Authored by:  Carlos Eduardo Aucique-Perez and Muhammad Adnan Shahid, Horticultural Sciences Department, UF/IFAS North Florida Research and Education Center – Quincy

Phytophthora species (spp.) are among the most destructive pathogens of citrus worldwide, causing significant yield losses across a wide range of production environments (Zhong & Nicolosi, 2022; Graham & Feichtenberger, 2015). These pathogens are highly adaptable and can persist in irrigated arid regions as well as high-rainfall systems. In citrus, Phytophthora spp. infect plants at all growth stages and nearly all tissues, including roots, stems, leaves, and fruit. Major diseases caused by these pathogens include root rot, foot rot (gummosis or collar rot), fruit brown rot, twig and leaf dieback (canopy blight), and damping-off in nurseries (Graham & Feichtenberger, 2015).

In Florida citrus production, Phytophthora nicotianae and P. palmivora are the species associated with foot rot, crown rot, and root rot, each characterized by distinct infection sites and symptom development (Dewdney, 2025). Foot rot typically develops at the base of trunk, crown rot occurs below the soil surface on susceptible rootstocks, and root rot affects fibrous roots, causing cortical decay and sloughing of tissues. These diseases have been widely reported since the 1980s and are strongly associated with the movement of infected nursery stock. Therefore, early detection in nurseries and orchards is essential to prevent establishment and spread, and integrated management practices are necessary to reduce tree decline and yield losses (Graham & Feichtenberger, 2015; Dewdney, 2025).

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Symptom Development in the Field

Phytophthora spp. infections develop progressively and are expressed as both above- and below-ground symptoms. Early above-ground symptoms include reduced vigor, chlorosis, and canopy thinning. As root systems decline, trees exhibit twig dieback, smaller leaves, premature defoliation, and reduced fruit size and yield. In advanced stages, severe canopy decline and tree death may occur (Figure 1). Under favorable environmental conditions, infection may also extend to fruit, producing brown rot lesions characterized by firm, brown decayed areas (Erwin & Ribeiro, 1996; Graham & Feichtenberger, 2015).

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Below ground, infected trees show a reduction in fine feeder roots, which become darkened, water-soaked, and decayed, often with a foul odor. As disease progresses, structural roots and the crown may be affected, resulting in bark lesions, cracking, and gummosis at the base of the trunk (Figure 2) (Timmer et al., 2000; Graham & Feichtenberger, 2015).

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Epidemiological Cycle

The pathogen survives in soil as resistant structures such as oospores and chlamydospores, enabling long-term persistence under unfavorable conditions (Erwin & Ribeiro, 1996). When environmental conditions become favorable, these structures germinate and produce sporangia that releases motile zoospores. Zoospores require free water for movement and actively locate and infect citrus roots. Infection is favored by saturated soils, moderate temperatures (~59-82° F), and host stress or root damage. Saline conditions increase susceptibility by stressing roots, whereas calcium-rich soils may suppress disease development. Seasonal activity differs among species: P. nicotianae is more active in warm conditions, while P. citrophthora persists during cooler periods but is reduced during hot summer conditions unless irrigation maintains moisture (Cacciola & Lio, 2008).

After infection, the pathogen colonizes root cortical tissues and progresses into the crown, reducing water and nutrient uptake and leading to above-ground symptoms such as chlorosis (loss of green coloration) and decline. Sporangia produced in infected tissues are dispersed through irrigation water, runoff, and rain splash, facilitating secondary spread within orchards. Infected nursery stock remains a major source of pathogen introduction into new areas. Recently, the Phytophthora–Diaprepes complex, involving root injury caused by Diaprepes abbreviatus larvae, further increases disease severity, particularly in poorly drained soils (Dewdney, 2025).

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Detection and Monitoring

Early detection of Phytophthora spp. is essential for effective management and relies on field symptom recognition followed by laboratory confirmation (Cacciola & Lio, 2008; Dewdney, 2025). Field diagnosis is based on characteristic symptoms of root rot, crown rot, and foot rot. In field and nursery systems, baiting techniques using citrus leaves or fruit are commonly used to detect pathogen presence in soil or water samples. Positive samples are confirmed using selective media such as PARP or dilution plating for isolation and identification. These methods provide basic detection but may not detect low inoculum levels. More sensitive approaches include immunological methods such as ELISA and molecular diagnostics such as PCR and qPCR, which allow species-specific detection and quantification. However, these techniques require specialized laboratory infrastructure (Cacciola & Lio, 2008).

Soil inoculum monitoring is an important decision-support tool. Samples are collected from March to November under the canopy in irrigated zones. For each 10-acre block, 20–40 subsamples are taken from the top 10 inches of soil and combined per microsprinkler area. Samples should be kept cool during transport (not refrigerated). Populations above 10–20 propagules/cm³ of soil are considered damaging in established orchards, while lower thresholds (3–5 propagules/cm³) are critical for nursery stock and new plantings (Cacciola & Lio, 2008; Dewdney, 2025). These samples can also be used for nematode analysis to assess combined soil pest pressure.

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Phytophthora diseases management

Management of Phytophthora diseases in citrus relies on an integrated strategy combining cultural, genetic, chemical, and monitoring approaches. Key practices include the use of certified disease-free nursery stock and resistant rootstocks, along with improved soil drainage and careful irrigation management to reduce soil saturation and conditions favorable for zoospore movement and root infection. Maintaining tree vigor through balanced fertilization further reduces host susceptibility. Fungicide applications in citrus groves should be determined by rootstock susceptibility, nursery infestation risk, and grove history of Phytophthora diseases. Preventive treatments are recommended for young trees on susceptible rootstocks, such as ‘Cleopatra’ mandarin and ‘Sweet orange’, while tolerant rootstocks should be treated only after foot rot symptoms appear. In mature groves, root rot management should be based on annual monitoring of Phytophthora spp. populations, with applications timed to spring and fall root flushes. Fosetyl-Al (Aliette), phosphite salts, mefenoxam (Ridomil Gold), and fluopicolide (Presidio) are effective fungicides, and their rotation is recommended to reduce fungicide resistance. Regular monitoring through field scouting and laboratory diagnostics, including baiting and molecular methods (PCR/qPCR), supports early detection and timely intervention. Overall, successful management depends on reducing inoculum sources, limiting conducive environmental conditions, and maintaining host resilience (Graham & Feichtenberger, 2015; Erwin & Ribeiro, 1996; Cacciola & Lio, 2008; Dewdney, 2025).

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References

Cacciola, S. O., & Lio, G. d. (2008). Management of Citrus diseases caused by Phytophthora spp. In A. Ciancio & K. G. Mukerji (Eds.), Integrated management of diseases caused by fungi, phytoplasma and bacteria (Vol. 3, pp. 115–146). Springer.

Dewdney, M. M. (2025). 2025–2026 Florida Citrus Production Guide: Phytophthora foot rot, crown rot, and root rot (CPG Ch. 31, CG009/PP-156, Rev. 8). University of Florida Institute of Food and Agricultural Sciences. 

Erwin, D. C., & Ribeiro, O. K. (1996). Phytophthora diseases worldwide. APS Press.

Graham, J. H., & Feichtenberger, E. (2015). Citrus Phytophthora diseases: Management challenges and successes. Journal of Citrus Pathology. iocv_journalCitruspathology_27203

Timmer, L. W., Garnsey, S. M., & Graham, J. H. (2000). Compendium of Citrus diseases (2nd ed.). APS Press.

Zhong, G., & Nicolosi, E. (2020). Citrus origin, diffusion, and economic importance. In A. Gentile, S. La Malfa, & Z. Deng (Eds.), The Citrus genome (pp. 1–18). Springer.

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