Reports say, it is seed-borne and seed transmitted

Xylella fastidisa is vector transmitted bacterium. Although the pathogen is considered not to be seed borne, transmission from seeds to seedlings of sweet orange has been reported by Li et al. (2003) but recently Helvécio D. et al reported that it is not transmitted by seeds (Europ. J. Pl. Path. 07/2014; 139(3). 

Thank you Syeda! But, they proved it only for citrus. Anyway...

Dear all, as far as I could find "true" transmission is only possible by phloem succing insects like cicacada. But contamination may also occur just by saprophytic development on the plant surface - must be rare, but can be possible. I saw some Information on this in a eview, but I cannot remember where it was. If I find some info - I will immidiately share it.  

Dear Heinz, thank you for you idea. I suspect that there are some difference in Xylella movement in annual herbs and woody plants. So far, nobody has confirmed that Xylella is capable to have saprophitic stage. The reference by Helvécio D. et al  (Europ. J. Pl. Path. 07/2014; 139(3). has shown that the bacterium was found in all parts of seeds, but  seedlings were still healthy. I am afraid that such result depends on plant species, and some other  plants can be infected via seeds. Especially annual herbs. 

So as Italy has recently reported outbreak of Xylella and this country is a "motherland" for many vegetables/ornamentals seeds produced by Russian and other seed companies,  I worry about spreading of the pathogen to South regions of Russia with seeds of annual crops like sunflower. 

Reports say, it is seed-borne and seed transmitted

Dear all,

The fact that Xylella infects sunflower doesn't surprise me, I expect that the host range of this bacterium is much wider than what is currently known.

I do agree when Alex says that there is not any proof of Xylella having  saprophytic Phase. Just consider that today, 130 years later from the appearance of Pearce’s disease, more than 70 plant species belonging to 35 families have been described as hosts of X. fastidiosa  where this bacterium colonizes endophytic tissues (http://edis.ifas.ufl.edu/in17).

What is interesting is the fact that disease symptoms due to Xylella are so far described on only a few species such as  Pierce’s disease of grapevine, citrus variegated chlorosis and sporadically on other plant species including alfalfa, almond, coffee, elm, maple, oak, oleander, peach, pear, plum and sycamore (Chatterjee et al., 2008). Strains of X. fastidiosa have been found in association to olive leaf scorch and branch dieback disease symptoms in California (Hernandez-Martinez et al., 2007).

As for the case of olive dieback in Italy, I am following this issues very closely and I suggest to pay a lot of attention as this is a VERY PARTICULAR case. Xylella was thought and reported as the causal agent of olive dieback in Italy based on the two hypothesis: i) because it has been isolated from the diseased olive plant tissues sampled there and ii) because olive leaf scorch and dieback due to Xylella have been previously reported from the California.

The first hypothesis is not reliable as this bacterium colonizes a range of plant species without causing any disease symptoms. The second hypothesis is contradicted  by the fact that in Italy unlike in California, in addition to Xylella, a wide range of microrganisms, including bacterium, fungii and viruses have been isolated from the diseased tissues and even the Koch's postulates have been confirmed with them.

Chatterjee, S., Almeida, R. P., & Lindow, S. (2008). Living in twoWorlds: The Plant and Insect Lifestyles of Xylella fastidiosa. Annual Review of Phytopathology, 46, 243–271.

Hernandez-Martinez, R., Cooksey, D. A., & Wong, F. P. (2007). Leaf scorch of purple-leafed plum and sweetgum dieback: two new diseases in Southern California caused by Xylella fastidiosa strains with different host ranges. Plant Disease, 93, 1131–1138.

Jay,  thank you for valuable information. Speaking about true reason of olive tree dieback in Italy: Xylella like many other vascular system pathogens (just for instance -Xanthomnas campestris on brassicas, because I have long-term data for this pathogen) do not cause rapid plant  death, but "open gates" for many secondary pathogens, often necrotrophic or make plant highly susceptible to water stress.   

Seeds dryied for propagation are usually treated. Sap in plant stocks and wild sap-sucking hoppers are elusive of any treatment. These insects require specific understanding tha is still missing.

I agree definitively with all Infos brought together - although long time since first recognition of this disease we know very little about the transmission and the ecology of this organism at so many different hosts. Also endophytic and symptomless development (yet not identified) should be kept in mind! Nature is not that simple ... ! Although not working with Xylella I will follow your discussion to learn a lot - thanks to all!

Prof. Dehne, you are quite right! Just one example:

George Hudler note (branchingout.cornell.edu/Back_Samples/20(5)/20(5)May31Insert.pdf ): "If I had it to do all over again (and I don’t) I’d do a thorough workup of affected trees for presence of a pathogen that causes marginal scorch in so many other species of trees (e.g. oak, elm, sycamore): Xylella fastidiosa. You can read more about that on pages 195 – 96 of our recently published “features” compendium, but be advised that so far, connection of horse chesntut leaf scorch with Xylella is only a hunch! "  And this disease affect 90% of trees in Moscow at hot summers...