Attempts for improving protocol to phenotype olive cultivars responses to Xylella fastidiosa infections

The incubation period of Xylella fastidiosa (Xf) in olive could last as long as >1 year, even in the case of the highly pathogenic Xf subsp. pauca, ST53.  Such long latency period is a major constrain when performing large screening program for searching potential resistant olive genotypes. In this work, we compared the incubation period of the infections on 9 grafted and self-rooted plants of the cultivar Cellina di Nardò. Plants were inoculated by caging on each plant, 10 specimens of Philaneus spumarius previously confined for bacterial acquisition on field-infected trees. Plants were maintained under controlled conditions and monthly sampled. One-month post-inoculation (pi), Xf was detected in 1 grafted and 1 self-rooted plant.  Six-months pi, 5 self-rooted plants and all 9 grafted plants were positive. One-year pi, 7 out 9 self-rooted plants tested positive, while 2 remained negative.
Initial symptoms (defoliation of the apical shoots) started to appear 5-months pi on 1 grafted plant, and then progressed with all grafted plants exhibiting symptoms within 8 months pi. 
Symptoms on the infected self-rooted plants were delayed compared to the grafted plants: initial symptoms appeared 8-months pi on 2 plants, and only at 15-months pi the remaining infected plants showed symptoms. The delayed appearance of the symptoms (approx. 6 months) correlated with a lower symptom severity on self-rooted plants. Interestingly, the roots of grafted plants harboured a higher bacterial population size then self-rooted plants.
The results of this experiment showed that: (i) vector-mediated transmission is an efficient mean for the inoculation of olive plants; (ii) the use of grafted plants may reduce the latency period in olive; (iii) the bacterium in olives successfully colonize the roots. Nevertheless, the differential plant response recorded in our experiment, once validated in field conditions, may provide useful indications when realizing new olive plantations in the infected area.