Dear Elsebeth,

It seems that the answer is yes. It can be measured in the leaves. I have copied some relevant text from a recent publication entitled "Conserved nematode signalling molecules elicit plant defenses and pathogen resistance " contained in the link depicted at the end of this answer:

Ascr#18 induces defense responses and enhances resistance

On the basis of the finding that ascr#18 is produced by all analyzed species of plant-parasitic nematodes, we asked whether this ascaroside is perceived by plants and affects plant-defense responses to diverse pathogens. Because ascarosides as the nematode signalling molecules may have direct effects on nematode pathogens that could confound detection of plant responses to ascr#18, we began by testing the effect of ascr#18 on defense responses of Arabidopsis to a bacterial (Pseudomonas syringae pv. tomato) and a viral pathogen (Turnip Crinkle Virus—TCV; Fig. 2). Since plants would naturally encounter nematodes via their roots,Arabidopsis roots were partially immersed in water containing different concentrations of ascr#18 for 24 h before leaves were inoculated with the pathogens. Root treatment with 1 μM ascr#18 reduced growth of virulent P. syringae, whereas a higher ascr#18 concentration (5 μM) was less effective (Fig. 2a). Root treatment with ascr#18 at 1 μM also enhanced resistance to virulent TCV (Fig. 2b–d, Supplementary Fig. 2.). Viral replication, as measured by the amount of viral coat protein (CP) in leaves (Fig. 2b), was reduced in both inoculated and distal leaves. Moreover, systemic spread of the virus was nearly abolished with only a trace of CP present in distal leaves of ascr#18-pretreated plants. Disease symptoms, including development of chlorosis on the inoculated leaves (Fig. 2c), leaf curling/crinkling and suppression of inflorescence development (Fig. 2d), were also reduced in ascr#18-pretreated plants.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4525156/

Hoping this will be helpful,

Rafik

I think Yes. Please have a look at this link.

http://www.annualreviews.org/doi/abs/10.1146/annurev.py.12.090174.000441?journalCode=phyto

Yes it can