Scientists Identify Insect That Transmits Killer Bacterium to Cucurbits

Since the first discovery of cucurbit yellow vine disease (CYVD) in Texas and Oklahoma in 1988, its cause has been a mystery. Now, a multidisciplinary team of federal government and university researchers has discovered a key missing piece in that puzzle.

The researchers have identified and characterized the bacterial culprit that causes CYVD and have fingered squash bugs as the primary carriers. The squash bug, Anasa tristis, has long been the scourge of gardeners and farmers, though never before implicated in transmitting a plant disease. The bacterium inhabits plant tissue called phloem. This was the first time a phloem-inhabiting bacterium has been implicated in a cucurbit crop disease.

The scientific collaborators include Agricultural Research Service entomologist Sam Pair and plant pathologist Benny Bruton of the South Central Agricultural Research Laboratory, Lane, Okla., Jacqueline Fletcher and Ulrich Melcher of Oklahoma State University at Stillwater and Forrest Mitchell of Texas A&M University in Stephenville.

Since November 2000, the scientists have acquired conclusive evidence that the squash bug can act as a vector of the bacterium that kills cucurbits. They confirmed their findings using a highly specific and sensitive DNA technique called polymerase chain reaction (PCR).

CYVD occurs frequently in squash, pumpkin, watermelon and cantaloupe, but it has not appeared in cucumbers. Symptoms of this disease are distinctly different from other vine declines of cucurbits. Affected plants show leaf yellowing, phloem discoloration and plant collapse.

Although first found in the Southern Plains, CYVD has now been confirmed in Tennessee and Massachusetts. Scientists speculate that misdiagnosis may be the reason the disease has not been found in other cucurbit-growing areas.

The squash bug is particularly hungry when it emerges from overwintering, so the scientists intend to exploit that behavior to control the bug and perhaps CYVD. They have found that using trap crops--small plantings of squash on the perimeters of melon fields--concentrates the pest insects in the squash, where they can be controlled with minimal insecticidal applications.

ARS is the U.S. Department of Agriculture’s chief scientific research agency.