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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

2013 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:

This research was conducted in support of NP303 objective 2C of the parent project. Systems approach research indicated three major critical control points for Phytophthora contamination in Oregon nurseries: contaminated ground, used containers, and infested irrigation water. Research in the last year has addressed management of each of these critical control points.

Contaminated ground: Inoculum of Phytophthora spp. in nursery soil and gravel beds leads to recurrent infection in many nurseries. Growers have few options of disinfesting contaminated ground because of increasingly strict regulations on the use of soil fumigants. We investigated soil solarization as a method for disinfesting compacted nursery soils from Phytophthora pini and P. ramorum in field trials in California and Oregon. Soil solarization under the test conditions in California appears to offer an effective means of eliminating P. ramorum from at least the upper layers (top 15 cm) of the soil profile, where most of the naturally occurring inoculum is located.

Used containers: Inoculum of Phytophthora pini and two other plant pathogens, Rhizoctonia solani, and Pythium irregulare, were placed in sachets as a bioassay for determining the effectiveness of solarization for disinfesting used containers. Sachets and dataloggers were placed in 1-gal containers at five different locations within pallets each holding 2100 nested pots. Pallets of pots were placed outdoors at each of four sites: the OSU Botany and Plant Pathology Farm, the Oregon State University (OSU) West Greenhouse area, and the OSU Hysslop Farm. One pallet was also placed inside a closed greenhouse at Hysslop Farm. After two weeks, dataloggers were removed and sachets were assayed for pathogen viability. P. pini was assayed by baiting with leaf disks and plating onto semi-selective media (PARPH).

Solarization of pots was inconsistent in eliminating soilborne pathogens except when pallets were placed inside the greenhouse. Although recovery of Phytophthora pini was 0% from solarized pots, it was only 23% from the lab, suggesting that another factor such as low moisture or immature chlamydospores reduced survival of this pathogen across locations. Temperature maxima ranged from 37°C to 73.5°C. Temperatures were highly variable among pot position within the pallet load and across locations. All of the pot positions of the pallet solarized within the greenhouse achieved at least 20 hours above 50°C. One factor that could have contributed to inconsistent kill was lack of moisture throughout the pallet load. Heat could also have been lost from the base of the pallet not covered by the plastic film.

Infested irrigation water: We tested the relationship between P. pini inoculum dose and disease severity in a detached rhododendron leaf assay. At inoculum doses below 10,000 zoospores mL-1, P. pini was unable to consistently infect leaves unless the leaves were wounded. Treatment of zoospores that resulted in encystment, such as pumping them through a spray apparatus, or physically agitating them by vortexing, also reduced their infectivity. These results suggest that infested irrigation water is unlikely to provide inoculum at levels necessary to cause foliar infection of P. pini under most circumstances. Dose-response relationships for other Phytophthora species have not been evaluated.

We also compared several commercial algaecides and a locally-produced rhamnolipid biosurfactant for their effectiveness in lysing zoospores of Phytophthora ramorum and preventing infection of rhododendron leaf disks in vitro. Pro-Clear (aluminum chlorhydrate) and Algae-Off (sodium carbonate peroxyhydrate) were effective at the labeled rate, as was the AGAE rhamnolipid surfactant at the 100 mg L-1 rate.

Future experiments Contaminated ground: Efforts are underway to model the survival of P. ramorum and P. pini in relation to soil temperature and moisture. Additional research is aimed at understanding how to maximize the effectiveness of solarization in killing soilborne Phytophthora species in commercial nurseries in California, Oregon, and Washington, and to determine how the presence of crushed rock on the soil surface affects soil heating. Used containers: Solarization of used containers will be conducted with the addition of moisture to improve the effectiveness of lethal heating. Irrigation water: Additional research will include comparison of the locally-produced biosurfactant and the recently labeled surfactant, Zonix, for effectiveness in lysing zoospores of P. ramorum.


Last Modified: 10/23/2014
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