|SHARMA-POUDYAL, DIPAK - WASHINGTON STATE UNIVERSITY|
|DU TOIT, LINDSEY - WASHINGTON STATE UNIVERSITY|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2016
Publication Date: 7/7/2016
Citation: Sharma-Poudyal, D., Paulitz, T.C., Du Toit, L.J. 2016. Timing of glyphosate applications to wheat cover crops to reduce onion stunting caused by Rhizoctonia solani. Plant Disease. 100:1474-1481.
Interpretive Summary: Rhizoctonia spp. cause stunting and patches in irrigated onions in the Columbian Basin of Washington State. The pathogen is carried over from a cover crop of wheat planted the previous fall, to hold the soil and keep it from blowing away. When the cover crop is sprayed out with herbicides, the pathogen can build up on the roots and bridge or transfer to the onion crop. This research looked at timing of Roundup applications before planting of onions, to see if a longer sprayout interval could reduce disease. This research showed that increasing the sprayout interval from 3 days to 19 and 27 days, disease could be significantly reduced.
Technical Abstract: Stunting caused by Rhizoctonia spp. is economically important in irrigated onion bulb crops in the semi-arid 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 the onion seedlings against wind- and sand-blasting, but not tall enough to shade the onion seedlings. This cover crop serves as a green bridge to carry Rhizoctonia spp. inoculum from the cereal roots to developing onion seedlings, 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 carryover 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; and 19, 10, and 3 days before onion seeding in 2012 and 2014, respectively. As the interval between herbicide application and onion planting was 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% in these trials. Similarly, the concentration of R. solani AG 8 DNA detected in soil sampled from the dead cover crop rows declined as the interval between glyphosate application to the cover crop and onion seeding increased in the 2012 trial, but not in the 2014 trial. The concentration of R. solani AG 3 and R. solani AG 8 DNA detected in soil sampled from the cover crop rows was significantly positively correlated with the number of patches of stunted onion plants (r = 0.49 and 0.61 at P = 0.039 and 0.008, respectively), total area of stunted patches (r = 0.50 and 0.66 at P = 0.035 and 0.003, respectively), and patch severity index (r = 0.49 and 0.64 at P = 0.038 and 0.005, respectively) in the 2012 trial; however, in the 2014 trial these correlations were only significant for R. solani AG 3 DNA concentration detected. The significant reduction in onion stunting resulting from a longer interval between herbicide application to the cover crop before onion planting provides a practical cultural method of minimizing the impact of stunting on onion bulb production.