New Plantings a Focus of Phytophthora Management

Plant pathologist Megan Dewdney put emphasis on new plantings during a mid-March discussion of phytophthora management for citrus trees infected with HLB. Dewdney is a University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) associate professor at the Citrus Research and Education Center, where she spoke.

Dewdney’s presentation was part of an OJ break hosted by UF/IFAS Extension multi-county citrus agent Chris Oswalt.

In a slide titled “Why the focus on new plantings?” Dewdney pointed out that citrus trees need strong root establishment prior to HLB infection. HLB reduces the efficiency of phytophthora management, and the cost effectiveness of management is uncertain once a tree is infected with HLB.

PREVENTION RECOMMENDATIONS
Dewdney advised growers to avoid rootstocks that are highly susceptible to phytophthora in sites with a history of the disease. She also recommended that the plant graft union be 6 to 9 inches above the soil. Other recommendations for phytophthora prevention at planting included:

• Adequate drainage and proper irrigation are essential.
• Keep the area around the trunk clear of weeds and avoid wounding.
• Remove trunk wraps early in spring and treat for fire ants, which feed on and damage bark.

DON’T ASSUME GROVES NEED TREATMENT
Dewdney advised growers not to assume their groves require treatment for phytophthora. She suggested they sample for the disease to find out how much inoculum is present. She said 10 to 20 propagules/cm³ soil of either the Phytophthora palmivora or Phytophthora nicotianae species is an actionable level.

According to Dewdney, P. palmivora is a major brown rot concern, and prefers poorly drained soils with high water tables, like those found in the Indian River region. P. nicotianae is a common cause of root rot statewide in Florida. Citrus yield reduction estimates for all phytophthora diseases are 3% to 6% per year, or approximately $20 million in crop loss, she added.  

FOCUS FUNGICIDES ON ROOT FLUSHES
Fungicides should be targeted at phytophthora during spring and fall root flushes, Dewdney suggested.

Learn much more about phytophthora management from the UF/IFAS 2022–2023 Florida Citrus Production Guide.

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