The wilt that wasn't

Since it became a widespread nuisance at the Quail Hollow Club in Charlotte, N.C., the pathogen some call “bacterial wilt” has skyrocketed into the spotlight of turfgrass pathogens. It’s become one of the Ten Most Wanted for golf course superintendents and turf researchers alike.

But those researchers still haven’t even agreed on what it actually is. Though they sometimes disagree on findings, this pathogen has split some labs into camps.
 

Does it exist? “Bacterial wilt is a real disease,” says Dr. Lane Tredway, senior technical field representative for Syngenta. “It’s a documented disease on annual bluegrass caused by the bacteria Xanthomonas translucens. The same bacteria was implicated in the cause of the widespread decline of Toronto creeping bentgrass in the early 80s. What we’re seeing today may or may not be related to that.”

Tredway, who headed the North Carolina State Turf Pathology program before recently joining Syngenta, says researchers are moving too fast when categorizing the pathogen.

“I think the mistake we’re making is we’re trying to put the story to rest too quickly,” he says. “It’s way too early to make any definitive conclusions as to how important these bacteria are, how widely distributed they are and, most importantly, how to diagnose them and manage them.”

But even at these early stages, the name doesn’t quite fit, says Dr. Rick Latin, professor of plant pathology at Purdue University.

“I’m not even sure I can call it bacterial wilt,” he says. “Bacterial wilt was identified as Xanthomonas, but here’s the issue: On creeping bentgrass, it’s caused by Acidovorax. They’re widely different pathogens and different species, so why would we call it the same thing?”

Latin refers to the disease as “bacterial decline,” differing from the original “wilt” and from the “bacterial etiolation” that Michigan State University’s Dr. Joe Vargas uses.

Superintendents with creeping bentgrass have seen symptoms of etiolation, thinning, yellowing and shallow rooting.

With any name, there’s no question that bacteria are involved. Turfgrass pathologists disagree on which is to blame, or how many could be. Vargas says the cause is Acidovorax avenae pv avenae, which he found in infected samples. But more important than the pathogen’s name is what it does to turf itself, and that can be backed up by observations.

“Where we have to start is with the symptoms,” says Dr. Nathanial Mitkowski, associate professor of plant pathology at the University of Rhode Island. “The thing we know for a fact, that is undisputable, is that it seems to occur on creeping bentgrass greens. The symptoms include etiolation, thinning, typically shallow rooting and yellowing.”

Etiolation, or a general yellowing and elongation of the plant, plays a big part within that list of symptoms. The pathogen can strike anywhere there’s creeping bentgrass, but the symptoms tend to be more severe in areas with more heat.

“In cooler climates, these symptoms may pop up for a week or two weeks in August or July, and then it’ll recover and go away, and you won’t see it again until next year,” says Mitkowski. “In warmer climates, like the Southeast, the symptoms may pop up as early as April or May. They’re obviously worse when you get to June or July, but they can go for months, through the height of the growing season.”

Though researchers have had the pathogen on their radar for much longer, it’s really only exploded onto the scene within the past two to three years, says Mitkowski. That growth coincides with two of the toughest summers on record for many golf courses, a correlation that isn’t lost on researchers.

“It’s first important to keep in mind that the last two summers have been extremely difficult from an environmental standpoint for golf course superintendents with persistent heat and, in some areas, prolonged drought conditions,” says Tredway.

According to Vargas, the damage comes from bacteria preventing turf from getting the necessary water for growth, especially during these heat-stressed times.

“We have … found xylem vessels clogged with bacterium,” says Vargas. “Xylem vessels conduct water from the roots up to the shoots. Clogged xylem vessels which limit the uptake of water, especially during warm weather of the summer, put the creeping bentgrass plants under tremendous stress and can result in their death.”

But the list of symptoms is almost the only thing on which researchers working on the pathogen can mostly agree. Only a few labs in the country are prepared to identify samples of the bacteria involved, and currently, the results are contested.
 

The battle for the bacteria. Originally, the symptoms, including etiolation, were believed to be caused by a connection to applications of Primo to creeping bentgrass. As more research was completed, more clues about the pathogen came into focus, shifting away from that explanation alone. When Vargas found Acidovorax under an electron microscope, he named it as the primary pathogen involved.

“We have isolated Acidovorax into pure culture and inoculated healthy creeping bentgrass plants, placed them in growth chambers and we were able to infect the plants with Acidovorax showing it is a pathogen capable of attacking creeping bentgrass,” says Vargas.

But finding it capable of attacking the turf isn’t the same as actually finding the cause of the symptoms until testing can duplicate those findings. Turfgrass researchers check those results against Koch’s postulates to determine whether something involved is a pathogen.

“You have to isolate it from where you found it, and then you have to put it back on a new plant and determine that you get the same symptoms,” says Mitkowski. “We’re talking about the proof of concept. Yes, this thing causes disease when we put it on uninfected grass. And Vargas did that, with some stipulations.

“This bacteria he found was a pathogen, but it didn’t produce the exact same symptoms that we saw in the field. In the greenhouse, sometimes it would just kill the grass. Sometimes it would just do a little bit of damage and knock out some of the leaf blades and the plants would recover. This is really where all the controversy is, though. It’s a known bacterial pathogen in other plants, but it is not producing the same symptoms we see in the field.”

Whether or not it definitively meets that standard, Vargas sees the bacteria producing results similarly in the greenhouse tests.

“Some of these infected plants were also examined under the electron microscope and again the xylem vessels were clogged with Acidovorax,” says Vargas.

The concept of the bacteria as the primary pathogen is also supported by some testing that shows results with application of an antibiotic to the turf (though these products aren’t currently legally allowed to be used for turf outside of experimental testing). When it’s been done experimentally, according to Mitkowski, the symptoms go away.

Even if bacteria is to blame for the symptoms, the disparity between what’s observed in the field and in the greenhouse means there’s another piece to the puzzle, says Tredway.

“There are a lot of possibilities,” he says. “There are bacteria that are known to induce these types of symptoms. There are fungi that cause etiolation in other crops. We’ve investigated the bacteria possibilities very heavily, and the problem is we cannot find any one particular bacteria that is consistently associated with these symptoms. We do find Acidovorax associated with the etiolated turf in a number of circumstances, but there are other occasions where we’ve not been able to find that particular bacteria and instead we find two or three others.”

Though there are other possibilities, Acidovorax is still a major player for Mitkowski.

“From my perspective, whenever I get samples of this etiolation, of this thing we call ‘bacterial wilt,’ I’d say 99.9 percent of the time I find this bacteria that Joe Vargas identified,” he says. “So that’s some empirical evidence, but it’s not experimental evidence.”
 

What's missing? “I definitely think it’s reasonable to assume there’s Acidovorax involved here,” says Latin. “But whether or not the Acidovorax is the aggressive primary pathogen that brown patch or pythium is? I would say no.”

Research into what role the bacteria (or another agent) plays in creating these symptoms is ongoing, but a few issues make study in the greenhouse difficult.

“It’s virtually impossible to mimic the stress and the conditions on a golf course putting green in a growth chamber environment,” says Tredway. “Even though we have control of the temperature, humidity and lights, we can’t do the close mowing and apply a lot of the stresses that a golf course superintendent does.”

That’s a problem for Mitkowski, as the most heavily affected turf he sees on courses is under multiple stresses.

“It may be the stress involved in a golf course,” he says. “They’ve got constant management; they’ve got lots of different fertilizers going on. When I go out to golf courses, almost always the bacterial wilt symptoms are occurring on the plants that are in the worst conditions: there’s too much shade, it’s too wet, there’s too much traffic.”

That stress is a possible missing piece, according to Latin – he argues there’s another factor that predisposes turf to infection by Acidovorax. As his research has continued, he’s observed high concentrations of the bacteria are required in turf to get some symptom expressions.

“Of course, you don’t need those types of populations when you’re wounding plants and likely introducing bacteria when we mow every day,” he says. “We’ve had two very stressful summers here in the Midwest, and that’s when the reports starting coming in.”

Turfgrass researchers are testing for other conditions and effective responses.

Stress could be a factor, but it’s not the only possibility, according to Mitkowski.

“The other explanation is there’s something else going on in the field that we’re missing in the greenhouse,” he says. “There’s a chemical that’s been applied, there’s been some suggestion that certain plant growth regulators or biostimulants that golf course superintendents use which aren’t being applied in the greenhouse are actually stimulating the bacteria to cause disease.

“It’s possible that there is another agent – that this isn’t one bacteria causing this disease and the bacteria is one piece of a multipiece complex.”

The question remains, but probably not for much longer. As more research is being done by the labs equipped to handle identifying the bacteria, researchers are actively testing in the field, where other possible conditions can be observed and applications tested.

“Hopefully, by the end of the summer, we’ll be able to say, ‘These are the applications we made that worked, these are the applications that had no effect and these are the applications that made it worse,’” says Mitkowski.

With the additional testing, more questions will be answered –whether it means Acidovorax will remain a top threat to creeping bluegrass or another factor is implicated.

“No one has observed these symptoms, isolated the bacteria in pure culture and put them back and recreated those symptoms in the field,” says Latin. “Once we’re able to do that, I think we’re going to learn a lot more about this particular pathogen and the extent to which this bacteria is a pathogen.
 

Kyle Brown is GCI’s associate editor.