To assess the presence of the xylem-limited bacterium Xylella
fastidiosa subsp. pauca strain CoDiRO in olive trees, a specific
sampling method was evaluated. Symptomatic and symptomless
plants were randomly selected in four olive orchards located in the
province of Lecce (Southern Italy). The crown of each plant was
subdivided into a lower and an upper portion; four samples were
collected from each layer in the main four cardinal directions. A
total of eight samples per plant, composed of one- or two-year-old
asymptomatic twigs, were collected next to branches showing leafscorch
symptoms. In this preliminary study, the null hypothesis was
tested. i.e. there is no difference between the lower and the upper
portions of the tree canopy and across the four cardinal directions. Samples (472), collected from 60 plants belonging to 11 different
olive cultivars, were tested by qPCR. Out of 236 samples taken
from the upper and lower parts of the canopy only 38.1% of lower
samples, in contrast to 56.8% taken from the upper crown layer,
were positive to the bacterium,. The McNemar test determined that
there is a statistically significant difference in the proportion of
positive samples between the upper and lower crown (p < 0.001).
The Cochran’s Q test was performed to evaluate differences in the
four cardinal directions. The null hypothesis suggesting there is
no difference across cardinal directions was confirmed (p = 0.097).
Based on these preliminary results, it appears that sampling should
be directed to the upper part of the canopy. However, further studies
are needed to improve the efficiency of the sampling technique