Investigations on the diagnosis, colonization, and epidemiology of grapevines with Pierce's disease

Pierce?s disease (PD) of grapevines, caused by Xylella fastidiosa, is devastating Texas vineyards. Two rapid diagnostic techniques, real-time polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA), were compared on the basis of cost, reliability, and their ability to quantify X. fastidiosa in diseased tissues. A high correlation was found between the two techniques for measuring bacterial titer in vitro. A similar relationship was not detected when applying the methods to diseased tissue. There was a 75% similarity between the techniques when used to diagnose PD in artificially infected grapevines. Where the two methods differed, real-time PCR was more successful in identifying plants known to be infected with the bacterium. In uninoculated grapevines, the two techniques were similar, where the positive rates were 7% and 4% for ELISA and real-time PCR respectively. In a second study, 3 grape cultivars, ?Cynthiana?, ?Cabernet Sauvignon?, and ?Chardonnay?, were inoculated with 2 isolates of X. fastidiosa to measure disease development and colonization by the pathogen. The bacteria colonized similar distances from the inoculation point over a 25 week period in all three cultivars. Real-time PCR and ELISA absorbance values suggest that the concentrations of bacteria ranged between 104 and 106 cells/ml in a 1.27 cm section of grapevine cane. Concentrations of bacteria didn?t vary based on distance from the inoculation point. Marginal leaf-scorch symptoms were seen on ?Cabernet Sauvignon? and ?Chardonnay? grapevines 9 weeks post-inoculation. Leaf-scorch symptoms were not observed on ?Cynthiana?. The vigor of all inoculated grapevines was reduced compared to negative control grapevines the season after initial infection. In a third study, a Texas vineyard planted in Viognier grapevines was surveyed for PD symptoms on 3 separate dates. In October 2003, 45/50 rows had significant aggregation of symptomatic grapevines according to Ordinary Runs Analysis. Aggregation of symptomatic grapevines was found down the row more often than across the row. The rapid rate of disease progress and mortality rate of vines in this vineyard suggest that vine-to-vine spread is occurring and that Viognier vines are highly susceptibly to PD.