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Project description

Pruning for control of Pierce's disease. (00FE007)
Program UC IPM competitive research grants program
Principal
investigator
A.H. Purcell, Insect Biology, UC Berkeley
Host/habitat Grapes
Pest Pierce's Disease Xylella fastidiosa
Discipline Plant Pathology
Review
panel
Applied Field Ecology
Start year (duration)  2000 (Two Years)
Objectives Determine if the recovery from Pierce's disease by severe pruning of grapevines continues on the regenerated vine growth for more than one season.

Test the repeatability of severe pruning to regenerate healthy vines from grapevines with Pierce's disease.

Test a new disease severity rating system to guide pruning vines with Pierce's disease.

Final report This project was a one-year extension of experiments to test severe pruning (just above the graft union) of grapevines with Pierce's disease (PD) as a way of regenerating healthy-appearing grapevines the next year. By the second year of our studies, most of the apparently recovered vines had PD symptoms. Some PD-vines that had no PD symptoms 2 years after severe pruning developed PD during the third year after pruning. The low incidence of newly diseased vines from 2000 to 2002 for most vineyards indicates that natural re-infection was unlikely to have caused this delayed reappearance of Pierce's disease on regenerated growth after severe pruning. The recurrence of PD symptoms the second or third year after severe pruning indicates that Xylella fastidiosa persisted in the vine trunk at levels that did not cause symptoms or allow detection by culture for one and sometimes two years. These results support previous research findings that rapid vegetative growth suppresses populations of X. fastidiosa and expression of PD symptoms in grapevines. Vigorous vegetative regrowth may have inhibited or outpaced growth and movement of X. fastidiosa the first year after pruning. The few new diseased vines in our seven vineyard plots in recent years precluded valid statistical studies to adequately test severe fall pruning with our new rating system that based the proximity of PD symptoms to the base of the canes and the distance of the symptomatic canes to the main trunk. It is not known if late summer or early fall pruning of vines with PD might increase the effectiveness of severe pruning in regenerating healthy vines in the Napa Valley. Growers should not use severe pruning to eliminate PD vines as a source of inoculum to reduce vine-to-vine spread of PD, such as may be caused where the glassy-winged sharpshooter is present in vineyards during winter and spring months. Instead, PD-vines should be completely removed.

Second-year
progress
Severe pruning (just above the graft union) of grapevines with Pierce's disease (PD) symptoms successfully regenerated healthy-appearing grapevines the next year (fall 1999), but by the second year most of the apparently recovered vines had PD symptoms. Some PD-vines that had no PD symptoms two years after severe pruning developed PD during the third year after pruning. The low incidence of newly diseased vines in 2000 and 2001 for most vineyards indicates that natural re-infection was unlikely to have caused this delayed reappearance of Pierce's disease on regenerated growth after severe pruning. The recurrence of PD symptoms the second or third year after severe pruning indicates that X. fastidiosa persisted in the vine trunk at levels that did not cause symptoms or allow detection by culture for one and sometimes two years. These results support previous research findings that rapid vegetative growth suppresses populations of X. fastidiosa and expression of PD symptoms in grapevines. Vigorous vegetative regrowth may have inhibited or outpaced growth and movement of X. fastidiosa the first year after pruning. It is not known if fall pruning prior to leaf abscission of vines with PD might increase the effectiveness of severe pruning in regenerating healthy vines. Growers should not use severe pruning to eliminate PD vines as a source of inoculum to reduce vine-to-vine spread of PD, such as may be caused where the glassy-winged sharpshooter is present in vineyards during spring months. Instead, PD-vines should be completely removed.

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