Mangrove CNP: A threat to Florida’s mangroves? 

Background: Mangrove trees are essential to our dynamic coastal and estuarine environments. Found throughout tropical and subtropical regions around the world, mangroves are trees which exhibit monumental resilience. They exist in sodden salty soils with fluctuating water levels, in the presence of fierce tropical storm winds and waves, flooded by freshwater during monsoonal rains, and squeezed by human development between these natural elements and the built environment. They sequester carbon, filter stormwater, control erosion, minimize wind and wave damage during storms, provide essential habitat for economically important fish species and protect water quality for near shore reefs and seagrass beds.

In Florida, we have three species of mangrove that are protected by state law.

Red Mangrove (Rhizophora mangle)

These are the mangroves people often envision when they think of mangroves, they have prop roots and long, dangling pencil-like propagules. They are generally found in standing water in Zones 10 and 11. These mangroves have a lower soil salt tolerance than other mangroves and therefore tend to inhabit lower portions of regularly flooded intertidal zones. Red mangroves are occasionally called the ‘walking trees’ due to their horizontal growth habit. Red mangroves reach between 20 and 75 feet tall. The national champion red mangrove is in Lee County Florida, documented at 64 feet tall.

Black Mangrove (Avicennia germinans)

These trees grow in Zones 8b to 11b and reach heights of 40 to 60 feet tall. This evergreen tree generally grows further inland than reds, where the roots are still inundated during high tide. Black mangroves have a higher salt tolerance than the other two species. Black mangrove trees have glands on their leaf surfaces that excrete excess salt, indigenous people gathered the leaves for this salt. Historically, the wood was an important fuel source for smoking fish. The bark was also used in the tanning process as a black dye for animal skins. Additionally, the flowers are a popular nectar source for honeybees, leading to the creation of ‘mangrove honey’.

White Mangrove (Laguncularia racemosa)

More of an upland species of mangrove, white mangroves are generally found further inland along the coast where it is irregularly flooded in the intertidal zone. These trees grow in Zones 9a to 11b and reach 30 to 40 feet tall. White mangrove trees produce hard, strong wood that has historically been used for lumber. Most commonly these trees were used as a fuel source and for tanning leather.

For more information on Florida’s mangroves, see my previous blog post

Current Status of Mangroves in Florida: Estimates for mangrove forest extent in Florida range from 100,000 acres to over 600,000 acres. In a study from 2015, conducted by the USDA Southern Research Station, Forest Inventory and Analysis methods were used to develop an estimate of 238,000 with 80% of those on publicly managed lands. (Brown, 2015) That leaves 47,600 acres of mangroves maintained on private property throughout Florida. While mangroves are protected from unregulated removal or destruction, trimming is allowed per the 1996 Mangrove Trimming and Preservation Act, as amended. This trimming can result in trees with mature height potential of over 35 ft. being managed at an overall height of 6 ft. Many of these mangroves exist along seawalls, in canals or surrounding docks. In Sarasota County, Florida, an estimated 90% of all privately maintained mangroves are maintained in a 6 ft. hedge configuration. (Sarasota County, 2019)  

While anecdotal evidence exists for the impact of trimming on mangrove health, few studies exist which empirically describe these impacts. From the limited research and application of arboricultural knowledge, trimming impacts may include reduced fecundity, carbon starvation, weak secondary shoot development and higher overall stress. Taken from these impacts it is assumed that indirect ecosystem services reduction will occur as a result of trimming, including marine habitat reduction, carbon sequestration reduction and reduced storm buffer capacity. (Medina-Irizarry and Andreu, 2022)  

What is Mangrove CNP: In early 2024, the Marine Resources Council, a non-profit organization in southeast Florida, noticed extensive die off in their mangrove nurseries. Upon investigation by University of Central Florida, Florida Institute of Technology and Fairchild Botanical Gardens, the culprit was identified as Mangrove CNP. Mangrove CNP is a suite of three fungal pathogens, Curvularia, Neopestalotiopsis and Pestalotiopsis which in healthy mangroves exist without causing disease. These fungi are known to be pathogenic and benign in mangroves and other crops throughout the world. 

The symptoms of Mangrove CNP include leaf yellowing, curling, thickening, and sudden leaf drop as well as stem lesions and ultimately death. Mangroves are more susceptible to the disease when they are young or under severe stress. Mangrove CNP has been identified as being implicated in mangrove decline in Vietnam and other parts of Southeast Asia with the first report in Florida in Miami in 2019. Similar to other forest pathogens, the fungus is likely a contributing factor, leading only to mangrove death after predisposing and inciting factors such as a harsh urban environment coupled with improper and extreme trimming. (Manion, 1991) A collaborative effort at identification and treatment in Florida has been initiated by the Marine Resources Council and University of Central Florida, which includes the use of antimicrobial chemical application to the affected mangroves. 

Recommendations for Mangrove Management: You can protect your mangroves by ensuring that they are managed appropriately to encourage health. Follow all applicable rules and regulations, while limiting any trimming during times of drought or immediately before tropical storms. Pay attention to pruning dose and timing, trimming in fall and winter months can be more stressful. Never remove a significant amount of foliage from mangroves that are exhibiting other stress symptoms such as slow/stunted growth, leaf yellowing or general lack of vigor. If you are planting new mangroves along your coastlines, be sure to inspect the young plants from the nursery closely for any of the indicative symptoms of CNP such as leaf yellowing, thickening, spots, or necrotic tissue on leaves and stems.  

If you are a professional mangrove trimmer looking for recommendations on best management practices, reach out to: alyvinson@ufl.edu (Hillsborough Co.) or aubeda@ufl.edu (Sarasota Co.)

See this website from the collaborative team researching treatment and monitoring the spread of the disease for more information on Mangrove CNP in Florida.

References: 

1. Gilman, E., et. al. (2023). Rhizophora mangle: Red Mangrove1. EDIS. https://edis.ifas.ufl.edu/pdffiles/FP/FP50200.pdf   
2. Andreu, M., et. al. (2022). Avicennia Germinans, Black Mangrove. EDIS.  http://edis.ifas.ufl.edu/pdffiles/FR/FR32100.pdf 
3. Vinson, A. (2019). Mysterious Mangroves. UF/IFAS Blogs. https://blogs.ifas.ufl.edu/manateeco/2019/06/14/mysterious-mangroves/  
4. Brown, M.J. (2015). Estimating Mangrove in Florida: Trials Monitoring Rare Ecosystems. USDA Forest Service. https://www.fs.usda.gov/pnw/pubs/pnw_gtr931/pnw_gtr931_111.pdf  
5. Medina-Irizarry, N., Andreu, M. (2022). The Impacts of Trimming Mangroves. EDIS. https://edis.ifas.ufl.edu/publication/FR448  
6. Report Mangrove CNP. (n.d.). Mangroves.report. Retrieved June 14, 2024, from https://mangroves.report/ 
7. Chesnes, M. (2024, May 12). This disease is the latest threat to Florida’s vulnerable mangrove trees. Tampa Bay Times. https://www.tampabay.com/news/environment/2024/05/13/florida-mangrove-trees-disease-plants-forests/ 
8. Nguyen, Huong & Hardy, Giles & Le, Tuat & Nguyen, Huy & Le, Duc & Nguyen, Thinh & Dell, B.. (2021). Mangrove Dieback and Leaf Disease in Sonneratia apetala and Sonneratia caseolaris in Vietnam. Forests. 12. 1273. 10.3390/f12091273. 
9. Nguyen HTT, Hardy GESJ, Le TV, Nguyen HQ, Le DH, Nguyen TV, Dell B. Mangrove Dieback and Leaf Disease in Sonneratia apetala and Sonneratia caseolaris in Vietnam. Forests. 2021; 12(9):1273. https://doi.org/10.3390/f12091273 
10. Manion, P.D. (1991). Tree disease concepts, 2nd edn. Prentice-Hall, New Jersey 

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