Impact Of Host Plant Xylem Fluid On Xylella Fastidiosa Multiplication ...

two non-infested vines of each variety. On all canes from the Coachella Valley, internodal distance and petiole weight were
measured and the number of leaves occurring at cane branches was counted.
Probability of PD Detection Based on Petiole Location
The probability of detecting PD from an individual petiole was greatest in basal portions of the cane (Figure 1). This result
follows the suggestion of Hill and Purcell (1995) that the newest growth would not likely contain bacteria because of the
incubation time required for spread. Our result is likely most applicable to chronic infections and this has been noted by
others (Feil et al. 2003), but not presented by our method of examining infection on a node basis along the length of entire
canes.
Probability of PD Positive
1.0
0.8
0.6
0.4
0.2
0.0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
A
1.0
0.8
0.6
0.4
0.2
0.0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
B
Node Position
Node Position
Figure 1. Probability (±SE) of positive PD detection at each node position (1 is most basal) for (A) Superior Seedless (n=6)
vines and (B) Chardonnay vines (n=5).
Morphology of Healthy and PD-infected Vines
We did not detect any differences in Perlette (Λ=0.57; df=4, 11; P>0.05; MANOVA) or Superior Seedless (Λ=0.89; df=4, 11;
P>0.05; MANOVA) varieties in internodal distance, petiole weight, petiole length, or number of leaves branching off of
canes between healthy and infected canes. We measured these factors with the intent to identify a morphological feature that
could aid in identifying infected vines, but no differences helpful for this purpose were found.
Effectiveness of PD Symptoms for Sampling
We photographed each leaf from each cane to evaluate the reliability of symptoms for use in identifying PD infected vines.
We have begun to examine the visual symptoms in relation to PD infection and will use image analysis to quantify foliar
symptoms. In preliminary observations, 9.5% of petioles from PD-infected vines were PD-positive and had asymptomatic
leaves, and 16.1% of petioles were PD-negative, but had symptomatic leaves. Healthy vines had 16.7% of petioles with
symptomatic leaves that were PD-negative. Generally, symptoms were more severe in basal portions of canes and the
likelihood of finding an asymptomatic positive petiole was greater on distal portions of canes (Figure 2).
CONCLUSIONS
• Samples taken from basal portions of grapevine canes were more likely to yield an ELISA positive result. We believe
this result applies primarily to chronically infected vines.
• We did not discover cane morphological differences between healthy and PD-infected vines that could be useful in
detecting PD infected vines.
• We are in the process of evaluating the relationship between PD foliar symptoms and PD infection and have observed
that the likelihood of a PD symptomatic leaf being negative for PD was greater than the likelihood of a PD asymptomatic
leaf being positive for PD. Also, distal portions of canes were more likely to be asymptomatic when infected with PD.
• Based on the potential for choosing symptomatic leaves that are PD-negative, we suggest taking petiole samples for PD
diagnostic tests from basal portions of grape canes to increase the likelihood of detecting PD positive vines.
- 267 -