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United States Department of Agriculture

Agricultural Research Service

Related Topics


Location: Emerging Pests and Pathogens Research

2012 Annual Report

1a. Objectives (from AD-416):
Both oomycetes and fungi can be serious pathogens of floriculture crops, causing diseases that destroy the aesthetic quality and marketability of these economically important plants. Since each production business grows numerous plant species, there are many different plant-pathogen combinations that can result in harmful diseases. Introductions of new cultivars, shifts in weather patterns, or a change in cultural practices can suddenly favor pathogen development and lead to explosive disease outbreaks caused by familiar but incipient pathogens. New pathogens frequently are encountered on floriculture crops due to the introduction of new species from exotic locations. The global movement of seeds, cuttings, and plants allows the global movement of pathogens, despite our best efforts to restrict importation of disease-causing agents through quarantines and other regulatory actions. This project will focus primarily on diseases of floriculture and nursery crops caused by Phytophthora spp. These oomycetes continue to impact these two segments of the “green” industry on an annual basis, causing significant economic losses every year. Early detection and accurate identification of the species of Phytophthora attacking specific host plants are important first steps to effective disease management. By identifying sources of primary inoculum (i.e., the inoculum that initiates infection and pathogenesis) and understanding the roles of other factors affecting disease outbreaks, one can take steps to prevent the pathogen from becoming established in a greenhouse or nursery or from spreading through a production facility. However, once Phytophthora spp. are present in a nursery, greenhouse, or landscape, alternative management strategies need to be available—including fungicides, cultural practices, host resistance, etc. New, improved, and innovative disease management strategies are needed to prevent serious economic losses to Phytophthora diseases on both herbaceous and woody ornamental crops. Specific objectives include: 1. Identification of new host-pathogen associations for Phytophthora spp. on floriculture and nursery crops in the Southeast. 2. Investigation of variation in virulence within a species—including host specialization 3. Identification of important sources of inoculum of Phytophthora spp. through improved methods of sampling and detection. 4. Evaluation of management strategies for Phytophthora spp. in nurseries and greenhouses. 5. Elucidation of the role of sciarid fungus gnats in development of Phytophthora root rot diseases in greenhouse crops.

1b. Approach (from AD-416):
Objective 1. Identify new host-pathogen associations for Phytophthora spp. on ornamental crops. We will utilize our extensive collection of Phtyophthora spp. to identify new host-pathogen associations. Initially, we will target P. cinnamomi and its hosts in SC. In cooperation with the Clemson University Plant Problem Clinic (PPC) and colleagues around the country, Phytophthora isolates have been collected from diverse ornamental plants for > 15 years. Isolates will be identified using molecular techniques and molecular identities will be validated and confirmed using traditional morphological and physiological characters. Experiments to confirm pathogenicity in new host-pathogen associations will be conducted in the greenhouse. Objective 2. Investigate variation in virulence and host specialization in P. nicotianae. P. nicotianae is the most important species attacking floriculture crops in SC and elsewhere in the Southeast. Previously, studies in our lab have demonstrated that isolates of P. nicotianae vary in virulence and exhibit host specialization. We will focus our efforts on petunias and annual vincas. Using isolates from our permanent collection, we will inoculate a set of standard cultivars of each host plant species and determine if isolates vary in virulence and if isolates recovered from one plant species can aggressively attack other plant species. Objective 3. Identify sources of primary inoculum. Production of ornamental crops is a multi-step process, with plugs, cuttings, and liners produced at one location and plants for wholesale or retail sale produced at another location. Therefore, it is possible that inocula of Phytophthora spp. are being moved along with propagation materials used to produce both woody and herbaceous plants. In collaboration with local nurseries and greenhouses, we will target several floriculture plant species and sample plugs coming from various vendors around the country. Plugs will be assayed for Phytophthora spp. using several different methods developed in our laboratory. Objective 4. Evaluate management strategies for Phytophthora spp. in nurseries and greenhouses. Minimizing losses to diseases in commercial production facilities requires effective disease management strategies. Managing Phytophthora diseases in nurseries, greenhouses, and landscapes continues to be a challenge. Using an established host-pathogen system (e.g., P. nicotianae on annual vinca or petunia), we will evaluate new fungicides that have been developed or are under development and compare these to industry standards. In addition, we will screen cultivars of floriculture plant species for resistance to P. nicotianae. Host plant resistance has the potential to effectively manage Phytophthora diseases over the long term. Objective 5. Elucidate the role of fungus gnats in development of Phytophthora diseases. Laboratory assays will investigate the effects of feeding by larval Bradysia impatiens on host plant susceptibility to Phytophthora nicotianae. Observed fungus gnat-Phytophthora-host plant interactions will be further investigated and ultimately taken into consideration in developing more effective IPM practices.

3. Progress Report:
Note: During the first year of the project (2011-12), two new MS-level graduate students were identified and enrolled to work on this project. One is supported fully by the project and the other receives partial support from the project, but his stipend is covered by other funds. The first student began graduate school in August 2010 and his thesis research is focused on this project. He is using our permanent collection of isolates of species of Phytophthora that has cultures recovered from ornamental plants in the southeastern USA (primarily from South Carolina) over the last 16 years (1996-2011). He identified 232 isolates that could be P. cinnamomi from a variety of host plants for his research. He has examined colony morphology and, sensitivity to the fungicide mefenoxam. In addition, he has extracted DNA from these isolates and examined RFLP fingerprints of the ITS region and had the ITS region sequenced, so we can determine isolate identities based on molecular characters. These data are being used to conduct a population genetics study on the inherent diversity within P. cinnamomi—a species of worldwide importance. We initially examined approximately 180 isolates, and 157 have been confirmed to be P. cinnamomi. So far, it appears we have two groups of isolates that differ both in colony morphology and ITS sequence. We tentatively have identified 23 host plants that previously had not been reported. The other new student, enrolled in graduate school in January 2011, and his thesis research is focused on this project. He is working closely with one of the largest producers of herbaceous perennial plants in the eastern USA. We are studying Phytophthora foliage blight on garden phlox caused by P. nicotianae. This disease has not been reported previously, but it causes serious economic losses at this nursery and other nurseries in SC on an annual basis. We are investigating factors that affect pathogenicity—including temperature, relative humidity, salt stress from fertilizer, type of inoculum, and site of primary infection. We currently are initiating a study to evaluate several of the newer fungicides developed for oomycete diseases. We have been working for several years to document host specialization in P. nicotianae. In 2011, we conducted both greenhouse and field trials in which petunia and annual vinca plants have been inoculated with isolates from both of these plants. Currently (2012), we are repeating this study. We hope to demonstrate conclusively that vinca isolates are more virulent on annual vinca and petunia isolates are more virulent on petunia. In addition, we are evaluating the susceptibility of 39 cultivars of petunia to petunia isolates of P. nicotianae.

4. Accomplishments

Last Modified: 10/19/2017
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