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In the Field:  Virus Elimination

Jim Harbage watches Longwood’s cannas (Canna x. generalis) like a vigilant parent who thinks a toddler might be coming down with something. The Research and Production Leader is skilled at recognizing the symptoms of plant viruses, and is part of a team working to eliminate them from Longwood’s stock.
 
“Typically, you’ll see streaks of white or yellowish coloring on the leaves,” Harbage says. “You’ll also see more mottling. Some symptoms show up in the flowers. If you hold them up to the light you’ll see breaks or variations in the color.”

The culprits include Bean yellow mosaic virus and Canna yellow streak virus, which are members of the potyvirus group, and Canna yellow mottle virus. The latter is thought to pass from adult to seed, making it especially difficult to tackle. These viruses typically don’t kill the plants, but do weaken them and make them less attractive.

Cannas fill a critical display need at Longwood, which is one reason that the viruses are of such concern. The plants provide height, bold texture, and rich tropical color. The canna display value is so strong that Longwood Gardens spent twenty years breeding new varieties for the displays, ending the project in the late 1980s. The growing popularity of cannas and the increase in international plant exchange inadvertently created an opportunity for rapid spread of the viruses worldwide.

Viruses can be carried by insects (aphids), and can also be spread mechanically. “This is what we think happened,” Harbage explains. “We used to grow a stock block in the nursery. The plants were close together, then we’d dig them up at the end of the summer. Just by using a spade we were moving viruses through the collection pretty quickly.”

To combat the viruses, Harbage and his colleagues use a broad spectrum of techniques ranging from basic garden hygiene to cutting-edge science.

“We now grow the plants in the greenhouse year-round in separate containers, which gives us more control of insects that spread the virus,” says Harbage. “We don’t have to dig the plants up, which prevents mechanical spread as well.”

Shoot tip culture is another technique. “You cut off the growing point in the rhizome, then disinfect it and put it in a defined media under sterile conditions,” he says. “Basically you’re growing a piece of the plant without any roots. Using this system, micropropagation, the shoot starts to grow. Then it sends up side shoots that you can cut off and put in another medium with different composition for rooting. When these root, you can gradually ease them into the greenhouse environment.”

Another scientific methodology with exciting possibilities is meristem culture. “The meristem is where new cells are produced,” explains Harbage. “In infected plants, the virus does not spread as fast as the new cells are being produced at the meristem, so if you harvest the meristem you’re finding a piece of tissue that isn’t infected yet. The cool thing is that you can take that small piece of tissue and get it to grow. Once it produces leaves, it’s like shoot tip culture. We can send samples for testing and find out if we were successful recovering virus-free tissue. Kate Prestowitz, one of our former graduate students, was able to recover potyvirus-free Cannas using meristem culture combined with heat treatments. She presented the paper at an American Society for Horticulture Science conference.” Harbage also shares Longwood’s findings with other horticulturists, recently presenting to the International Plant Propagator Society.

A final approach Harbage’s team uses to combat canna virus issues is raising plants from seed.  Nature has provided plants with a mechanism for surviving virus infestation by blocking the movement of many viruses from the parent plant into seed produced by the plant.  So if you collect seed from a virus infected plant and grow it out, chances are it will be free from virus as long as you protect it from re-infection.  This technique has been used at Longwood to recover virus-free seedlings from plants infected with Bean yellow mosaic virus and Canna yellow streak virus. At Longwood, seeds are collected and grown. Individual plants with desirable traits are selected from among hundreds of seedlings. Those are grown in quarantine to preserve their virus-free nature. The most desirable seedlings are multiplied by division (clonally) to a point where new cultivars are now ready to go on display.

“We’ve gone back into the canna breeding process, and are now evaluating plants for naming and commercial release,” Harbage says. “Things have come full circle, and we’re excited about the possibilities.”

Visit http://www.longwoodgardens.org/HighlightedGeneras_1_3_2_3_2_1.html for information about Longwood’s cannas.