Etiological Role of the Xylem-Limited Bacterium Causing Pierce's Disease in Almond Leaf Scorch

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Abstract
A xylem-limited bacterium resembling the bacterium that causes Pierce''s disease (PD) of grapevines was isolated from 17 of 20 almond trees (P. amygdalus) with almond leaf scorch (ALS) symptoms but not from 16 symptomless trees in 2 orchards in California [USA]. The bacterium was isolated from symptomatic trees in both orchards throughout 1 summer on 22 of 23 (95.7%) and 34 of 60 (56.7%) attempts. PD or ALS symptoms developed in their respective hosts when inoculated with bacterial strains from either host. Injections with a syringe and hypodermic needle were an efficient means of inoculating both hosts. ALS symptoms were more extensive and the bacteria were reisolated more frequently when trees in varietal blocks were inoculated with bacteria by injection than by leafhopper (Draeculacephala minerva) transmission. Few 1st yr infections survived the winter.