Role of SystemicAgrobacterium tumefaciensPopulations in Crown Gall Incidence on the Walnut Hybrid Rootstock ‘Paradox’

Abstract

Yakabe, L. E., Parker, S. R., and Kluepfel, D. A. 2012. Role of systemic Agrobacterium tumefaciens populations in crown gall incidence on the walnut hybrid rootstock 'Paradox'. Plant Dis. 96;1415-1421 Greater than 75% of English walnut production in the United States occurs on the walnut rootstock Juglans hindsii x 7. regia 'Paradox', which is highly susceptible to infection by Agrobacterium tumefaciens. When seed were germinated and grown in the presence of A. tumefaciens, in the absence of wounding, 94% of the seedlings exhibited tumors while 89% contained systemic A. tumefaciens populations. When seedlings were wound inoculated, A. tumefaciens established endophytic populations in stem tissue and often migrated from the site of infection. Distribution of A. tumefaciens in the stem was random and may exhibit seasonal variation. A. tumefaciens populations in root tissue were more readily detected than in stem tissue and may serve as a reservoir for subsequent infection of the aerial portions of the tree. Importantly, 7% of inoculated, asymptomatic seedlings contained endophytic populations of A. tumefaciens. In all, 17% of seedlings inoculated as seeds developed galls at secondary stem-wound sites. These results provide an ecological and epidemiological foundation upon which to modify existing tree-handling practices in both nursery and orchard production environments to manage crown gall incidence.California walnut growers are responsible for 99% of walnut producdon in the United States (11). Approximately 75% of the industry uses the hybrid rootstock Juglans hindsii x J. regia 'Paradox' (C. Leslie, personal communication), which is highly susceptible to crown gall (1,11,24,30). Crown gall has been cited by the walnut industry as one of the top diseases affecting walnut nursery and orchard plantings across all growing regions in California (D. Ramos, personal communication).Crown gall incidence on walnut trees has been related to soilborne populations of Agrobacterium tumefaciens which colonize and infect root and crown tissue resulting in hypertrophie and hyperplastic growth visualized as galls (11,24,30). For this reason, the California walnut industry relies on preplant fumigation of orchard and nursery soils to manage soilborne populations of A. tumefaciens. Cultural practices also have been modified to reduce tree wounds and improve post-cold-storage handling of planting stock, which serves to limit infection by A. tumefaciens (11,30). Unfortunately, despite these efforts, crown gall control has been inconsistent.Recent observations by nurserymen and farm advisors support the hypothesis that walnut trees harbor systemic populations of A. tumefaciens. Supporting evidence includes high crown gall incidence on seedlings grown in methyl-bromide-fumigated nursery blocks. This is significant because methyl bromide fumigation has been shown to be effective in reducing A. tumefaciens populations in soil (43). In a recent crown gall outbreak, galls were initially observed at wounds incurred during the grafting proc...

Soil Inoculation and Blocker-Mediated Sequencing Show Effects of the Antibacterial T6SS on Agrobacterial Tumorigenesis and Gallobiome

“…The soil inoculation method was optimized based on the method used for agrobacterial infection of pea and walnut ( 46 , 47 ). Overnight (14 to 16 h) culture of the agrobacterial strain in 12 mL of 523 was were centrifuged at 6,000 × g for 10 min.…”

Development of a soil inoculation method coupled with blocker-mediated 16S rRNA gene amplicon sequencing reveals the effect of antibacterial T6SS on agrobacteria tumorigenesis and gallobiome composition

“…Soil inoculation method was performed essentially following the method used for A. tumefaciens on pea and walnut (44, 45). Overnight (14-16 hr) culture of A. tumefaciens strain in 12 ml of 523 broth were centrifugated at 6,000 × g for 10 min.…”

“…tumefaciens on pea and walnut (44,45). Overnight (14-16 hr) culture of A. tumefaciens strain in 12 ml of 523 broth were centrifugated at 6,000 × g for 10 min.…”