A decade ago, the plant disease rapid blight was discovered and characterized.
Although there are hundreds of known plant diseases, to find a previously unidentified disease causing serious damage to turf is highly unusual. But the nature of the disease was rarer still because none of the usual suspects – fungi, bacteria, nematodes or viruses – were involved. Instead, the microbe that causes rapid blight was found to be most closely related to an obscure group of ocean-dwelling organisms known as marine slime molds, none of which had ever been found to attack land plants.
The path towards the identification of rapid blight and the development of management practices for its control involved a unique scientific collaboration among golf course superintendents, university researchers and a privately funded consultant who played detectives.
In the 10 years since its discovery, the incidence of rapid blight has increased. It has now caused significant turf damage in 11 states. Recently the disease was also identified from colonial bentgrass and Poa annua greens in England.
Beginning
In the winter of 1995, on the greens of the Santa Ana Country Club in Santa Ana, Calif., golf course superintendent Dave Zahrte was alarmed. Over a single weekend, patches of dead turf appeared on his 19 Poa annua putting greens. He quickly sent a diagnostic sample to Larry Stowell, Ph.D. of the PACE Turfgrass Research Institute in San Diego.
Stowell examined the sample under the microscope and immediately found that it did not resemble the usual winter disease suspects, such as Fusarium patch. Stowell also ruled out insects, nematodes, cultural problems and nutrient imbalances. The only abnormality found was the presence of many thin walled, spindle-shaped cells, which unfortunately didn’t help point the way to a diagnosis. In fact, it was clear that the organism did not resemble any other type of documented turfgrass pathogen.
Determined to find the identity of the causal agent, Stowell sent samples throughout the scientific community. Mycologists suggested that the organism might be protozoan, a single-celled animal. Protozoologists countered suggesting the organism was most likely a chytridiomycete, which is a primitive fungus. In the end it was discovered that the organism was a unique pathogen of cool-season turfgrasses and that nothing like it had attacked land plants before.
While the researchers continued to sleuth out the identity of the disease pathogen, a number of things happened. Stowell and Bruce Martin, Ph.D. of Clemson University named the disease rapid blight, to match the way the disease acts on turf. The late Houston Couch, Ph.D. was consulted. He suggested mancozeb as a possible treatment for rapid blight. He was right; mancozeb worked and became the first effective method for its control. Superintendents from around the country submitted samples helping to establish what turf types were most susceptible to rapid blight. Mick Twito of Estrella Mountain Ranch in Goodyear, Ariz., submitted samples from perennial ryegrass. Tommy Witt of Cassique Golf Course on Kiawah Island, S.C., submitted a sample of rough bluegrass, thereby adding more grasses to the susceptibility list. Bentgrass also was attacked, though symptoms were not as severe. A PACE working group of superintendents from 60 golf courses around the country that were affected by the disease was formed to help fund research.
One by one, golf courses in Arizona began showing symptoms of the disease on greens, fairways and even roughs. That prompted Mary Olsen, Ph.D. to join the detective hunt. The University of Arizona Extension plant pathologist initiated work in the laboratory and field, and prepared high magnification micrographs of the pathogen. Still the identity of the culprit wasn’t found. And then, it was, and quite by accident.
Discovery
One day Olsen showed her photos of the pathogen to Robert L. Gilbertson, Ph.D., professor emeritus of mycology at the University of Arizona. Gilbertson said he thought the spindle-shaped cells resembled Labyrinthula zosterae, an obscure organism that causes wasting disease in aquatic eelgrass. Olsen was stunned. But DNA work conducted by Paul Peterson, a post-doctoral researcher in Martin’s Clemson lab, confirmed Gilbertson’s diagnosis. Olsen named the rapid blight causing organism Labyrinthula terrestris. It is the only Labyrinthula known to attack a land plant; all others occur in marine environments.
That Gilbertson was even able to recall Labyrinthula is astounding given its obscurity. Labyrinthula is a marine slime mold that doesn’t excite the interest of most scientists. Gilbertson, a mycologist, had read about and remembered Labyrinthula. His interest, coupled with an encyclopedic mind, cracked the case wide open.
Fighting rapid blight
With the disease pathogen identified, control measures also became fine-tuned. Kurt Desiderio, a working group member from Saticoy Country Club in Somis, Calif., was the first to note that trifloxystrobin (Compass) was effective at stopping the disease on his course. This was a surprise, since repeated tests with the closely related azoxystrobin (Heritage) had yielded no positive results. Desiderio’s observations have since been confirmed in replicated trials conducted by Martin at Clemson, who also showed that pyraclostrobin (Insignia) was highly effective against rapid blight. As a result of these efforts, the labels of Compass, Insignia and many mancozeb products have now been adapted to include rapid blight use recommendations.
Since PACE’s Stowell first looked at the rapid blight pathogen under the microscope, a lot has been learned about the disease. Rapid blight affects a broad range of cool season turfgrasses that show an increased severity of symptoms as soil salinities increase. Soil salinity problems are likely to increase in the future as competition for high quality water, increased use of recycled water on golf courses, and drought conditions occur. As a result, superintendents need to develop management strategies that cope with the potential for increased rapid blight attacks. In the meantime, an integrated approach that has proven successful has been identified that relies on a combination of cultural practices and chemical control. For more information on the approach, contact the PACE Turfgrass Research Institute at www.paceturf.org.
For more on the prominent role played by superintendents, read an article outlining the history and incidence of rapid blight at the American Phytopathological Society Web site (www.apsnet.org).
There is one interesting question about rapid blight that remains to be answered: How did Labyrinthula jump from the marine environment to terrestrial plants?