Scout, treat early to reduce 'purple top' problems in spuds

By Renee Sterns

Contributing Editor

Northwest potato growers have the beet leafhopper, and the devastating disease it can spread, under control--for now. A surprise outbreak of "purple top" in 2002 caused substantial damage to potato crops in Oregon and Washington state. Government, university and private industry researchers quickly mustered a team that pinpointed the cause, and growers have held damage to manageable levels in the years since.

But another troublesome year may lie ahead. And researchers still have plenty to learn about the leafhopper and the phytoplasma--a minute bacterium--it spreads in order to fine-tune control measures.

The 2002 outbreak showed up in almost all of the 180,000 potato-growing acres in the Columbia Basin, affecting both quality and yield in that year's crop, says Philip Hamm, Extension plant pathologist at Oregon State University's Hermiston Agricultural Research and Extension Center.

"Purple top" gets its name from leaves that turn purple in late stages of the disease. An earlier stage, when the plant yellows and produces small, curled leaves, prompts the alternate name, "potato yellows."

Aerial tubers form on stems above the ground. Yields from infected plants are severely limited, and the harvested potatoes have high sugars and poor specific gravity, says Andy Jensen, research director for the Washington State Potato Commission.

The aster-yellows phytoplasma once was believed to be the sole cause of purple top. But the 2002 outbreak turned up a new culprit, a phytoplasma currently known as beet leafhopper transmitted virescence agent, or BLTVA, passed along when infected beet leafhoppers feed on potato plants.

Early-season pyrethroid sprays so far have provided sufficient control, the researchers say. "Early control is the most important," Hamm says, because the younger the infected plants, the greater the disease's impact on the crop.

Start scouting in May

Growers should monitor fields in May and June for the presence of the leafhoppers, placing yellow sticky traps an inch or two off the ground. That height is critical to ensure catch, Jensen says.

Until researchers add more data to their pest model, their best recommendation is to begin spraying as soon as the beet leafhoppers show up in traps. Not all of the insects may carry BLTVA, but for now only a polymerase chain reaction lab test can determine whether a leafhopper is infected, says Joe Munyaneza, research entomologist with the U.S. Agriculture Department's Agricultural Research Lab in Wapato, Wash.

Direct feeding by the leafhoppers doesn't damage potato crops, he says. Rather, it's the infection they transmit that causes concern if they're infected with the phytoplasma.

Determining threshold numbers for sprays is one of Hamm's goals.

"It's not the number of leafhoppers you have but how many infected leafhoppers you have," he says. Knowing when to expect the main concentration once the first leafhoppers appear in traps will help growers focus their controls; a better grasp of the insect's life cycle and infection rates also means a firmer idea of when growers no longer need to spray.

Are some varieties more susceptible"

Meshing with this effort are Munyaneza's plans this year to look at potato varieties for particular susceptibility or resistance to the disease. That would also allow growers to tailor their control programs.

Pyrethroid sprays are relatively inexpensive controls, but they do add up and the researchers hope to reduce the number of applications needed. While resistance doesn't appear to be a concern, broad-spectrum pyrethroids may affect predatory insects and lead to mite and aphid problems, Munyaneza says.

Clearer guidelines on when and how often to spray might prevent mite and aphid outbreaks, he says.

Leafhoppers and mustards

Beet leafhoppers prefer plants in the mustard family as hosts. Stands around potato fields may serve as breeding grounds, but because the insects also are able to fly long distances, eradicating local host plants may not provide enough protection, Munyaneza says.

In addition, mowing down neighboring stands "only takes away their home" and pushes leafhoppers into potato fields, Jensen says. Russian thistle and kochia are also favored hosts.

Fall rains increase the number of host plants, and leafhopper populations appear to expand along with them.

Conditions may be right for another leafhopper boom this spring, and growers should be prepared, he says.

Why the purple top problem now?

Why beet leafhoppers carrying BLTVA turned into such a sudden and severe problem is unknown, Hamm says. A run of mild winters may have created a population explosion, or the switch to softer, more targeted insecticides may have left the leafhoppers unchecked.

"I'm so impressed with the team we built here in short order to determine the problem and what to do about it," he says.

Work remains on fleshing out the pest's life cycle, determining economic thresholds and refining spray guidelines, "But we've done a lot the last couple of years," Munyaneza says.

"We've made a lot of progress."