Infection of Grapevines by SoilborneAgrobacterium tumefaciensBiovar 3 and Population Dynamics in Host and Nonhost Rhizospheres

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Abstract
Hosts (Vitis vinifera ''Chardonnay'') and nonhost (Avena sativa ''Astro'') rhizospheres enhanced survival of Agrobacterium tumefaciens biovar 3 (AT3) in artificially infested unpasteurized field soil in a greenhouse study; populations in host rhizospheres were greater than in nonhost rhizospheres. Populations declined over time in both fallow soil and oat rhizospheres but not in grapevine rhizospheres. AT3 survived 10 mo in association with 54% of grapevines assayed; 35% of these developed tumors, and systemic infection were detected in 10%. Populations of rifampicin-resistant AT3 did not change over 10 wk in Concord grapevine (Vitis labrusca L.) rhizosphere; 60% of Concord seedlings in soil infested with 108 cfu/g became systematically infected within 10 wk, but none became systemically infected at lower levels of soil infestation (104, 102, and 1 cfu/g).