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Two New Races of Phytophthora phaseoli from Lima Bean in Delaware
T A Evans1, C R Davidson1, J D Dominiak1
1Department of Plant and Soil Sciences, University of Delaware, Newark 19717.
Plant Disease
|March 2, 2019
Summary
New races (E and F) of downy mildew (Phytophthora phaseoli) have emerged, overcoming resistance in lima bean cultivars. These new races threaten commercial lima bean production, necessitating updated disease management strategies.
Area of Science:
- Plant Pathology
- Agricultural Science
Background:
- Downy mildew, caused by Phytophthora phaseoli, is a significant disease affecting lima bean (Phaseolus lunatus L.) production on the U.S. East Coast.
- Commercial lima bean crops in Delaware, Maryland, and New Jersey have faced serious threats from this disease for the past five years.
- Current management strategies include resistant cultivars and copper hydroxide fungicides, which are being challenged by new pathogen races.
Purpose of the Study:
- To identify and characterize new pathogenic races of Phytophthora phaseoli affecting lima beans.
- To assess the susceptibility of existing lima bean cultivars to these newly identified races.
- To provide insights into the potential environmental factors influencing the prevalence of different Phytophthora phaseoli races.
Main Methods:
- Isolation and identification of Phytophthora phaseoli from infected lima bean pods.
- Determination of pathogen races using a modified cultivar differential system.
- Evaluation of in-field weather station data in conjunction with disease occurrence.
Main Results:
- Two new pathogenic races of Phytophthora phaseoli, designated E and F, were identified in Delaware in 1995 and 2000, respectively.
- Race E infects lima bean cv. 184-85, and Race F infects lima bean line BG2-408, both previously resistant cultivars.
- In the 2000 growing season, 86% of collected isolates were Race E and 5% were Race F, indicating their prevalence.
- Races E and F may possess higher temperature optima for infection compared to Race D.
Conclusions:
- The emergence of new Phytophthora phaseoli races (E and F) poses a significant challenge to current lima bean disease management.
- Existing resistant cultivars are susceptible to these novel races, highlighting the need for continuous monitoring and development of new resistant varieties.
- Understanding the environmental factors, such as temperature, associated with new races is crucial for predicting and managing downy mildew outbreaks.
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