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United States Department of Agriculture

Agricultural Research Service

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Research Project: INOCULUM THRESHOLDS NECESSARY FOR INFECTION OF SELECTED HOST SPECIES BY PHYTOPHTHORA RAMORUM

Location: Foreign Disease-Weed Science

2013 Annual Report


1a.Objectives (from AD-416):
The objectives of this proposal are to characterize inoculum density relationships between P. ramorum and selected Eastern US forest and nursery hosts. We will determine the number of sporangia required to cause infection on several major Eastern forest species. Species to be evaluated include chestnut oak (Quercus prinus), northern red oak (Quercus rubra), red maple (Acer rubrum), native mountain laurel (Kalmia latifolia), and Rhododendron 'Cunningham's White'. For each host, we plan to determine the minimum amount of inoculum necessary for infection and characterize the relationship between inoculum density and symptom development in forest hosts. Results will help workers predict the likelihood of P. ramorum spreading from potentially infected ornamental nursery hosts to Eastern forests and be useful in developing pest risk assessments.


1b.Approach (from AD-416):
We will use our specialized containment facilities to investigate the relationship between inoculum density and disease for P. ramorum on five selected host species. All plant inoculations with P. ramorum will be conducted inside level BSL-3P plant disease quarantine containment greenhouses and laboratories. We will perform whole-plant and detached leaf experiments to determine the minimum number of sporangia required to cause disease on stems and foliage of three important Eastern forest species as well as the important understory species Kalmia latifolia (mountain laurel) and rhododendron 'Cunningham's White'. Stems and foliage will be inoculated using a range of inoculum concentrations from 0 to 5000 sporangia per ml, and plants placed in dew chambers for 7 days at 20ºC. Following incubation in dew chambers, lesion areas will be assessed by scanning leaves and using image analysis software. Detached leaf experiments will also be performed to allow use of a wider range of inoculum densities than would be feasible using whole plants.


3.Progress Report:

Sporangia produced by six varied P. ramorum isolates were quantified, combined, and used to inoculate detached leaves of forest species of interest. The goal of these studies is to evaluate susceptibility and inoculum density relationships between the sudden oak death pathogen (P.ramorum) and selected host species prevalent in the Eastern US. Detached leaves of Sassafras albidum (sassafras) and Lonicera maackii (Amur honeysuckle) were inoculated with P. ramorum sporangia at concentrations of 0, 100, 1000, and 3000 sporangia per milliliter, and incubated in 9 x 13-inch Rubbermaid containers at 20 C in darkness for 7 days. Following incubation, symptoms were recorded by scanning the leaves, and the percentage of infected leaves determined. These two species were both found to be highly susceptible over the range of inoculum concentrations used. Preliminary inoculum density studies were also performed using detached leaves of honey locust (Gleditsia triacanthos), yellow poplar (Liriodendron tulipifera), flowering dogwood (Cornus florida), and mockernut hickory (Carya tomentosa). In addition to determining susceptibility of these species in future experiments, we will analyze whether the response to increasing inoculum levels varies among species.


Last Modified: 10/23/2014
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