DETECTION AND IMPLICATION OF EARLY AGROBACTERIUM TUMEFACIENS INFECTION OF PARADOX SEEDS AND SEEDLINGS

Paradox (Juglans hindsii × J. regia), the dominant rootstock used in California, USA walnut production, has many desirable horticultural characteristics, but is highly susceptible to crown gall. Crown gall, caused by the bacterium Agrobacterium tumefaciens, is not consistently controlled with soil fumigation. This observation, in conjunction with observations of galls occurring in above ground grafting wounds, led to the hypothesis that A. tumefaciens inoculum may be endophytic in walnut seedlings. Paradox seeds collected directly from commercial Paradox mother trees (i.e., Juglans hindsii) were free of A. tumefaciens. Sampling conducted at multiple times during seed production revealed that seed-borne A. tumefaciens populations increased as a function of contact time with the orchard floor. In greenhouse experiments, seeds inoculated with A. tumefaciens produce seedlings with galls at the crown and/or roots in the absence of artificial wounding. Endophytic populations of A. tumefaciens were also found in shoot and root tissue. These data suggest that A. tumefaciens inoculum may exist as a resident endophyte and that timely seed collection, limiting contact with the orchard floor, may be an important first step in preventing crown gall disease of walnut.