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United States Department of Agriculture

Agricultural Research Service

Related Topics

Research Project: Tracking and Managing Diseases of Floriculture Crops Caused by Oomycetes and Fungi

Location: Biological Integrated Pest Management Unit

2011 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

Two MS-level graduate students were identified and enrolled at Clemson University in 2011 to conduct research under objectives 1 and 2. One student is fully supported by the project and the other receives partial support. Objective 1. Identify new host-pathogen associations for Phytophthora spp. on ornamental crops. More than 200 isolates of P. cinnamomi from a variety of host plants have been identified from our permanent culture collection of Phytophthora spp. isolates recovered from ornamental plants in the southeastern USA over the last 15 years. DNA extractions are in progress, and we will soon begin DNA sequencing to confirm identities of the isolates and initiate a population genetics study. Morphological and physiological features of the isolates will also be examined. Objective 2. Identify sources of primary inoculum. Working closely with one of the largest producers of herbaceous perennial plants in the eastern USA, we will be tracking the production of two perennial plants and one bedding plant that annually have problems with Phytophthora diseases. By following plants through the production cycle, we hope to identify the critical control points where primary infection occurs, so effective management strategies can be implemented. The nursery has been making excessive applications of fungicides during the final stages of production with less than satisfactory results. Objective 3. Investigate variation in virulence and host specialization in P. nicotianae. We have completed a greenhouse study in which three cultivars of annual vinca (Titan Blush, Cora Burgundy, Cora Lavender) were inoculated with 40 isolates of P. nicotianae. The Cora line of annual vinca was developed to be resistant to P. nicotianae. Isolates varied significantly in virulence, cultivars varied significantly in susceptibility, and there also was a significant interaction between isolate and cultivar—i.e., the virulence profiles of P. Nocitianae isolates varied with the host plant cultivar. Isolates could be separated into three virulence categories: weak, intermediate, and aggressive. Weak isolates did not cause symptoms on Titan Blush—the susceptible cultivar, intermediate isolates caused disease on Titan Blush, and aggressive isolates caused disease on all three cultivars. We are in the process of completing this study. We have been working for several years to document host specialization in P. nicotianae. Greenhouse and field trials are being conducted in which petunia and annual vinca plants were inoculated with isolates from both of these plants. We hope to demonstrate conclusively that vinca isolates are more virulent against annual vinca and petunia isolates are more virulent against petunia. Objective 4. Evaluate management strategies for Phytophthora spp. in nurseries and greenhouses. Studies are nearing completion—see report under objective 3 (research under objectives 3 and 4 is closely related). Project activities have been monitored by the ADODR via e-mail communications and telephone conversations.


Last Modified: 4/24/2014
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