In recently infected plants the yellowing is mostly in a ring beginning under the bark, with a normal white-colored cylinder of tissue inside the yellowed outer layer of wood. Dwarf disease progressively worsens over 1-2 years after first symptoms and eventually kills plant. Noticeable dwarfing requires 6-9 months after inoculation in the greenhouse, probably longer in the field. Several methods are now available in research labs. AgDia sells a detection kit.
Cultures take about a week to confirm.
Contamination of cultures may obscure results. Low sensitivity but usually adequate for alfalfa except for some infected plants that do not show symptoms. PCR a molecular technique that amplifies genes unique to Xylella . Most sensitive method and may soon be available in commercial labs. Serological methods can be used to confirm that the bacteria you see are X. fastidiosa.