UGA researchers to study transmission of human pathogen to coral reefs

The spread of lethal diseases from animals to humans has long been an issue of great concern to public health officials. But what about diseases that spread in the other direction, from humans to wildlife? A multidisciplinary team of researchers at the University of Georgia has just been awarded a five-year $2 million Ecology of Infectious Diseases grant from the National Science Foundation and National Institutes of Health to study the first known case of such a “reverse zoonosis” that involves the transmission of a human pathogen to a marine invertebrate, elkhorn coral.

White pox disease has devastated coral reefs throughout the Caribbean and Florida Keys, and is believed to be responsible for much of the coral reef loss there since 1996. White pox disease is caused by a human strain of the common intestinal bacterium Serratia marcescens, which causes the hospital infection serratiosis. Historically, many emerging human diseases, such as AIDS and Ebola, have come from the natural world. The researchers are concerned that the transmission of Serratia marcescens from humans to elkhorn coralmay indicate the beginning of a new phenomenon of diseases jumping from humans to wildlife.

The UGA team will investigate the mechanisms of transmission of white pox disease and the factors that drive its emergence in marine animals. “This bacterium has jumped from vertebrate to invertebrate, from terrestrial to marine, and from anaerobic to aerobic environments,” said James W. Porter, associate dean of the Odum School of Ecology and the team’s leader. “Triple jumps like this are rare.” Understanding the modes of transmission will allow the scientists to attempt to predict future impacts of the disease and to begin to develop effective control strategies.

The scope of the team’s research will extend beyond gaining an understanding of the impact of white pox disease on elkhorn coral and how to counter it. The most likely source of the pathogen for coral reefs is under-treated human sewage, so the study will also explore the intersection of public health practices and environmental health outcomes.

“This investigation addresses not only environmental protection, but also the socio-ecological determinants of coastal zone protection,” said Porter. “This includes the cost of wastewater treatment infrastructure. Given a reliance on tourism by most Caribbean countries, this study addresses a disease system that is of great economic importance and public health concern to developing nations.”

The complexity of the problem required assembling a team of researchers from different scientific disciplines. “The Odum School is extremely well-positioned to lead this study,” said Dean John Gittleman. “Working effectively in collaboration with units from across campus is one of our strengths, and aquatic ecology, theoretical ecology and disease ecology are three of our areas of particular depth that facilitate such interdisciplinary team building.”

Porter, who has spent decades studying coral reefs in the Florida Keys and the Caribbean, said that this is the most exciting and groundbreaking study of his career. “This is science in action to save an endangered species and a threatened ecosystem,” he said. “We are linking good public health practices to effective environmental protection.”

Microbiologist Erin K. Lipp, associate professor of environmental health science in the College of Public Health, will be looking at the genetic diversity of Serratia marcescens to determine which of its different strains are pathogenic to corals, and why. She will collect and analyze samples to determine how the different strains of the bacterium are related, and will then conduct challenge experiments. “We’ll inoculate fragments of corals with different strains and see which cause signs of disease,” said Lipp. “If we can identify strains that do versus those that don’t cause disease, we can then conduct genetic comparisons to isolate the genes that are responsible.”

Assistant professor Andrew W. Park, who has a joint appointment in the School of Ecology and College of Veterinary Medicine department of infectious diseases, will use the data gathered by Porter and Lipp to create models to inform analysis of the spread of the disease. “My part of the project is about making sense of the data in terms of transmission,” said Park. “We’ll use the modeling to help test the hypothesis that there is variation for resistance to the bacteria and explore different candidate hypotheses for how the disease spreads. The pathogen can be spread in different ways—forinstance, by predatory snails, or through water currents. We’re trying to untangle all those competing explanations.”

John Wares, assistant professor of genetics in the Franklin College of Arts and Sciences, will be looking into the interactions of the pathogen with the microbial environment of coral reefs in the Caribbean. Unlike humans, corals do not have classic immune systems, with white blood cells to take on and destroy invading bacteria. Instead, they appear to rely primarily on external defense systems, such as beneficial bacteria that live on their surface. Wares will investigate this system to determine whether healthy bacterial communities can defend corals from disease. “This is essentially high-tech community ecology,” said Wares. “I’ll be looking at what organisms are living on the coral and what role they play in promoting coral immunity.” He said he is excited about the opportunity to use next-generation genetic sequencing, through the Georgia Genomics Facility at UGA. “In the past, we might have been limited to looking at a sample of a few hundred microbes from a given sample of the community,” he said. “For this study, we can study tens of thousands from each sample. It will be very powerful.”

Understanding the transmission process is critical, but Porter said that the study has wider implications. “By incorporating the role of land use practices and water quality into our environmental models of disease prevalence and transmission, this project will have particular significance for sustainable development activities and coastal-zone carrying capacity studies worldwide,” he said. “The modeling element of this study connects disease transmission with water quality, climate variability and patterns of human population density. We expect to show that if you upgrade land-based wastewater disposal systems you improve survival of economically important natural resources such as coral reefs.”

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Fight spring, summer turfgrass diseases now

Fall is a great time to guard against spring and summer diseases on warm-season grasses.
Spring dead spot, or SDS, is one of the most common and important diseases on bermudagrass in Georgia. It is difficult to manage without an integrated approach. The disease is most common on intensively maintained turf like golf courses or lawns.

SDS causes dead patches

The characteristic dead patches appear in the spring when the grass is breaking dormancy, and the problem can persist well into summer. The fungus that causes the disease attacks the roots and stolons in the fall and winter. This makes the grass more vulnerable to winter freeze damage, which leads to the dead patches of grass.

Late September through October is the best time to apply preventative fungicide applications if SDS has been a problem this past season. But this won’t provide complete control. Most infections can be eventually eliminated over a period of years by combining fall fungicide applications with sound cultural practices.

But maintaining a disease free lawn in the coming years can only be accomplished by eliminating the stress that allowed the disease organisms to attack the lawn in the first place. Lawns are stressed by poor soil conditions combined with an imbalance of nutrients. Compaction, poor drainage and thatch thicker than one inch are linked to SDS outbreaks.

Follow these tips

Applying nitrogen late in the season or excess nitrogen, especially with a potassium deficiency, can encourage the development of disease. An integrated management program to improve the lawn’s health includes the following steps:
• When planting new lawns, use cold tolerant cultivars.
• Aerate and remove thatch regularly.
• Irrigate deeply and less frequently. (Once per week in the absence of adequate rain.)
• Mow at the recommended height. Low-mowing height stresses lawns.
• Monitor pH and nutrient levels on a regular basis with soil tests. Keep potassium and phosphorus in balance with nitrogen.
• Maintain a pH between 5.5 and 6.0 if disease has been a problem. The pH can be lowered by using ammonium sulfate as a nitrogen source.
• Apply moderate levels of potassium in September and October to increase cold hardiness. If a deficiency of potassium is indicated on a soil test, two applications of potassium sulfate or potassium chloride can be applied at a 3 to 4 week interval for a total of 1 lb. of K2O per 1,000 sq. ft. Excess potassium should be avoided as it can also encourage disease.
• Do not apply nitrogen after August. Nitrogen should be added in recommended amounts in late spring and early summer. Use moderate amounts of nitrogen during the summer so that excess nitrogen is not carried over into the fall.
• Apply fungicides in late September or October if SDS was a problem the previous spring.

Other warm-season grasses, such as zoysia, centipede and St. Augustine, will also benefit from these general recommendations to prevent diseases like take-all and Rhizoctonia large patch. Follow recommendations for fertilizer applications for the particular grass species. A pH of approximately 6.5 is generally optimum for warm-season grasses.

See these sites for more help

For more information on maintaining turfgrass in Georgia, see the website www.Georgiaturf.com. For fungicide recommendations, contact your local University of Georgia Cooperative Extension agent or consult the Georgia Pest Management Handbook for Homeowners at www.ent.uga.edu/pmh/.

By Elizabeth L. Little 


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Dealing with wet gardens and landscapes

Most gardeners view rainfall as a good thing. But too much of a good thing, namely rain, can be bad.
Disease is always an issue when there is abundant moisture and plants don’t have time to dry out. Many ornamentals, particularly annuals and tender perennials, suffer in the form of leaf spots and root rot. If annuals are not planted on raised beds, too much rainfall can cause them to die.

Pale, yellow coloring is a result of wet roots and leached nitrogen from the soil. Light applications of fertilizer will sometimes help perk up annuals, provided the rainfall levels off.

Leaf spots and other fungal diseases can be controlled through sanitation and occasional use of fungicides. Picking off infected leaves and removing heavily diseased plants will help to curtail the problem.

Some plants and vegetables have been affected by strong winds in combination with the wet soils. This has caused many plants to lean over. As long as the root system has not detached, the plants can be gently stood back up by hand. Then, lightly step on the opposite side of the plant root ball.

In some cases, it may be necessary to use a temporary staking system and guy wires to encourage a plant to grow back in the right direction. If you use wires, protect the plant with some form of a rubber collar such as an old water hose.

Small plants may be stood back up with the help of a single stake or even tomato cages. Corn that has blown over will often stand itself up in a few days and still produce decent ears.

Vegetable gardens also need attention. Weeds seem to love the wet conditions and most likely are thriving. Control weeds through light tilling and hand pulling. Weeds pull nutrients from the soil and will stunt vegetable plants if left unchecked.

As the summer progresses, keep an eye on your tomatoes. If the rains continue, tomato plants will be especially vulnerable.

Prune off diseased foliage to encourage new growth. Many tomatoes will exhibit growth cracks near the top of the fruit as a result of too much rain.

While they may not look pretty, these tomatoes are still perfectly fine to eat. Varieties that put out one or two big harvests should be removed after production to avoid buildup of diseases or insects.

It seems as though it is difficult to have a summer that has the right amount of rainfall. We either get too little or too much all at once. By paying close attention to landscapes and gardens during times of stressful conditions, you can help your plants survive.

By Robert Westerfield
University of Georgia

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UGA Leads Effort to Swat Down Major Vegetable Disease

In the Southeast, thrips are tomato and pepper farmers’ No. 1 enemy. The tiny, plant-feeding bugs carry a disease that can devastate their crops. A $1.75 million grant will help experts with the University of Georgia and other universities in the region develop ways to stop the damage.

“The key pest for Georgia, north Florida and up through the Carolinas are no doubt thrips-vectored viruses,” said David Riley, a research entomologist with the UGA College of Agricultural and Environmental Sciences. “It’s the one that will make or break the crop.”

Over the next four years, Riley will lead a multistate, interdisciplinary team of experts from CAES, the University of Florida, Clemson University and North Carolina State University. The goal is to drastically reduce the risk and damage of the tomato spotted wilt virus. The U.S. Department of Agriculture Cooperative State Research, Education and Extension Service will fund the effort.

Each university included in the grant has faculty working on ways to control TSWV and thrips in their state. This grant will provide the funding and direction needed to coordinate the efforts into strategies farmers can use throughout the region, said Terry Kelley, a vegetable horticulturist with UGA Cooperative Extension.

“Several individual variables that impact tomato spotted wilt virus on tomatoes and peppers have been identified in recent years. This grant will provide the means to look at combinations of these variables and determine the best overall system to use in combating it,” Kelley said. “Hopefully the result will be a strategy that growers can employ to reduce the impact of TSWV every season on their farms."

TSWV is mainly carried by tobacco thrips and Western flower thrips. The disease costs tomato and pepper farmers in Florida, Georgia, South Carolina and North Carolina as much as $100 million in preventive management and damage annually. The four states combined produce half of the nation’s fresh tomato and pepper supply, worth $1.3 billion annually. “This is the hot zone for tomato spotted wilt virus in the U.S.,” Riley said.

The virus can destroy 20 percent to 30 percent of a field in any year or all of a field in a bad year, said Bill Brim, a vegetable farmer in Tifton, Ga. The few TSWV-resistant varieties available to farmers now can succumb to the virus over time.

“This virus can devastate us.” said Brim, who is president of the Georgia Fruit and Vegetable Growers Association. “We’re excited about this grant. The research and work that will come from it will help us stay ahead of the game on this in the future.”

Riley hopes to develop a risk index, or a planning tool, that farmers can use to score their crop’s chances of getting the disease. By combining resistant plants, different types of plastics to cover planting beds, thrips population predictions, chemicals and chemical application times, farmers can reduce the damage the disease causes.

In the mid-1990s, CAES experts developed a similar index for the disease in peanuts. It helped farmers drastically reduce the damage it causes that crop annually.

“We won’t eliminate the virus. It will always be around,” Riley said. “But I am confident in time we will be able to eliminate the damage it causes.”

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