Question
I have an oleander hedge which has thinned down from being lovely and thick. It is dropping some of its leaves because the leaves are turning yellow and motley with brown marks. I have searched on the internet and fear it may be something called leaf scorch which is fatal to the plant. Is this disease only in the USA? I live in Australia where oleander are numerous and I haven't seen it on oleander before.

Answer
I cannot find any reports that the pathogen that causes oleander leaf scorch (Xylella fastidiosa) is present in Australia.  It is likely that the problem you are observing is due to another problem such as environmental stresses, root rot or fungal leaf spots.  Since you have done some research and feel that the symptoms look similar to those caused by this pathogen, I would recommend that you contact Plant Health in your country.  On the website pasted below, you can find contact information and a phone number to call (1800 084 881).  I suspect that if you contact them, they will help you figure out what may be causing the symptoms.  Please let me know if the link does not work for you.

http://www.daff.gov.au/animal-plant-health/pests-diseases-weeds/plant