Scientists from the USDA’s Agricultural Research Service have teamed with a West Coast university to develop a new and highly detailed genetic way to trace the spread of Agrobacterium, one of the world’s most important bacterial plant pathogens.
“Agrobacterium causes crown gall, growths or galls on affected plants - plant tissue that grows in an unorganized manner disrupting water and nutrient flow,” says Melodie Putnam, Director of the Oregon State University Plant Clinic.
“On grapes, crown gall causes poor growth or can be lethal to young plants while older plants may have reduced yield, reduced life span, or increased susceptibility to other diseases,” she says. “Crown-gall bacteria are systemic within a grape plant, meaning once it is infected, it will always be infected. This negativity impacts the ability to graft new varieties on infected rootstocks or train up new vines in an attempt to get away from the disease.”
Appearance of the pathogen can be particularly problematic in vineyards for a variety of reasons: “First, because this is a soil-borne bacteria, symptoms may not be visible,” says Putnam. “Second, there is no ability to forecast when infection is likely. Third, there are no universal control products, and fourth, when disease might occur, and which plants might be diseased, is unpredictable.”
Putnam’s advice? “Growers need to rigorously scout plants for unusual growth and act quickly to avoid spreading the pathogen. The difficulty, particularly with grapes, is that crown-gall looks a lot like callus tissue and since both occur at graft union, it’s difficult to tell them apart. If the material is nursery stock, it should be destroyed because there is no cure. If the plant is older and already planted out, it should be flagged and monitored so it can be promptly removed if it begins to fail.”
OSU professor Jeff Chang adds, “What this pathogen does is unusual amongst all pathogens in that it genetically transforms its host and in doing so changes the way the plant grows. It basically causes a cancer to the plant that grows uncontrollably, and for young plants, is deadly.”
Like the postal service
The newly discovered tracking mechanism is analogous to the postal service, according to Alexandra Weisberg, a biologist at the university.
“Current methods of detecting pathogens reveal only zip codes while the method we have developed reveals a deeper layer of data, the full name and address of the sender which allows us to track movement of bacteria and its plasmids and deduce logical pathways by which pathogens get into plants,” Weisberg says.
“Agrobacterium is a significant pathogen of grapes because, as a soil-borne pathogen, it senses when a plant is around and can swim toward those plants, colonizing and growing on the roots,” adds Chang.
“When the plant is wounded — like when a harvester comes through or during grafting procedures — that wound represents a vulnerable point for the pathogen to enter,” he says.
Understanding the genetic basis for how pathogens like Agrobacterium evolve and diversify in agricultural settings provides a new foundation for determining their spread and assessing risks of future outbreaks and thus helping growers use appropriate strategies to limit spread, Weisberg explained.
Not only will this new approach to whole genome sequencing, made possible by dramatic improvements in DNA sequencing in the early 2000s, be invaluable to grape growers, the virulent root-causing pathogen responsible for crown-gall disease also affects walnut growers and fruit tree orchards, so the new key to tracking disease outbreaks benefits many.
For more news on pests, disease management and other issues affecting vineyards, subscribe to the bi-monthly newsletter The Grape Line.