California Ag Network

Tag: Fungus

  • War on Weeds Takes Toll on Beneficial Bacteria in the Soil

    Ithaca, N. Y., (September 28, 2017) – As farmers battle in their above-ground war on weeds, they may inadvertently create underground casualties – unintentionally attacking the beneficial bacteria that help crops guard against enemy fungus, according to Cornell University research.

    Specifically, Cornell researchers found negative consequences of the weed-killing herbicide glyphosate on Pseudomonas, a soil-friendly bacteria.

    “Beneficial Pseudomonas in the soil can help crops thrive. They can produce plant-stimulating hormones to promote plant growth and antifungals to defeat problematic fungi – such as Pythium and Fusarium – found in agricultural soil, but previous studies reported that the abundance of beneficial bacteria decreased when the herbicide glyphosate seeps underground,” said Ludmilla Aristilde, assistant professor of biological and environmental engineering. “Our study seeks to understand why this happens.”

    Soil bacteria require their proteins – composed of amino acids – and their metabolism to support cellular growth and the production of important metabolites to sustain their underground fight. But glyphosate applied to crops can drain into the soil and disrupt the molecular factories in the bacterial cells in some species, interfering with their metabolic and amino acid machinery.

    The new findings show that glyphosate does not target the amino acid production and metabolic gadgetry equally among the Pseudomonas species. For example, when Pseudomonas protegens, a bacteria used as a biocontrol agent for cereal crops, and Pseudomonas fluorescens, used as a fungus biocontrol for fruit trees, were exposed to varying glyphosate concentrations, the researchers noted no ill effects. However, in two species of Pseudomonas putida, used in soil fungus control for corn and other crops, the bacteria had notably stunted growth, said Aristilde, who is a faculty fellow at Cornell’s Atkinson Center for a Sustainable Future.

    “Thus, if a farmer is using Pseudomonas fluorescens as a biocontrol, then it is probably okay to use glyphosate,” Aristilde said. “But if the farmer uses Pseudomonas putida to control the fungus in the soil, then glyphosate is more likely to prevent the bacteria from doing its job.”

    The study offers molecular details for why glyphosate adverse effects on Pseudomonas are species-specific. “That’s actually good news because – as a society – we will likely not stop using herbicide completely,” said Aristilde. “If that is the case, farmers need to know which beneficial soil biocontrol they’re using can be susceptible. If they’re using a strain that is susceptible and conflicting with their herbicide application, then it is a problem. That’s the bottom line.”

    “Glyphosate-Induced Specific and Widespread Perturbations in the Metabolome of Soil Pseudomonas Species” was published in Frontiers of Environmental Science. The research was funded by the U.S. Department of Agriculture’s National Institute of Food and Agriculture; the National Science Foundation; and the Academic Venture Fund at Cornell’s Atkinson Center for a Sustainable Future.

  • Fungal Hunter Prowls Soil for Plant Pathogen

    Beltsville, Maryland, (August 7, 2017) – Viewed through a microscope, the fungus Trichoderma asperellum resembles little more than delicate branched filaments. But make no mistake; it is a formidable hunter—of other microorganisms, that is.

    Since 2006, Agricultural Research Service (ARS) plant pathologist Tim Widmer and colleagues have conducted laboratory and field tests to gauge T. asperellum’s potential to biologically control Phytophthora ramorum. A plant pathogen, P. ramorum is the culprit behind sudden oak death, a disease of oak and other hardwood trees in coastal forests of California and Oregon.

    Nursery growers are familiar with a different manifestation of P. ramorum, known as “ramorum blight.” The disease afflicts rhododendron, viburnum, camellia, and other woody ornamental plants.

    Chemical fumigation and soil sterilization are two common methods of keeping nursery stock blight free-and compliant with federal and state quarantine regulations meant to prevent the pathogen’s spread.

    Now, in collaboration with BioWorks, Inc., of Victor, New York, Widmer is evaluating ways to commercially formulate the T. asperellum fungus as a biobased alternative to such soil treatments, which can be costly (upwards of $3,900 an acre for some fungicides), dangerous to use, and harmful to beneficial soil organisms. The effort includes trials funded by the U.S. Farm Bill.

    “This fungus, T. asperellum, is a mycoparasite, meaning it will actually attack and kill P. ramorum. It does this by penetrating different spore forms that the pathogen uses to survive and reproduce,” says Widmer, who is with the ARS Foreign Disease-Weed Science Research Unit in Fort Detrick, Maryland.

    In petri dish experiments at Widmer’s laboratory, and later in trials with potting mix at the National Ornamentals Research Site at Dominican University of California in San Rafael, the biocontrol fungus reduced P. ramorum levels by 60 to 100 percent, depending on which of the 12 strains of T. asperellum were tried. A chemical fungicide achieved similar results, but its effects proved temporary: 8 weeks later, the pathogen reemerged in the potting mix. Widmer suspects the fungicide temporarily halted the growth of the pathogen, but didn’t kill it.

    Under unfavorable soil conditions, P. ramorum can survive within thick-walled survival pods called “chlamydospores.” But T. asperellum specializes in breaching such defenses. In its branched form, called “mycelia,” the fungus coils around the chlamydospores and degrades their cell walls with powerful enzymes. The biocontrol fungus then enters the chlamydospores to feed, germinate, and start the cycle over again until little or no pathogen remains in the soil.

    Indeed, in small-scale nursery trials conducted with the California Department of Food and Agriculture, drench treatments of the fungus reduced the pathogen to undetectable levels during the study’s entire 3-month monitoring period.

    Buoyed by those results, Widmer and his colleague Gary Samuels, a now-retired ARS mycologist, applied for and received a patent on the top-performing fungal strain, known as “04-22.”

    BioWorks has since licensed the strain and is collaborating with Widmer on developing and registering formulations that can be applied to nursery soils and potting mixes. Developed commercially, it could give growers another tool for battling ramorum blight, helping to safeguard vulnerable ornamental crops and forests alike. — By Jan Suszkiw, ARS Office of Communications.

     

    Left: Microscopic image of a Phytophthora ramorum spore. Right: Microscopic image of the Trichoderma asperellum fungus attacking a Phytophthora ramorum spore. (Photo by Tim Widmer D3826-1 | D3827-1)

    Tanoak tree leaves killed by Phytophthora ramorum. (Joseph O’Brien, USDA Forest Service, D3825-1)