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United States Department of Agriculture

Agricultural Research Service

Research Project: Efficacy of a Systems Approach for Producing Nursery Stock Free of Plant Pathogenic Phytophthora Species

Location: Horticultural Crops Research

2012 Annual Report


1a.Objectives (from AD-416):
To determine the effectiveness of a systems approach for producing nursery stock free of plant pathogenic Phytophthora species and to compare it to the current system by measuring the frequency of infestation by Phytophthora species before and after implementation of the systems approach.


1b.Approach (from AD-416):
Symptomatic and asymptomatic plant tissue will be sampled for the presence of Phytophthora. Soilborne Phytophthora will be sampled by removing cores from potting media (container stock) or soil (field-grown plants). Water samples from water sources will be baited with rhododendron leaves. Documents SCA with Oregon State Univ.


3.Progress Report:

Establish disease threshold levels for waterborne Phytophthora spp. P. pini (formerly P. citricola) is one of the most common Phytophthora species in Oregon nurseries sampled. We conducted detached leaf assays with Rhododendron ‘Catawbiense Boursault’ to determine disease threshold levels from waterborne inoculum of P. pini. Leaf dips in zoospore inoculum of at least 10,000 zoospores per mL is required for consistent symptom development under optimal environmental conditions, with a disease incidence threshold of about 2,000 zoospores per mL. Threshold dose is influenced by plant factors (wounded vs. nonwounded leaves) as well as the degree of mechanical agitation of zoospores subjected to pumping through irrigation systems. Disease development was most extensive for nonagitated zoospores, and was only slightly reduced by moderate agitation which resulted in zoospore encystment. Zoospore inoculum subjected to pumping through an irrigation apparatus under controlled conditions in the greenhouse and lab resulted in drastically reduced symptom development. Flow cytometry and scanning electron microscopy were employed to quantify and visualize effects on zoospore inoculum.

Mitigate contamination of healthy plants from outdoor infested soil/gravel beds. Contaminated ground is the most common critical control point for Phytophthora contamination in Oregon container nurseries (Parke et al. 2008). Recurrent infections can occur when healthy container plants are set on ground that harbors infested plant debris and spores of the pathogen. Water runoff from infested soil can also lead to contamination of recycled water used for irrigation. Effective ways to decontaminate infested nursery soil continues to be one of the most formidable challenges in limiting the persistence and spread of Phytophthora diseases, and this is particularly critical for the quarantine pathogen P. ramorum. Approximately 86% of soilborne P. ramorum inoculum in Washington nurseries was from the organic layer or the top 0-5 cm of soil, with no detections occurring below 10 cm (Dart et al. 2007). Chemical fumigants and drenches were investigated for elimination of P. ramorum from soil (Yakabe & MacDonald 2010). While several fumigants were effective, the authors noted that many nurseries cannot fumigate because conventional shank fumigation equipment is not designed for use in compacted, gravel-laden soil. We are investigating soil solarization as a means to disinfest nursery beds. Plots were established in two locations: NORSDUC in San Rafael, CA, and the Botany and Plant Pathology Farm in Corvallis, OR. Infested leaf inoculum of P. ramorum (CA) and P. pini (OR) was placed in sachets and buried in soil columns at 5, 15, and 30 cm depths. Half of the plots were covered with 6-mil infrared, anti-condensation clear plastic film and the other half were left uncovered. Temperature and water content at each soil depth are being monitored continuously with TDR reflectometers connected to a datalogger. After solarization, the biocontrol agent Trichoderma asperellum will be added to a portion of both solarized and nonsolarized plots. Sachets will be recovered at periodic intervals so that leaf inoculum can be plated to determine pathogen viability. Microbial community changes resulting from solarization and addition of the biocontrol agent will also be determined from soil samples collected over time.

Mitigate contamination of healthy plants from used containers. Growers of container plants re-use pots as a cost-saving measure. While this practice also conserves energy and reduces waste, we have shown that debris in re-used pots harbors Phytophthora species and other soilborne pathogens and weed seeds. These contaminants can cause disease and weed problems for plants subsequently grown in these pots, increasing pesticide use and labor costs, and potentially spreading diseases from nursery to nursery. Washing of pots followed by treatment with a chemical disinfectant is prohibitively expensive for most growers. Pasteurization with aerated steam is an effective means of disinfesting pots, but most nurseries lack the equipment required to accomplish this. We are testing solarization as a low-tech method for disinfesting used containers. Used 1-gallon pots were obtained from a commercial nursery. Pots (2,000) were nested, and stacked on each of four pallets. Sachets containing infested leaf inoculum of Phytophthora pini, Rhizoctonia solani, and Pythium debaryanum were placed inside pots at assigned locations within each pallet load along with iButton temperature recorders. Each pallet was covered with 6-mil clear plastic polypropylene (Thermax) placed inside each of three closed, non-air-conditioned greenhouses at the Hysslop field station, Corvallis. An additional pallet was placed outside the greenhouses. After two weeks, the sachets and temperature recorders will be recovered, and the viability of pathogen tested by growth on selective media.

Determine diversity and distribution of Phytophthora species in nurseries. We sampled Phytophthora species in several nurseries and determined species identity using Phytophthora-ID (Grünwald et al. 2011). P. syringae and P. plurivora were the most abundant taxa found. Furthermore, each nursery had a unique and distinct Phytophthora community. We are further determining if Phytophthora populations are clonal and whether populations are differentiated among nurseries. This work will improve best management practices by establishing patterns of pathogen movement within and among nurseries. This research was conducted in support of objective 2C of the parent project.


Last Modified: 9/23/2014
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