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United States Department of Agriculture

Agricultural Research Service

Related Topics

Research Project: Biology, Pathology, and Epidemiology of Emerging Oomycete Pathogens

Location: Foreign Disease-weed Science Research

2013 Annual Report

1a. Objectives (from AD-416):
1: Identify key components of the biology, pathology, and epidemiology of emerging oomycete pathogens as the basis for improved management strategies. 1A - Determine whether Phytophthora sp. inoculum levels on host plants and in soil are reduced by treatment with selected chemical and biological control agents. This sub-objective addresses the need for methods to remediate occurrences of P. ramorum in nurseries in both the Western and Eastern U.S. to allow continued operation of these nurseries. 1B - Improve methods for detection of Phytophthora sp. in soil and investigate physical and chemical factors that affect detection. This subobjective addresses requests from our APHIS partners for research addressing improved soil detection of P. ramorum and knowledge of detection limits and factors affecting detection. 1C - Determine key elements in spore germination and host infection by selected oomycete pathogens. For this subobjective, we are addressing additional key stakeholder priorities, including determining modes of seed transmission of P. ramorum, factors affecting chlamydospore germination, and studies on infectivity of P. kernoviae, P. pinifolia, and Sclerophthora rayssiae var. zeae.

1b. Approach (from AD-416):
Using specialized containment facilities, we will obtain data in key research areas to assist in managing emerging oomycete pathogens including Phytophthora ramorum, P. kernoviae, P. pinifolia, and Sclerophthora rayssiae var. zeae. To address the need for methods to remediate occurrences of P. ramorum in nurseries in both the Western and Eastern U.S., we will use a novel root inoculation assay to measure the effects of selected chemicals and biological control agents on Phytophthora sp. inoculum levels on host plants and in soil. We will also develop baiting assays to improve methods for detection of Phytophthora sp. in soil and investigate physical and chemical factors that affect detection. We will use a variety of experimental approaches in specialized laboratory and greenhouse facilities to study chlamydospore germination and the potential for seed transmission of P. ramorum, as well as the infectivity of P. kernoviae, P. pinifolia, and Sclerophthora rayssiae var. zeae. Understanding key features of biology, pathology, and epidemiology of the selected pathogens will contribute to development of targeted management practices and recommendations.

3. Progress Report:
An evaluation of a commercial formulation of Trichoderma asperellum to remediate Phytophthora ramorum-infested soil under nursery conditions completed the first year trials in California. Results showed that the formulated T. asperellum survived well in soil and showed some level of control but was not better than the crudely formulated wheat bran colonized by T. asperellum. Seven endophytes (fungi that live within a plant without causing apparent disease) that showed some inhibitory activity towards P. ramorum on agar cultures were selected to test in planta. The endophytes were introduced into the Rhododendron cuttings by wound inoculation. New growth of the rooted Rhododendron cuttings was inoculated with P. ramorum. Disease symptoms were significantly reduced on leaves infected with two of the seven endophytes. The active compounds have been isolated and identified from culture filtrates of the endophytes, which may lead to a new biological control method to protect plants from P. ramorum infection. Boxwood blight was first reported two years ago in the US, seriously threatening the boxwood nursery industry as well as historic gardens. We investigated the ability of the pathogen to survive in leaves, twigs and as discrete microsclerotia at different moisture levels and temperatures over time. Results indicate the pathogen survives poorly at constant temperatures of -10 C or 30 C, and survives better in sand at water-carrying capacity than in drier sand. Following 5.5 months of incubation, sporulation from microsclerotia and infected plant tissue was still abundant from material incubated in wet sand at 0, 10 and 20 C. This information will be useful in developing best management practices in nurseries and historic sites containing boxwood. Further studies investigated the effect of temperature on germination of chlamydospores of P. ramorum. At specific temperatures between 5 and 30 C, substantial variation in germination percentage was observed, particularly within the NA-1 clonal lineage of P. ramorum. At 5 C no more than 3 percent germination was observed, and this was for isolates of the EU-1 clonal lineage. No germination was obtained at 30 C for any of the isolates tested. Phytophthora pinifolia is a foreign pathogen that has devastated pine plantations in Chili in recent years, and is of great concern to US regulatory agencies. Using our specialized containment facilities, we obtained consistent infection of host plants by P. pinifolia using a newly developed protocol. It was determined that a high concentration of pathogen spores in conjunction with wound sites and a prolonged dew period are necessary for infection. The successful protocol we developed for infection of a variety of economically important tree species with P. pinifolia provides a much-needed tool for assessing the risk this pathogen poses to US forest species.

4. Accomplishments
1. Minimum inoculum dose needed for root infection by the Sudden Oak Death organism. Given the spread of Phytophthora in nurseries through irrigation runoff and given the frequent detection of P. ramorum in stream water in the West Coast and in several eastern states, it is vitally important to understand the routes by which the pathogen can infect plants. ARS researchers at Ft. Detrick, Maryland determined the concentration of pathogen spores necessary to infect roots of two common plants in the nursery industry: Viburnum and Rhododendron. The results, obtained with a wide range of pathogen strains, showed that plant roots can be infected in runoff infested with as little as 5 spores per milliliter. The results demonstrate that numbers of spores produced in runoff from infected plants vastly exceed the number needed for infection of healthy nursery plants. These studies provide the first estimates of numbers of P. ramorum spores needed to infect nursery plant roots, and will provide the basis for recommendations aimed at preventing spread of the disease in the nursery industry.

Review Publications
Widmer, T.L., Dodge, S.C. 2013. Can Epiphytes reduce disease symptoms caused by Phytophthora ramorum. Biological Control. 65:135-141.

Preuett, J.A., Collins, D.J., Luster, D.G., Widmer, T.L. 2013. Screening selected Gulf Coast forest species for susceptibility to Phytophthora ramorum. Plant Health Progress. PHP 2013-0730-01-RS.

Wang, X., Zhu, X., Tooley, P.W., Zhang, X. 2013. Molecular cloning and functional analysis of three genes encoding polygalacturonase-inhibiting proteins from Capsicum annuum, and their relation to increased resistance to two fungal pathogens. Plant Physiology. 81:379-400.

Tooley, P.W., Browning, M.E., Leighty, R.M. 2013. Inoculum density relationships for infection of some Eastern forest species by Phytophthora ramorum. Journal of Phytopathology. 161:595-603.

Last Modified: 05/24/2017
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