citrus from the Ridge to the Valley. Her article follows.
As we move into September, it is essential to remember that brown rot can impact yield, especially early varieties. Brown rot is caused by either Phytophthora nicotianae or Phytophthora palmivora. These species are in groves across the state, with P. nicotianae being more prevalent on the Ridge and P. palmivora in higher populations on Flatwoods soils. These are soil-borne pathogens that can be moved into the canopy by rain splash. Once the pathogen comes in contact with the fruit, it has the potential to cause brown rot if the fruit is not protected. The two species move differently in the canopy, with P. nicotianae typically affecting fruit on the lower part of the canopy. P. palmivora can essentially move up through the canopy by rain splash from infected fruit to healthy fruit.
Fruit drop can also contribute to the sources of inoculum in the grove. Once fruit hits the ground, it can become infected with P. palmivora. This rotting fruit gets splashed with rain, which can reach the lower hanging fruit in the canopy. Unfortunately, fruit drop late in the season due to HLB is difficult to avoid and can contribute to this problem. However, low-hanging fruit may be knocked off the tree during the growing season by equipment such as herbicide booms. Cultural practices such as skirting can help by raising the height of the lower-level branches to allow for equipment and to reduce fruit drop. This spatial difference can also lessen the inoculum in the tree coming from the soil or infected fallen fruit because the lower hanging fruit is farther from the ground.
Brown rot tends to be more of an issue with early varieties but can still occur with mid-season and late varieties. Brown rot symptoms can occur between mid-August through October. Inoculation of healthy developing fruit can occur as early as July and is not easy to detect until fruit starts to color.
If you have had brown rot in the past, chances are you have already applied a preventative application of a fungicide in late July. The products listed in the 2021-22 Florida Citrus Production Guide are Aliette, Phostrol and ProPhyt, which are systemic and offer fruit protection for 60 to 90 days. Copper fungicides applied in August can offer 45 to 60 days of protection.
As we move into the late summer and early fall, the amount of rainfall we have can impact the incidence of brown rot you may experience in your grove. If there is more rain than usual, there is a greater chance of brown rot occurring. Monitor your grove to determine if additional chemical controls may be needed in October due to rainfall and increasing incidences of brown rot. You may choose to apply the systemic fungicides mentioned earlier at half the label rate or copper fungicides. Always read the label of the product you choose to account for preharvest intervals during this time.
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