Identification, laboratory, greenhouse and field handling of Aphanomyces euteiches on pea (Pisum sativum)

Authors: ZITNICK-ANDERSON, KIMBERLY - North Dakota State University; Porter, Lyndon; Hanson, Linda; PASCHE, JULIE - North Dakota State University

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/12/2021
Publication Date: 9/13/2021
Citation: Zitnick-Anderson, K., Porter, L.D., Hanson, L.E., Pasche, J.S. 2021. Identification, laboratory, greenhouse and field handling of Aphanomyces euteiches on pea (Pisum sativum). Plant Health Progress. 22(3):392-403. https://doi.org/10.1094/PHP-01-21-0011-FI.
DOI: https://doi.org/10.1094/PHP-01-21-0011-FI

Interpretive Summary: Aphanomyces root rot is a major plant disease found commonly on peas, alfalfa and lentils throughout the world. Plants under field conditions are infected either by survival structures in the soil called oospores or by swimming spores called zoospores. Common symptoms of the pathogen on peas are infected roots that turn a honey-brown color and yellowing or dieing of lower leaves. The pathogen is capable of killing plants before they mature. Many of the plants susceptible to the pathogen are part of the legume family that are mostly plants containing seeds in pods. The pathogen can be isolated from the soil by collecting organic matter in the soil containing the oospores and placing the matter directly on healthy roots under moist conditions and isolating from the infected tissue that develops. The pathogen can also be isolated directly from infected roots from the field by using a selective-medium that contains specific chemicals that mostly promote only the growth of the pathogen on agar. Specific anatomical structures of the pathogen commonly observed under the microscope are described, as well as molecular techniques that can use the pathogens DNA to properly identify its presence in soil or plant tissue. Methods for storing the pathogen for long periods (one year) are discussed, as well as inoculation techniques that can be done in the greenhouse or field to assess plants for resistance to this pathogen or to assess chemical treatments that could be beneficial in managing the disease. Techniques to determine the inoculum levels of the pathogen in grower fields are also discussed. Determining inoculum levels can assist growers in making management decisions such as crop rotations. Aphanomyces has two different what are called virulence types, Type 1 or Type III, and the means to determine the virulence types is discussed. Identifying virulence types is important in determining which varieties of a certain crop will grower better or worse in certain fields.

Technical Abstract: Aphanomyces root rot caused by Aphanomyces euteiches is a major root rot pathogen on pea, lentil and alfalfa worldwide. The pathogen can survive in field soil via oospores for prolonged periods of time. The primary inoculum of A. euteiches are swimming biflagellated zoospores. The pathogen causes a golden-brown root rot on peas that can lead to chlorosis and necrosis of lower leaves and eventual plant death. The pathogen is capable of infecting 44 species of plants across 27 genera. A. euteiches can be isolated from soil using a wet-sieving technique and from infected tissue using a selective medium containing metalaxyl, benomyl, and vancomycin followed by hyphal tipping. Morphological features that characterize A. euteiches can generally be found on seven-day-old agar plates infested with the pathogen and include: coenocytic hyphae,oogonia, aplerotic or plerotic oospores, monoclinous or diclinous paragynous antheridia, and under appropriate aqueous conditions, zoosporangia and zoospores can be observed. The pathogen can be identified using universal internal transcribed spacer region primers, SCAR markers, and species-specific primers. Techniques such as the Frozen Soil Method and the Agar Slant Method have been developed to store A. euteiches successfully for one year while maintaining pathogenicity and aggressiveness. Reliable inoculation techniques using either dried grain inoculum colonized with the pathogen or zoospores developed via a Corn Kernal Method, Pea Broth Method, or Wheat Leaves Method, have been successful in evaluating host genetic resistance and chemical management solutions under greenhouse conditions. Effective field infestation techniques such as the Oatmeal Broth and Vermiculite Method and Cornmeal-sand Mixture Method have worked to conduct research under natural environmental conditions. Techniques have also been developed to assess the inoculum potential in grower fields using a soil-based method where plants are grown in field soil under greenhouse conditions and by determining oospore concentrations in the soil. There are two different virulence types of the pathogen classified as Type I and Type III and pea differentials have been developed to categorize isolates into these types.