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.

DECEMBER 20, 2022

Researchers analyze performance of bacterium in combating coffee rust

by Ricardo Muniz, FAPESP

Researchers analyze performance of bacterium in combating coffee rust
The research is part basic science, investigating the bacterium’s resilience in a hostile environment—coffee leaves—and part biotech, seeing whether the bacterium inhibits the development of a pathogen. Credit: Jorge Mondego/IAC

A new study has analyzed the potential of a bacterium for biological control of the fungus Hemileia vastatrix, which causes coffee rust, a major challenge for Brazilian coffee growers. An article on the study is published in the journal BMC Microbiology.

The symptoms of coffee rust are yellow spots like burn marks on the leaves of the plant. The disease impairs photosynthesis, making foliage wither and preventing bean-producing cherries from growing until the tree resembles a skeleton. It is typically controlled by the use of copper-based pesticides, which can have adverse effects on the environment.

“This was a basic science study, in which we set out to understand the behavior of bacteria that inhabit the leaves of coffee trees. First of all, there are several compounds that are harmful to bacteria and can be used to attack them,” said Jorge Maurício Costa Mondego, last author of the article.

“Second, leaves are environments that undergo significant environmental pressures, such as sunlight and rain. We wanted to understand how bacteria that live on coffee leaves can withstand both the compounds produced by the coffee plant and the stresses of rain and sun,” he said.

Besides this basic science front, the study also addressed applied science challenges. The researchers decided to find out whether bacteria that inhabit coffee leaves can combat the fungus that causes coffee rust. The first step consisted of identifying the expressed sequence tags (ESTs) of Coffea arabica and C. canephora produced by the Brazilian Coffee Genome Project (Projeto Genoma EST-Café).

“I was the first author, alongside Ramon Vidal, a professor at UNICAMP, of an article in which we compiled the sequences expressed by C. arabica. It was published in 2011. We weren’t yet thinking in terms of metagenomics, but that’s what we did, more or less accidentally,” Mondego said.

Accidental metagenomics

The researchers found sequences they considered contaminating in the midst of the coffee leaf ESTs. “We took these sequences, fed them into the database, and concluded that they appeared to be from Pseudomonas spp, a genus of bacteria.,” Mondego said. “This stimulated the curiosity of our research group, which was led by Gonçalo Pereira, also a professor at UNICAMP. We asked ourselves, ‘What if we’ve done metagenomics without meaning to? Do these bacteria really live on coffee leaves?'”

At the time, Mondego was already a researcher at IAC. A few years later, he was able to join forces with Leandro Pio de Sousa, first author of the article published in BMC Microbiology. Sousa was a student who had a scientific initiation scholarship and now holds a Ph.D. in genetics and molecular biology from UNICAMP.

“I invited Leandro to work with me on this study, which was designed to see if Pseudomonas really does live on coffee leaves. If so, the previous findings would be confirmed. He agreed immediately,” Mondego said.

They isolated bacteria from the coffee leaves and put them in a culture medium. Under ultraviolet light, it is possible to characterize Pseudomonas, which looks purple and can easily be selected in the medium. “We collected the bacteria, extracted their DNA and sequenced one, which we called MN1F,” he said.

They made several interesting discoveries about MN1F, which has a secretion system that reflects its need to survive in a hostile environment full of fungi and other bacteria. “The secretion system produces antibacterial and antifungal compounds. That suggested it could be used for biological control,” Mondego said. They also detected a number of proteins associated with protection against water stress.

The next step entailed physiological experiments, whereby bacteria were cultured in different media to confirm the researchers’ observations regarding the genome. “The biological experiments proved several inferences correct. We showed that the bacterium does indeed have a considerable capacity to withstand strong osmotic pressure, which can be considered analogous to the effects of drought on coffee leaves,” Mondego explained. “Furthermore, MN1F is capable of degrading phenolic compounds that can be harmful to it. It breaks down these compounds from the plant and converts them into material for its own survival.”

The researchers then conducted a battery of tests to find out if MN1F could be used for biological control, preventing or inhibiting the development of H. vastatrix, the fungus that causes coffee rust. The tests took place under greenhouse and laboratory conditions, including an attempt to inhibit in vitro germination of the fungus. In all of the experiments, the bacterium proved capable of inhibiting the development of spores (reproductive units) and mycelium (the filamentous network containing the fungus’s genetic material).

More information: Leandro Pio de Sousa et al, Functional genomics analysis of a phyllospheric Pseudomonas spp with potential for biological control against coffee rust, BMC Microbiology (2022). DOI: 10.1186/s12866-022-02637-4

Journal information: BMC Microbiology 

Provided by FAPESP 

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Fungus that eats fungus could help coffee farmers

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Bacterial wilt still threatens crops in tropical and subtropical areas

For tomatoes and more than 200 other plant species, bacterial wilt is an extremely destructive disease. Although some may not show symptoms, they keep the pathogen alive in the soil. The disease is caused by Ralstonia solanacearum, a bacterium found mainly in moist, hot areas.

The first sign of R. solanacearum is plants that start to wilt during the day and recover by nightfall. Later, they remain wilted and die. The disease usually starts in patches and spreads fairly rapidly to neighboring plants. It is almost impossible to control once it takes hold.

Fortunately for farmers, resistant genes are available. However, there are several races of R. solanacearum, with only one occurring in South Africa, namely race 1 biovar 2. It is therefore crucial to choose a tomato variety that has resistance to this race, or it will be ineffective. Varieties with the resistant gene can extend the harvest season into warmer conditions, but when the soil becomes too hot, the gene becomes less effective.

The resistant gene for race 1 biovar 2 was developed at the Agricultural Research Council station in Mbombela, and the resistant variety is named Rodade. This gene is used by other countries for our strain of the pathogen, and was hailed as a major breakthrough at the time.


Publication date: Thu 22 Dec 2022

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Secretion secrets revealed: Pathogen effector characterization for a devastating plant disease

Date:November 22, 2022Source:American Phytopathological Society

Summary:A recent study has discovered and characterized secreted proteins from the pathogen Candidatus Liberibacter solanacearum. These proteins, called effectors, offer clues into the manipulation tactics this bacterium uses to subdue its plant host. The study found that these effectors can be present in both the plant and insect host.Share:


Sometimes the most niche plant pathogens pack the greatest punch. Such is the case for the Florida citrus industry, which has seen a 70% decline in its orange production since the introduction of Huanglongbing (citrus greening) in 2005. This disease is caused by the bacteria Candidatus Liberibacter asiaticus, which spreads via a flying insect — unlike most bacterial plant pathogens. When the insect feeds on the sugary sap of a plant, it deposits the bacteria into the veins of the plant, directly into the phloem, which allows the bacteria to follow this transport highway throughout the plant.

A close relative of the citrus greening pathogen, Candidatus Liberibacter solanacearum (CLso), is a newly emerging pathogen of tomato and potato. As this bacterium cannot survive outside of its hosts, very little is known about it, including how it causes disease. A recent study led by Paola Reyes Caldas, of the University of California, Davis, has discovered and characterized secreted proteins from the pathogen CLso. These proteins, called effectors, offer clues into the manipulation tactics this bacterium uses to subdue its plant host.

Newly published in Molecular Plant-Microbe Interactions, the study found that these effectors can be present in both the plant and insect host. Once inside the plant, these effectors can target various parts of the cell such as the iconic chloroplast, which are critical for the plant to perform photosynthesis. Additionally, these effectors are mobile in that they can travel from one plant cell to another. Corresponding author Gitta Coaker comments, “These effectors can also move from cell to cell, which could explain how Liberibacter can manipulate the plant while remaining restricted to the phloem. Unlike effectors from culturable leaf colonizing bacteria, the majority of Liberibacter effectors do not suppress plant immune responses, indicating that they possess unique activities.”

Whether these unique activities alter the phloem environment or insect attractiveness to facilitate pathogen spread remains to be seen, but this research offers an exciting starting point to unravelling this complex disease. Once targets of these effectors are identified, genetically engineering these important crops to prevent manipulation could be a fruitful solution to managing these diseases.

Story Source:

Materials provided by American Phytopathological SocietyNote: Content may be edited for style and length.

Journal Reference:

  1. Paola A. Reyes Caldas, Jie Zhu, Andrew Breakspear, Shree P. Thapa, Tania Y. Toruño, Laura M. Perilla-Henao, Clare Casteel, Christine R. Faulkner, Gitta Coaker. Effectors from a Bacterial Vector-Borne Pathogen Exhibit Diverse Subcellular Localization, Expression Profiles, and Manipulation of Plant DefenseMolecular Plant-Microbe Interactions®, 2022; DOI: 10.1094/MPMI-05-22-0114-R

Cite This Page:

American Phytopathological Society. “Secretion secrets revealed: Pathogen effector characterization for a devastating plant disease.” ScienceDaily. ScienceDaily, 22 November 2022. <>.

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