Project Details:

Characterization of Phytophthora sojae and Phytophthora sansomeana populations in the North Central region & an assessment of management strategies

Parent Project: This is the first year of this project.
Checkoff Organization:North Central Soybean Research Program
Categories:Soybean diseases
Organization Project Code:NCSRP
Project Year:2017
Lead Principal Investigator:Anne Dorrance (The Ohio State University)
Co-Principal Investigators:
Alison Robertson (Iowa State University)
Douglas Jardine (Kansas State University)
Martin Chilvers (Michigan State University)
Kiersten Wise (Purdue University)
Emmanuel Byamukama (South Dakota State University)
Santiago Mideros Mora (University of Illinois at Urbana-Champaign)
James Kurle (University of Minnesota)
Dean Malvick (University of Minnesota)
Loren Giesler (University of Nebraska)
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Information and Results

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Project Summary

Phytophthora sojae is present in many fields across the North Central region and Ontario, Canada, and had been managed very successfully with the deployment of single resistance Rps genes (Rps1a, Rps1c, Rps1k, Rps3a, and Rps6) as well as partial resistance (field resistance or tolerance). However, there are an increasing number of reports where varieties are sold that have Rps genes that are no longer effective towards the regional population or lack sufficient levels of partial resistance. The consequence is stand loss through damping-off or the development of stem rot throughout the season which results in lower yields and added weed pressure.

The most recent survey of P. sojae pathotypes (races) was completed during 2012-2013 with funding provided by Monsanto in which 213 unique pathotypes were identified among 873 isolates collected from 202 fields in 11 states. Two key findings from this sampling included: i) a greater number of regions have a higher proportion of isolates with virulence to key Rps genes, such that the resistance gene will no longer be effective as well as, ii) isolates across the region continue to increase in complexity. This indicates that the ability to recycle Rps genes is highly unlikely. There were several challenges in this study, the first was baiting from soil during a drought year (2012) such that there was low recovery; due to sample processing few isolates were recovered (WI); and few fields were actually sampled (MI, MO, NE, and SD all had less than 10 fields).

In another recent study funded by soybean checkoff dollars (2010-2013), the genotypic diversity among P. sojae populations was examined. That genetic diversity exists in this pathogen has been highly controversial, but with new markers, and in-depth sampling of populations recovered in the 2000s, regional populations of P. sojae in the Midwest were identified. As a soil borne organism, it was believed that this P. sojae should be clonal (every individual is the same within a field and between neighboring fields) – but the findings from this study clearly indicated that the P. sojae populations in the Midwest are not clonal.

These findings indicate that a more thorough assessment of the P. sojae populations in the North Central region is needed. In addition, new Rps genes have been identified but their effectiveness is still highly questionable. Among the 15 Rps genes that were identified prior to 2001, only 5 were effective and incorporated into varieties.

A second Phytophthora, P. sansomeana, also appears to be contributing to seed and seedling damping-off of soybean at greater incidence than previously thought, both from surveys previously reported in Illinois (Malvick), Ohio (2003 and 2015 data), and region-wide (Chilvers USDA-AFRI project data). This is intriguing as P. sansomeana has a larger host range (corn, douglas fir, soybean). As part of this proposes study, state populations of P. sansomeana will also be examined for host range, resistance in current soybean cultivars, and efficacy of seed treatment fungicides.

Project Objectives

Recover Isolates (Obj 1 & 3)
Evaluate sources of resistance
Evaluate Fungicide Sensitivities
Combination of host resistance/seed treatments
Outreach/Information transfer
Crop Protection Outputs
Outreach/preliminary findings
Soybean Breeders Workshop

Project Deliverables

1. Recover P. sojae from fields within each state and characterize for pathotype and genetic diversity.
• Isolate collection of >2000 isolates (200 isolates per state)
• Improved understanding of diversity of pathotypes across region will inform of areas were Rps genes can and cannot be deployed
• Improved knowledge of genetic diversity across the region

2. Evaluate the new sources of resistance to these regional populations.
• Identify Rps genes that are effective for each of collaborating states
• Improved deployment of soybean varieties with Rps genes across the region will result in fewer losses due to Phytophthora root rot

3. Recover P. sansomeana from fields within each state and characterize for host range, genetic diversity.
• Isolate collection of >600 isolates (50 isolates per state)
• Improved understanding of phenotypic and genetic diversity within this species
• Improved knowledge of distribution and prevalence of the pathogen across the region
• Knowledge of effectiveness of Rps genes against pathogen

4. Establish sensitivities (EC50) values for P. sojae and P. sansomeana isolates recovered from fields towards the new active ingredients ethaboxam, strobilurin, and oxathiapiprolin fungicides.
• Establish baseline levels of fungicide sensitivity to ethaboxam and oxathiapiprolin before these active ingredients are widely used.
• Monitor sensitivity to strobilurins and detect reduced sensitivities that may have occurred in past few years based on published data (Broders et al 2007; Matthiesen et al 2016; Rojas et al, submitted) due to widespread use in seed treatments containing strobilurins.
• Information on placement of active ingredients across the region

5. Comparison of new seed treatments on varieties with different resistance packages (Rps gene(s)/partial resistance) in field trials.

6. Evaluation of potential herbicide interactions with the development seed rot and stand loss.
• Field trials will be established at least 2 locations per state during 2017 and 2018.
• Effect of different resistance packages on soybean disease and midseason PRR.
• Detect interactions between pre-emergence herbicides and watermold pathogens that increase risk of stand loss due to seedling disease
• Economic benefits/costs for the each management tactics.

7. Development of Crop Protection Network Outputs on the Management of Phytophthora spp. that infect soybean in the North Central and Ontario regions.
• Development of management guide

Progress of Work

We are very excited to report that we are quickly reaching our first goal of sampling fields for baiting and have reached 92 fields sampled in 6 states for 920 soil samples (10 per field). Fields have already been identified to complete the sampling this spring and baiting has begun on some of the fall samples.
Lines to develop into differentials carrying putative new sources of resistance will be the focus of the next few weeks. With the goal for seed increase this summer.
The team will have a conference call on April 3 to finalize field studies for the summer. Our first goal is to evaluate the interaction of herbicides and the development of Phytophthora seedling blight.

Project administration - all of the subcontracts are now active.
Kentucky (Carl Bradley) will have a similar project funded from KY

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Final Project Results

Benefit to Soybean Farmers

Performance Metrics

Project Years