Grape disease remains a mystery.

For many years a devastating disease has plagued grapes in California and southern states from Florida to Texas. It is most common in European, or vinifera, wine grapes.

Pierce's disease symptoms vary with species and cultivar, but are typified by marginal browning of leaves in late summer and fall, and death of vines, according to John Hartman, Extension plant pathologist for the University of Kentucky College of Agriculture. "Leaves often fall from the vine at the point of attachment to the petiole, leaving the petiole still attached to the shoot."

Hartman went on to say that delayed shoot growth, leaf mottling and dwarfing of new shoots may be observed in spring and early summer. He said the disease progresses along the grape vine with symptoms developing in adjacent leaves, along the shoot both above and below the initial point of infection.

"Late in the season, wood on affected canes fails to mature normally, leaving green `islands' of tissue which persist into the dormant season and can be seen on canes throughout the winter," Hartman said. "Initially, only one or a few canes on a vine will show foliar and wood symptoms. Symptoms will be more pronounced in vines that were stressed by high temperatures and drought conditions."

Pathologists are not sure if the disease has become established in the wild in some states.

"Pierce's disease is not carried from infected vegetation to grapes or from diseased grapes to healthy grapes by insect vectors," he said. "We know little about which vectors would be involved in transmission."

Possible vectors include spittle bugs, sharpshooters, such as the glassy-winged sharpshooter which is causing problems in California, or treehoppers or other xylem feeding insects.

The disease also can be transmitted by grafting, so in areas where the disease is present keep the removal of infected vines to a minumum.