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United States Department of Agriculture

Agricultural Research Service


Location: Foreign Disease-weed Science Research

2008 Annual Report

1a. Objectives (from AD-416)
Oomycete plant pathogens cause a wide range of serious diseases of great concern to U.S. agriculture, and some are of recent emergence or reemergence. Obtaining knowledge of the biology, taxonomy, ecology, and pathology of selected diseases caused by members of this group is key to developing improved management practices and pest risk assessments which will allow damage caused by such diseases to be reduced. Our first objective is to determine basic knowledge of the biology, taxonomy, ecology, and pathology of selected Oomycete pathogens as the basis for development of improved control/management strategies and pest risk assessments. Our second objective is to determine the response of selected host species to inoculation with selected Oomycete pathogens to determine susceptibility of plant species of agricultural and commercial importance as well as naturally occurring plant species. Our proposed studies will focus on sudden oak death caused by Phytophthora ramorum and brown stripe downy mildew of maize caused by Sclerophthora rayssiae var. zeae. Attainment of our objectives will benefit U.S. agriculture by contributing new knowledge regarding biology, infectivity, spread potential, and survival ability of destructive Oomycete pathogens thus facilitating the development of improved regulatory, management, and control strategies.

1b. Approach (from AD-416)
Utilize specialized plant pathogen containment facilities to investigate and characterize biology, taxonomy, ecology, and pathology of selected Oomycete diseases considered to be a threat to the U.S. Maintain isolates of exotic and emerging endemic Oomycete pathogens and elucidate factors related to their life cycles, host range, infectivity, and survival characteristics. Assess factors critical in epidemiology of diseases caused by exotic and emerging Oomycetes using a variety of scientific methods and approaches.

3. Progress Report
For our first objective, we performed in vitro assays that have identified ten different soil and epiphytic leaf microorganisms that are inhibitory towards the sudden oak death pathogen, P. ramorum. Three isolates have been chosen for further testing on whole plants. Formulations for application of the antagonists have been tested to study their impact on spore germination, growth, sporulation and host toxicity. In addition, five endophytes from Viburnum cultivars showed in vitro inhibition towards P. ramorum growth. Work is progressing on developing tissue culture plants of Rhododendron and Viburnums to test these endophytes. Four Trichoderma asperellum isolates were also selected for further testing based upon their in vitro mycoparasitism against P. ramorum. Tests have been started to determine if adding T. asperellum to P. ramorum-infested soil or infected leaves will remove the pathogen from the infested material. We also demonstrated the presence of P. ramorum sporangia on infected roots and chlamydospores in infected host plant roots. We showed that sporangia of P. ramorum can be detected in runoff from plants with root infections. Spread of the pathogen from nursery sites to natural areas through contaminated runoff has been observed in Florida and Mississippi. There is also uncertainty about the role of chlamydospore dormancy in the long-term survival of P. ramorum. We studied vital staining to determine the status of non-germinating chlamydospores, and found that fluorescein diacetate derivatives show promise in determining whether spores are living or dead. We adapted and optimized a real-time polymerase chain reaction (PCR) assay to detect P. ramorum, for use with a Cepheid 'SmartCycler' machine, the type of machine used by many university and state laboratories whose job is pathogen detection. This work will allow these groups to use our assay, which targets a DNA sequence different from that used in other PCR assays. The assay we developed can be used alone or with other assays to provide a secondary determination for borderline samples. We have developed techniques for producing oospores and of P. kernoviae, a foreign pathogen which is a threat to U.S. forests. Studies of oospore maturation are also in progress. We have initiated a long term experiment to examine the impact of high and low temperatures on the survival of P. kernoviae oospores and data have been collected at the three month time point. In addressing our second objective, we have nearly completed screening of over twenty cultivars of Viburnum for their susceptibility to P. ramorum. In addition, we have screened 25 plant species which comprise part of the understory of Eastern oak forests for their susceptibility to P. ramorum and evaluated the propensity for the pathogen to sporulate on each species. This information will be of use in attempts to survey Eastern US forest regions for the presence of P. ramorum, and in pest risk assessments. NP 303 Component 2, Problem Statement 2C.

4. Accomplishments
1. Adaptation of a Phytophthora ramorum detection test to the SmartCycler. We adapted and optimized a previously described detection test for the sudden oak death pathogen, P. ramorum, for use with a 'SmartCycler' machine made by Cepheid Corporation. This machine is available more widely to end users such as state and national regulatory agencies, the Forest Service, and state and university plant diagnostic clinics, than are other machines for which the test was originally developed. The test was proven to be specific for the target pathogen, P. ramorum and did not result in false positive reactions with closely related species of Phytophthora which have proven to cross-react with other tests. The Smartcycler-based test was also more efficient and easier to use than when performed using previous equipment and can employ ready-to-use freeze-dried components in bead form. This accomplishment will broaden the use of the detection assay among end users and provides an improved detection assay which is easier and more efficient to use to inspect nurseries and survey wild areas for the presence of P. ramorum.

2. Screening of understory plant species for susceptibility to Phytophthora ramorum. P. ramorum is an oomycete plant pathogen that has caused substantial economic losses to the US nursery industry, and also threatens oak forests of the Eastern US. We evaluated the susceptibility of 25 plant species which make up a portion of the understory in forests of the Eastern US, to infection by P. ramorum. Fourteen species in addition to the control showed infection of over 90% of their leaves. The number of airborne spores (sporangia) and long lived resting spores (chlamydospores) produced on the plant species varied greatly as well. The results show that many common understory species in Eastern US forests are susceptible to P. ramorum, and will help state and federal regulators in assessing the threat posed by this pathogen to forest ecosystems. This research falls under Component 2 of the National Program 303 (Plant Diseases) Action Plan: Biology, Ecology, Epidemiology, and Spread of Plant Pathogens and their Relationships with Hosts and Vectors, and addresses Problem Statement 2C: Population Dynamics, Spread, and Epidemiology of Pathogens.

5. Significant Activities that Support Special Target Populations

Review Publications
Browning, M.E., Englander, L., Tooley, P.W., Berner, D.K. 2008. Survival of Phytophthora ramorum hyphae following exposure to temperature extremes and various humidities. Mycologia. 100:236-245.

Last Modified: 2/23/2016
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