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United States Department of Agriculture

Agricultural Research Service

Related Topics


Location: Emerging Pests and Pathogens Research

2013 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:
Morphological and physiological diversity among isolates of Phytophthora cinnamomi from ornamental plants in South Carolina: Researchers at Clemson University (Clemson, SC) studied morphological and physiological diversity among 142 isolates of P. cinnamomi from ornamental crops that were collected over the last 17 years (1995-2011). Identities of all isolates were confirmed using molecular methods. One isolate was determined to be P. cinnamomi var. parvispora (Pcp). Three morphologically distinct groups were identified based on mycelium growth habit on PARPH-V8 selective medium: aerial (122 isolates), appressed (17 isolates), and dwarf (2 isolates). These groups had significantly different growth rates; aerial isolates grew fastest and dwarf isolates grew slowest. Aerial and appressed isolates produced very few sporangia, but dwarf isolates and the single isolate of Pcp produced abundant sporangia under laboratory conditions. All isolates were sensitive to the fungicide mefenoxam at 100 ppm. The A2 mating type (91%) was much more common than the A1 mating type (9%); 6 of 13 A1 isolates came from camellia. Distinct morphological groups can be used for routine diagnostics, and there was no evidence of reduced sensitivity to the fungicide mefenoxam in this species. Host plant associations for isolates of P. cinnamomi from ornamental plants in South Carolina: Researchers at Clemson University (Clemson, SC) have examined diversity of ornamental plants from which 142 isolates of P. cinnamomi. These isolates were recovered from 56 named and 16 unnamed species, 46 genera, and 31 families. A large portion of the samples came from the five counties in the northwestern part of the state—where temperatures are more moderate and an area within the native range of many species in the family Ericaceae, the family to which the most number of diseased plants in this study belonged. They found 33 host plants that had not been reported previously. A manuscript reporting these two accomplishments is in preparation. Genetic diversity among isolates of P. cinnamomi from ornamental plants in South Carolina: Researchers at Clemson University (Clemson, SC) studied genetic diversity among 142 isolates of P. cinnamomi from ornamental crops that were collected over 17 years. DNA sequences at ITS 1 and 2 region were examined for all isolates; then, DNA sequences for four loci (cox1, cox2, ß-tub, and rps10) were examined for a set of 61 representative isolates. This is the first study to find significant correlations between genotypic and phenotypic characters, other than mating type, among a population of P. cinnamomi. Taxonomic subgroups within P. cinnamomi may exist. The one isolate of P. cinnamomi var. parvispora was genetically and morphologically distinct and may be a separate species. One manuscript is in preparation, and an abstract covering all three accomplishments was published: Schreier, S.G.I., and Jeffers, S.N. 2013. Characterization of Phytophthora cinnamomi from ornamental crops in South Carolina. Phytopathology 103:S2.128-129. Another graduate student is completing research on Phytophthora foliage blight of garden phlox caused by P. nicotianae. This disease has not been reported previously, but it causes serious economic losses at several nurseries in South Carolina 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 also studied variation in cultivar susceptibility and isolate virulence.

4. Accomplishments

Last Modified: 2/23/2016
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