In Florida, the soilborne phytophthora pathogens — primarily Phytophthora nicotianae and Phytophthora palmivora — are the leading causes of root rot, crown rot and foot rot. Two University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers recently wrote about phytophthora management in cold-hardy citrus. The authors are Assistant Professor and Extension Specialist Muhammad Shahid and postdoc Carlos Aucique-Perez, both with the North Florida Research and Education Center.
Slightly edited excerpts follow:
THE THREE FACES OF ROT
While often grouped together, each form of phytophthora presents a distinct pattern.
Foot rot typically develops when the scion is infected near the soil line, resulting in bark lesions that can progress downward toward the bud union.
Crown rot occurs below the soil surface, leading to basal bark decay, particularly in susceptible rootstocks.
Root rot targets the fibrous feeder roots, causing the tissue to become soft and water-soaked. This decayed tissue often sloughs off, exposing the white, thread-like central stele of the root.
SPOTTING THE SYMPTOMS
Early field identification is critical to prevent tree loss. Above-ground symptoms often begin with reduced tree vigor, leaf chlorosis (yellowing) and canopy thinning.
As root function continues to decline, growers may observe twig dieback, smaller leaves and premature leaf drop. In advanced stages, the canopy will show severe decline, complete defoliation and reduced fruit yield. Below ground, infected trees show a marked reduction in fine feeder roots.
Growers suspecting infection should look for darkened, decayed root tissues that may emit a foul odor. Internal examination of the trunk base
may reveal necrosis (darkened areas) in the stem-root transition zone. Another tell-tale sign is gummosis, where sap oozes from bark lesions and cracks at the base of the trunk.
SAMPLING AND THRESHOLDS
To make informed management decisions, growers should monitor soil inoculum levels. Soil samples containing fibrous roots should be collected from March to November. For every 10-acre block, combine 20 to 40 subsamples taken from the top 10 inches of soil within the irrigated zones.
Generally, population levels above 10 to 20 propagules per cm³ of soil are considered damaging for established trees. The threshold for new transplants is lower, at three to five propagules per cm³.
MANAGEMENT STRATEGIES
Effective management relies on a combination of cultural and chemical controls:
- Water management is the most critical preventive step. Ensure proper soil drainage and ensure microsprinklers do not directly wet the tree trunks.
- Always use rootstocks with known tolerance or resistance to phytophthora.
- Maintain a soil pH between 5.5 and 6.5 and consider organic amendments like biochar to improve soil structure.
- Manage root-damaging pests like the diaprepes root weevil since their damage facilitates infection.
- If thresholds are exceeded, fungicides such as mefenoxam, phosphites or newer oomycota fungicides like oxathiapiprolin may be required.
- For the latest chemical application schedules and action thresholds, growers should consult the current Florida Citrus Production Guide.
Source: UF/IFAS
Share this Post