Rio Grande Valley citrus growers were told about a new strain of citrus disease discovered in South Texas during the semi-annual meeting of Texas Citrus Mutual (TCM) last week, but the potentially bad news was buffered by encouraging reports of new defense systems being developed to fight the spread of Huanglongbing (HLB), or Citrus Greening Disease.
TCM members gathered in Weslaco to hear updates about the disease first discovered in citrus groves in the Valley in January last year, one of many topics covered at the meeting. The disease, first discovered in a citrus grove near San Juan, caused Texas Agriculture Department and USDA officials to declare a quarantine of a five mile area last year limiting movement of citrus nursery stock and fruit containing leafy material. HLB is the disease that caused widespread destruction of citrus groves in Florida in the 1990s that resulted in multi-billion dollars losses to the industry.
Dr. John da Graca, Director of Texas A&M Citrus Center in Kingsville, told the assembly about the new strain of the disease found in the Valley last month.
“There are now three known species of the Liberibacter genus, the Liberibacter africanus, Liberibacter asiaticus, and the newly discovered one, Liberibacter americanus, which we discovered about two weeks ago in the Texas Valley,” da Graca reported.
He said the African and Asian strains are very similar, but africanus prefers higher, cooler elevations. It was the more common Asian strain found in Texas last year and previously in Florida and several other states in North America that infected a citrus tree in the Valley last year. California detected its first case of Liberibacter asiaticus this year. But until recently, the new strain, Liberibacter americanus, has never been reported in North America.
“This is cause for concern, but we are early in our evaluation of this new strain, so we don’t know a lot about what this means to citrus growers in Texas. But we have confirmation now from multiple labs that a solitary case of the new strain was found in the Texas Valley,” he told TCM members.
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Dr. da Graca has been working on citrus research from the Kingsville citrus center in recent years. He received his Ph.D. from University of Natal (South Africa) and was appointed to faculty of Texas A&M University-Kingsville Citrus Center in 1999. He currently holds the rank of Professor and Center Director in the Department of Agronomy and Resource Sciences and the Citrus Center, respectively.
New strain first found in Brazil
The new strain was initially discovered in Brazil in 2005, but the strain of Liberibacter americanus found in Texas doesn’t appear to have come from South America.
“At this point we are not certain how this strain arrived in the Valley,” da Graca said. “But we have been collecting leaves from plants in the area where the psyllid carrying the new variety was found—bags and bags of leaves in fact—and we may know something more in a week or so. But so far the strain has not been detected in plant material, only in the psyllid, or carrier.”
Dr. da Graca warned it is too early to speculate about the impact of the discovery. In Brazil, the americanus strain, like Liberibacter africanus, favors cooler weather and is not very resistant to heat as is the Asian strain. He says in Brazil the American strain virtually becomes dormant when summer arrives and returns with the arrival of the cooler season. But since the strain discovered in Texas is slightly different than the americanus strain in Brazil, “we aren’t certain what to expect.”
“It’s possible this may be the only psyllid that will be trapped to indicate presence of the new strain in the Valley; we are just too early on to know very much about what to expect until we do more research,” he said.
He also said the americanus strain, until now, has only been found in Brazil. But one instance of discovery in China was initially reported a couple of years ago, but subsequent testing failed to confirm the strain and since then, no other reports of americanus has surfaced in China.
“The same thing could happen here in Texas.” he said.
As far as the latest HLB update in the Valley is concerned, trees infected with HLB have been destroyed, an aggressive trapping program is ongoing, and so far damages from the disease have been limited. In addition, new research in addressing methods to fight the spread of the disease is promising.
“New research involving psyllid predators, a wasp from Pakistan, is extremely promising and we have been working on a fungus that also shows some promise at fighting the spread of the disease,” da Graca reports.
He said tests with wasp predators indicate they may contribute up to a 70 percent success rate in controlling psyllid populations and is hopeful research with a fungus will add to control efforts in the future.
“That doesn’t give us 100 percent control, which we would like to have, but any help is good when it comes to managing the spread of the disease,” he said.
The Florida citrus industry has suffered an estimated $4 billion in losses over the last five production years and an estimated 6,611 lost jobs during the same period. Losses in Texas have been greatly minimized by early detection efforts and an aggressive voluntary spraying program by Valley citrus producers even before the disease was discovered last year.