|PATZEK, L - Washington State University|
|DU TOIT, L - Washington State University|
|JONES, S - Washington State University|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2013
Publication Date: 12/20/2013
Citation: Patzek, L.J., Du Toit, L.J., Paulitz, T.C., Jones, S.S. 2013. Stunting of onion caused by Rhizoctonia spp. isolated from the Columbia Basin of Oregon and Washington. Plant Disease. 97: 1626-1635.
Interpretive Summary: Bare patch of onion is a recent disease that has appeared in in the semi-arid Columbia Basin of Oregon and Washington, in which patches of severely stunted onion plants developed following rotation with winter cereal cover crops. We isolated Rhizoctonia groups from many fields in 2009 and 2010, identified them with DNA sequencing, and tested their pathogenicity in greenhouse experiments.R. solani AGs 2-1, 3, 4, and 8, as well as W. circinata var. circinata, and binucleate Rhizoctonia AG E were able to cause stunting. However, based on testing with a range of inoculum densitsies, Rhizoctonia solani AG-8 was the most virulent.
Technical Abstract: During 2009 and 2010, 45 isolates of Rhizoctonia spp. were recovered from onion bulb crops in the semi-arid Columbia Basin of Oregon and Washington, in which patches of severely stunted onion plants developed following rotation with winter cereal cover crops. Characterization of isolates recovered from naturally infested soil and roots was performed by sequence analysis of the ribosomal DNA (rDNA) internal transcribed spacer (ITS) region, with the majority of isolates (64%) identified as R. solani. In steam-pasteurized field soil, stunting of onion was caused by isolates of R. solani AGs 2-1, 3, 4, and 8, as well as W. circinata var. circinata, and binucleate Rhizoctonia AG E evaluated at 13/8 and 15/15°C day/night temperatures typical of spring planting conditions in the Columbia Basin. Isolates of R. solani AG 5, as well as binucleate AG A and I were nonpathogenic. The most virulent isolates belonged to AG 8, although an AG 3 and and an AG E isolate were also highly virulent. Isolates of AG 2-1 and 3 caused moderate levels of disease, while isolates of AG 4 and W. circinata var. circinata caused low levels. Emergence was reduced by isolates of AG 2-1, 3, and E. When the various AGs were grown at temperatures ranging from 5 to 30°C, the relative growth rate of the Rhizoctonia isolates was not significantly positively correlated with virulence on onion within an AG.