Mulberry Leaf Scorch, New Disease Caused by a Fastidious, Xylem-Inhabiting Bacterium

Abstract

Mulberry leaf scorch (MLS) symptoms appeared in the Washington, DC, area by mid-July. Leaves showed marginal desiccation with subsequent necrosis of desiccated tissues and development of a chlorotic halo separating necrotic from unaffected, green tissues. Leaves in all stages of symptom development occurred on the same branch. Severely affected leaves abscised prematurely, and limited branch dieback was observed. The disease occurred at epiphytotic leveles (125 of 160 trees examined) in Alexandria, VA [Virginia, USA] and symptomatic trees were observed as far north as southern New York. A gram-negative, xylem-inhabiting bacterium morphologically similar to and serologically related to the Pierce''s disease and elm leaf scorch bacteria was isolated from 19 of 22 MLS-affected plants by incubating wood chips in supplemented PW broth or PD-2 broth (5-7 days). Isolation of the MLS bacterium from petioles was achieved on semisolid PD-4 and supplemented PW media (18+ days) with bacteria extracted from petioles by centrifugation or crushing. Each of three isolates of the MLS bacterium was inoculated via root and stem inoculations into four mulberry seedlings. Six months after inoculation, the MLS bacterium was reisolated from 10 symptomatic and one symptomless seedling. No bacteria were isolated from four buffer-inoculated, symptomless control seedlings. Isolates of the MLS bacterium were successfully subcultured on nutrient agar and were morphologically and serologically indistinguishable from isolates grown on supplemented PW or PD-4 media. The MLS bacterium is the least fastidious member of the fastidious, xylem-inhabiting bacteria.