Timing of Glyphosate Applications to Wheat Cover Crops to Reduce Onion Stunting Caused by Rhizoctonia solani

Abstract

Stunting caused by Rhizoctonia spp. is economically important in irrigated onion bulb crops in the semiarid Columbia Basin of Oregon and Washington, where cereal winter cover crops commonly are planted the previous fall to prevent wind erosion of soil. The cover crop is killed with herbicide application just before or shortly after onion seeding, so that the dead rows of cereal plants provide a physical barrier tall enough to protect onion seedlings against wind and sand blasting but not tall enough to shade onion seedlings. However, the cover crop also serves as a green bridge for Rhizoctonia spp. on cereal roots to colonize the onion roots, potentially resulting in severe stunting of onion seedlings. To determine the effect of timing of application of the herbicide glyphosate to reduce this green bridge effect and, subsequently, onion stunting, three herbicide application intervals preceding onion planting were evaluated in a grower’s onion field in each of 2012 and 2014 in the Columbia Basin. The wheat cover crop was killed with a glyphosate application 27, 17, and 3 days before onion seeding in 2012 and 19, 10, and 3 days before seeding in 2014. As the interval between herbicide application and onion planting increased from 3 days to 19 and 27 days, the number of patches of stunted onion plants decreased by ≥55%, total area of stunted patches decreased by 54 to 63%, and patch severity index decreased by 59 to 65%. Similarly, the Rhizoctonia solani AG 8 DNA concentration in soil sampled from the dead cover crop rows declined as the interval between glyphosate application and onion seeding increased in the 2012 trial but not in the 2014 trial. R. solani AG 3 and AG 8 DNA concentrations in soil sampled from the cover crop rows were significantly positively correlated with the number of patches of stunted onion plants (r = 0.490 and 0.607 at P = 0.039 and 0.008, respectively), total area of stunted patches (r = 0.496 and 0.659 at P = 0.035 and 0.003, respectively), and patch severity index (r = 0.492 and 0.635 at P = 0.038 and 0.005, respectively) in the 2012 trial; however, these variables were only correlated significantly with R. solani AG 3 DNA concentration in the 2014 trial. Increasing the interval between herbicide application to the cover crop and onion planting provides a practical management tool for stunting in onion bulb crops.

Time Interval Between Cover Crop Termination and Planting Influences Corn Seedling Disease, Plant Growth, and Yield

“…Previously, Bakker et al (2016) showed that rye cover crop roots are hosts to pathogens that can infect corn seedling roots and that the densities of some of these pathogens on rye roots increased with time after cover crop termination as the roots decomposed. Additionally, other studies (Dabney et al 1996;Sharma-Poudyal et al 2016;Smiley et al 1992) have also demonstrated with different plant species that increasing the time interval between termination of a cover crop or volunteer plants and planting of the main crop decreased seedling disease and increased growth of the main crop. Thus, it seems likely that increased infection rates by corn seedling root pathogens is a potential cause of corn growth and yield reductions following a rye cover crop.…”

“…They hypothesized that a longer time interval between termination of volunteer wheat and planting of a spring barley crop allowed less wheat growth, which reduced the mass of wheat roots serving as a food base for soilborne pathogens and allowed more time for competing soil microorganisms to reduce pathogen inoculum levels in the dead wheat roots. Similarly, Sharma-Poudyal et al (2016) found that increasing the time interval between herbicide application to a wheat cover crop and onion planting reduced the infection of onion roots by Rhizoctonia spp. Finally, Dabney et al (1996) reported that sorghum root disease incidence decreased when the time interval between spraying legume cover crops and planting sorghum was increased.…”

How glyphosate affects plant disease development: it is more than enhanced susceptibility

“…A recent study with onion further supports a need to evaluate timing of glyphosate application. 67 Wheat is grown as a cover crop in onion production in Washington state, USA, and this cover crop provides protection of young onion plants. Killing the wheat with glyphosate within a few days of planting onion results in more stunting of onion caused by Rhizoctonia.…”

Beheersing van bodempathogenen via bodemgezondheidsmaatregelen : Een overzicht van de beschikbare kennis voor een selectie van akkerbouwgewassen met hun bijbehorende bodemziekten

“…• Meer Rhizoctonia solani in gerst (Smiley et al, 1992) en ui (Sharma-Poudyal et al, 2016) als snel gezaaid werd na inbrengen van met glyfosaat doodgespoten wintertarwe (Smiley et al, 1992).…”

“…• Het snel na doodspuiten met glyfosaat inbrengen van gerst (Smiley et al, 1992), ui (Sharma-Poudyal et al, 2016) en wintertarwe (Smiley et al, 1992) leidde tot meer Rhizoctonia solani (lakschurft).…”