Avoid Phytophthora When Planting

Phytophthora infection can lead to severe stunting in newly planted citrus trees, researcher Megan Dewdney told growers and others at a Jan. 19 presentation at the Citrus Research and Education Center in Lake Alfred, Florida. She said stunted trees never thrive or produce adequate fruit. Dewdney is a University of Florida Institute of Food and Agricultural Sciences plant pathologist.

She offered numerous steps to prevent phytophthora infection at planting, starting with not planting trees exhibiting damage from the disease. Growers should avoid rootstocks that are highly susceptible to the disease in sites with phytophthora history, Dewdney added. Additional steps include planting the graft union 6 to 9 inches above the soil, providing adequate drainage and proper irrigation, and keeping the area around the trunk clear of weeds. Avoid wounding the trunk and remove trunk wraps early in spring. She also recommended treating for fire ants, which feed on and damage bark.

Cultural practices are the first line of defense against the disease, Dewdney emphasized. She said fungicides should only be used when all other measures have failed.

A slide in Dewdney’s presentation asked, “Why the focus on new plantings?” Answers included:

• Trees need strong root establishment prior to being infected with HLB.
• HLB reduces the efficacy of phytophthora management.
• The cost-effectiveness of management practices is uncertain once a tree is infected with HLB.

According to her presentation, Phytophthora palmivora is at high levels in Southwest Florida. There has been a doubling of blocks infested with P. palmivora in the past year. Dewdney suggested sampling to determine inoculum pressure before deciding to treat for the disease and applying fungicides targeting spring and fall root flushes.

The 2021-22 Florida Citrus Production Guide includes a section on phytophthora management. Dewdney advised growers to refer to the guide for treatment options.

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