Major plant scourge at last identified.

Major plant scourge at last identified

For more than a century, farmers have been fighting Pierce's disease of grapes, phony peach disease and several "scorches" -- diseases that leave trees looking parched. But decades of study didn't discover the small viruses thought to cause at least some of these blights. Now a researcher has identified and named the common culprit: Xyella fastidiosa. It's not a virus but the first pathogenic bacterium affecting plants to be discovered in almost 50 years.

The bacterium is so small it passed through filters made to trap such pathogens, leading to suspicions it was a virus, explains John L. Wells, a plant pathologist at the USDA's Eastern Regional Research Center in Philadelphia. While the organism physically resembles that causing Legionnaire's disease in humans, Wells says that genetically it bears no resemblance to any known bacteria. He called it Xylella to denote where it resides -- in the xylem, the woody conduit through which nutrients pass up a plant's stem or trunk.

Transmitted by insects, principally leaf hoppers, Xylella has very catholic tastes. It stunts ragweed and Johnson grass, yellows periwinkle and causes scorches in almonds, plum, sycamore, oak, maple, mulberry--even stately American elms on the Mall in Washington, D.C. Cherry trees have carried the highest concentrations of the pest, although they show no symptoms. In most cases, Xyllela appears to blight by disrupting a plant's hormone balance, Wells says. In fact, hormone treatment has put some affected trees, including peaches, in remission. In most cases, however, diseased plants must simply be pulled out. But help is on the way. Grafting roots of resistant plants to susceptible trees appears to offer one solution; another is to treat plants with antibiotics.