2012 Annual Report
1a.Objectives (from AD-416):
1) Identify sources of A. tumefaciens inoculum in/on hybrid seeds and seedlings;
2) Identify how and where A. tumefaciens is maintained on/in the Paradox hybrid seed;
3) Demonstrate methyl bromide alternatives for use in Paradox rootstock seedling production under nursery conditions.
1b.Approach (from AD-416):
Under nursery conditions we will quantify A. tumefaciens populations on Paradox hybrid seed as function of how and when the seed is harvested from mother trees at cooperating nurseries. Using sensitive detection protocols we will document the population dynamics of A. tumefaciens in/on hybrid seeds, and the effect indigenous soil-borne microbial communities have on those populations and crown gall incidence under nursery conditions.
The agreement was established in support of objectives 1 (subobjective 1.3) and 3 (subobjective 3.4) of parent project, which are to improve management strategies for key soil borne diseases of tree fruit and nut crops, and to characterize the etiology, biology and ecology of Agrobacterium tumefaciens. The goal of this project is identify sources of A. tumefaciens infection of walnut trees under both production and nursery conditions. A. tumefaciens is the causative agent of crown gall disease. ARS scientists at Davis, California, identified temperatures which restrict A. tumefaciens in-vivo, i.e. in planting and graft materials, and in-vitro.
Pre-graft treatment of dormant graft wood was able to reduce subsequent crown gall formation at the graft union. ARS scientists at Davis, California, also demonstrated contact with the orchard/nursery floor by either hybrid walnut seed or graft wood (almond,walnut, cherry, and peach) collected during the dormant season results in A. tumefaciens contamination levels which are dependent on the length of time the material is in contact time with the soil, i.e. the shorter soil contact time, the lower A. tumefaciens infection. Significant contamination can occur in as short of time as 3 hours. These results have resulted in altered nursery practices which now limit A. tumefaciens infections and crown gall formation. ARS scientists also have identified an effective inexpensive positively charged surfactant (i.e. detergent) which lowers populations of A. tumefaciens below detection limits on/in plant tissue and on solid surfaces such as grafting tools.
Most importantly, this detergent was more effective at killing A. tumefaciens than bleach solutions in the presence of high concentrations of organic matter which commonly occurs in nursery operations. ARS scientists at Davis, California, also demonstrated the ability of A. tumefaciens to move systemically in the vascular tissue of walnut trees. In addition, A. tumefaciens was able to survive in the plant tissue for long periods of times prior to inciting crown gall development. These two discoveries, systemic movement and latent infections (i.e. symptom development), have impacted the approaches needed to control crown gall incidence.