Location: Floral and Nursery Plants Research
Project Number: 8020-22000-042-10-S
Project Type: Non-Assistance Cooperative Agreement
Start Date: Sep 1, 2018
End Date: Aug 31, 2020
Objective 1: Identify the biological parameters affecting environmental persistence of R. solanacearum Race 3 biovar 2 strains. Objective 2: Identify the traits that underlie the unique cold virulence of R3bv2 strains. Objective 3: Identify the basis of latent infection of plants by R3bv2 strains of R. solanacearum.
Effective phytosanitation remains the best way to prevent introduction of R3bv2 to the U.S. Although R. solanacearum Race 3 biovar 2 (R3bv2) has long been a high-concern pathogen for potato and ornamental growers worldwide and was listed as a U.S. Select Agent in 2002, there are no validated methods for eradicating this pest from environmental sources of infection. We will define the conditions necessary to kill several representative strains of R3bv2 in water, on surfaces, and in debris of infected plants. Methods used will include heat treatment, dessication, UV irradiation, chemical cell lysis, chloride, and various commercial forms of hydrogen peroxide. The goal is to publish a set of peer-reviewed and validated eradication protocols useful for growers, researchers, and regulatory scientists and officials. This objective extends research initiated under USDA-ARS SCA 58-1230-8-446. The supposedly unique cold tolerance of R3bv2 strains is the basis of their Select Agent designation. Understanding the fundamental biology underlying this trait is therefore of considerable interest. By comparing the bacterial genes expressed at cool and warm temperatures in R3bv2 strains, we identified several candidate cool tolerance genes, notably a mannose-binding lectin (LecM) that we found was necessary for full virulence at cool (but not warm) temperatures and also for normal persistence in potato tuber tissue at storage temperatures. We will use defined LecM mutants of R3bv2 to test the hypothesis that at cool temperatures this protein helps R3bv2 cells attach to plant surfaces and to other bacteria, thereby increasing adaptation to cold environmental conditions. R3bv2 easily forms latent infections of many ornamental plants, most notably geraniums. This trait makes it hard to identify infected plants. Shipping cuttings from infected but symptomless plants can unknowingly disseminate this pathogen. We will test the hypothesis that during latent infections, R3bv2 do not activate the PhcA regulator that “turns on” the pathogen’s virulence functions. The goal of this objective is to use chemical treatment to activate R3bv2 virulence, making latently infected plants easy to see and remove.