How you can help Guam's dying gagu

Caused by bacterial wilt and wetwood bacteria, droplets of ooze often form in declining ironwood trees.

Editor’s note: This is the first in a two-part series about plant diseases on Guam and Micronesia.What happened to the huge, lush, towering, 100-year-plus gagu, or ironwood trees, that commonly dotted the island’s landscape at the University of Guam, Tiyan, Windward Hills Country Club golf course and elsewhere?

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Andersen Air Force Base’s Palm Tree Golf Course won an environmental innovation award for its handling of the coconut rhinoceros beetle.

Known by its scientific name Casuarina equisetifolia, ironwood trees are tightly integrated into Guam’s environment and local culture.

It is a hardy, pioneer, salt-resistant tree that occurs on the island’s main soil types: limestone, volcanic, and coral sand. It is propagated for windbreaks, erosion control, and urban landscapes.

Because C. equisetifolia is the dominant tree species on many of the sandy beaches of the Mariana Islands, it has become an important perching tree for the white-collared kingfisher (Halcyon chloris), the Mariana fruit-dove (Ptilinopus roseicapilla), and the white fairy tern (Gygis alba), which commonly lays eggs in the trees.

 There’s a fungus among us

It has been continually propagated since the 1600s. Due to its buoyant cones, it likely floated to Guam’s sandy beaches thousands of years ago on currents from the central Indo-Pacific coastline.From these cones, seeds were shed and grew into trees. Over time, ironwood became one of Guam’s prominent members of the halophytic (sea-salt adapted) vegetation type.Based on what we now know, Guam’s healthiest trees tend to occur in natural areas, near the coastline and in areas not prone to drought.Cocos Island and Ritidian are just a few of the places where healthy coastal stands of ironwood can still be found.

How you can help Guam's dying gagu

Huge, healthy ironwood trees still dominate the shoreline of Ritidian Point in northern Guam.

Farmer seeks help

In 2002, local grower Bernard Watson contacted University of Guam professor Robert Schlub about a group of five ironwood trees in one of his windbreaks that exhibited symptoms of rapid yellowing and death. Death occurred within a few weeks of symptom onset.This was totally unexpected because the trees in question were only 10 years old.Cross-sections of these trees exhibited areas of wetwood that were dark, water-infused, and radiated from their centers. Droplets of bacterial ooze appeared inside and outside the wetwood stained areas.Also appearing on Watson’s farm in 2002 were trees with the same cross-sectional symptoms but this time it was accompanied by thinning foliage and a much slower lethal decline.

 How to manage plant diseases

This latter condition was quickly discovered in other areas of Guam and was coined “ironwood tree decline” by Schlub and Zelalem Mersha, a former UOG post-doctoral fellow now working as a Virginia State University research and extension plant pathologist.Unraveling the cause or causes of IWTD would become a major focus of Schlub’s work at the University of Guam for the next two decades.In the course of the investigation, the Guam team would join forces with researchers from institutes in California, Georgia, Florida, Hawaii, Louisiana, South Africa, China and Australia.Many possible causes of IWTD have been eliminated by the team over the years.

Links to the disease

Age was ruled out as an IWTD contributor, when trees of varying ages began dying in areas where decline was most severe. The failure to find a correlation between the presence of beetles or nematodes (microscopic worms) ruled them out as causing IWTD.The normal appearance of tree buds and young foliage eliminated the likelihood of viruses being involved.Seeing no link between typhoon damage and decline in tree surveys in 2008 and 2009, Typhoons Paka in 1997 or Pongsona in 2002 were eliminated as causing Guam’s ‘sick’ trees.

 Plant diseases to watch for in the rainy season

Over time, five things were consistently linked to IWTD: The presence of termites on the side of trees. The occurrence of wood-rot fungi at the base of trees.The exposure of trees to harmful landscaping practices and the presence of bacterial ooze in tree cross-sections caused, namely the bacteria that causes wetwood and the bacteria that causes bacterial wilt.Bacterial wilt is caused by bacteria within the Ralstonia solanacearum species complex.

Trees decline in 13 years

We now know that Guam’s wilt pathogen strain is the same one that has been killing trees in China and India for decades.One of the team’s most recent projects included a resurvey of 200 trees that were part of a survey of 1,427 conducted in 2008-09. The project was funded by the McIntire-Stennis Cooperative Forestry Research Program (project GUA932, accession no. 1017908), under the U.S. Department of Agriculture’s National Institute of Food and Agriculture.The results suggest that the decline of Guam’s ironwood trees that began in 2002 is continuing to this day, and that trees with severe wilt symptoms or that are nearly dead have high bacterial wilt infection levels.From the data, it is reasonable to expect that half the trees that appear healthy today in areas of decline such as at the University of Guam campus or Fort Soledad will begin showing symptoms of decline over the next 13 years and that trees already suffering from IWTD will likely be dead or nearly dead within the same time period.

Foliage thinning is one of the ominous signs that this ironwood tree on the University of Guam campus is suffering from decline.

How you can help

Several steps have been taken to reduce the impact of IWTD.Hundreds of trees from various countries have been planted to add new genes to Guam’s tree population through cross-pollination.Some of these trees have been used to establish new ironwood windbreaks and others have been used as replacement trees in windbreaks with high levels of decline.Professionals and the general public are now being advised to reduce lawnmower and weed-trimmer damage to roots and the base of trees as a means to reduce infection and spread of fungi and bacteria.

To reduce transmission of Ralstonia and wetwood bacterial ooze when pruning, individuals are instructed to disinfect all tools.

How you can help Guam's dying gagu

Huge, healthy ironwood trees still dominate the shoreline of Ritidian Point in northern Guam.

The public is also being advised to remove severely declined trees as a means to protect nearby healthy trees.Planting healthy, young trees of different varieties or hosts is the quickest way to restore areas with high decline.

Robert L. Schlub is a plant pathology professor and extension specialist, and Elizabeth Hahn and Julia Delorm are extension associates with the Cooperative Extension Service at the University of Guam’s College of Natural and Applied Sciences.


Welcome to the discussion.

Open Letter on the crucial role of fungi in preserving and enhancing biodiversity

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Published on14.12.2022



When we think of forests we usually think of trees, plants and animals. But forests could not exist without fungi, which lie at the base of the biodiversity webs that support much of life on Earth.

Most fungi live as branching, fusing networks of tubular cells known as mycelium which can make up between a third and a half of the living mass of soils. Globally, the total length of fungal mycelium in the top 10cm of soil is more than 450 quadrillion km: about half the width of our galaxy. These networks comprise an ancient life-support system that easily qualifies as one of the wonders of the living world. Despite that, fungi represent a meagre 0.2% of our global conservation priorities.  

Fungi are largely invisible ecosystem engineers that have shaped life on Earth for more than a billion years. In fact, around 500 million years ago, fungi facilitated the movement of aquatic plants onto land, fungal mycelium serving as plant root systems for tens of millions of years until plants could evolve their own. This association transformed the planet and its atmosphere – the evolution of plant-fungal partnerships coincided with a 90% reduction in the level of atmospheric carbon dioxide. Today, most plants depend on mycorrhizal fungi – from the Greek words for fungus (mykes) and root (rhiza) – which weave themselves through roots, provide plants with crucial nutrients and defend them from disease.

Put simply, fungal networks embody the most basic principle of ecology: that symbiosis is fundamental to life on earth. Plants supply carbon to their fungal partners in exchange for nutrients like nitrogen and phosphorus – much of the phosphorus that makes up the DNA in your own body will have passed through a mycorrhizal fungus. In their exchange, plants and fungi engage in sophisticated trading strategies. The influence of these quadrillions of microscopic trading decisions spills out over whole continents. Globally, at least 5 billion tons of carbon dioxide are allocated from plants to mycorrhizal networks each year.

A call to action

A paradigmatic but often forgotten example of the keystone role of fungi is in the world’s forests, which are among the most important biological systems on our planet. They are our largest terrestrial carbon sink and the main terrestrial source of precipitation and oxygen. They house much of the planet’s biodiversity, serving as irreplaceable libraries of different ways to rise to the challenge of living.

However, current biodiversity, climate change, and sustainable food strategies, including forest restoration efforts overlook fungi and focus overwhelmingly on plants (flora) and animals (fauna). We urgently need to add a third “F” – funga – to create holistic conservation strategies that simultaneously address the triple planetary challenges of climate change, biodiversity loss and food security.  

Fungi must be incorporated into law-making and decision-making in international environmental treaties and frameworks, as well as national agricultural and environmental laws and policies, and local conservation and environmental initiatives. We invite the leaders meeting in COP 15 to start this process by adding fungi to the Post-2020 global biodiversity framework. Fungi have long sustained and enriched life on our planet. It’s time they receive the attention they deserve.

This open letter was written by:

Marc Palahí, Director European Forest Institute
Toby Kiers, Director Society for the Protection of Underground Networks
Merlin Sheldrake, author of Entangled Life
Giuliana Furci – Executive director, Fungi Foundation & co-chair IUCN SSC Fungal Conservation Committee
Robert Nasi, Chief Executive Officer, CIFOR-ICRAF
César Rodríguez-Garavito, Professor of Clinical Law and Director, Earth Rights Advocacy Clinic, New York University School of Law

Photo: Carolina Magnasco/Fungi Foundation


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