Effects of Cold Temperatures on Pierce’s disease

We are investigating the response of Pierce’s diseased grapevines in the field and laboratory-greenhouse to cold – especially freezing – temperatures. The geographical distribution of PD, its therapy by freezing treatments, and the failures of most late season infections to survive the following winter all suggest that cold temperatures play a key role it the spread of PD in temperate climates, but more information is needed to determine the underlying cause of freezing mortality of the causal bacterium, Xylella fastidiosa. In field studies, we used infective insect vectors to inoculate X. fastidiosa into 12 vines each month at University viticultural plots in Oakville and Davis from April through August, 1997. We recorded symptoms and attempted to culture X. fastidiosa from inoculated vines at both sites in the fall of 1997 to determine infection rates. April and August inoculations at Oakville were lower than planned. We will record symptoms and diagnose inoculated vines for infection with X. fastidiosa in the fall of 1998 to reconfirm the recovery of most late season infections of X. fastidiosa that have been previously reported (in 1979) by observing symptoms. During January and February, we took cuttings from inoculated canes and rooted them in the greenhouse to assess the rate of spread of X. fastidiosa from the point of insect infection, based on the date of inoculation. We inoculated an additional set of Ruby cabernet vines in the Central Valley (Kearney Agricultural Station) and Cabernet Sauvignon vines at UC Davis in April, with additional inoculations planned for June and July, 1998. Some vines were inoculated with infective insect vectors and other vines with needle inoculations of cultured X. fastidiosa. These vines will be assessed in fall, 1998 and again in the fall of 1999 for continued survival of X. fastidiosa. We prepared pot-grown grapevines with Pierce’s disease, allowed them to become dormant outdoors, and tested the effects of various laboratory freezing treatments on the survival of X. fastidiosa. We completed freezing treatments over a range of temperatures in March, 1998. Preliminarily, we have not detected bacterial growth in about 70%of the plants, but we will assess the plants again in July. In addition, we are recording the effects of temperature on the survival or cultured cells of X. fastidiosa in various solutions to compare to mortality rates in dormant grapevines.