Yield responses of three onion cultivars to stunting caused by Rhizoctonia spp. in the Columbia Basin of Oregon and Washington, 2012.

Authors: SHARMA-POUDYAL, DIPAK - Washington State University; Paulitz, Timothy; Porter, Lyndon; EGGERS, JORDAN - Oregon State University; HAMM, PHILLIP - Oregon State University; DU TOIT, LINDSEY - Washington State University

Submitted to: Plant Disease Management Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2013
Publication Date: 3/11/2013
Citation: Sharma-Poudyal, D., Paulitz, T.C., Porter, L., Eggers, J., Hamm, P.B., Du Toit, L. 2013. Yield responses of three onion cultivars to stunting caused by Rhizoctonia spp. in the Columbia Basin of Oregon and Washington, 2012 Plant Disease Management Reports. No. 7:V048.

Interpretive Summary: Rhizoctonia spp. cause patches of stunted onion plants in onion bulbs crop in the Columbia Basin of Washington and Oregon when onion crops are planted in sandy soils of this semi-arid region following winter cereal cover crops. A herbicide application is used to kill the cereal cover crop, usually just prior to planting onion seed, to avoid the cover crop competing with the onion crop. Plants in the stunted patches do not resume the normal growth and development compared to adjacent healthy plants, resulting in reduced bulb size in the patches. Field surveys were carried out to assess the effects of onion stunting caused by Rhizoctonia spp. on the yield of each of three onion cultivars grown in the semi-arid Columbia Basin of Oregon and Washington in grower-cooperator, center-pivot irrigated fields. The cultivars evaluated were Mercury, a red cv, Tamara, a yellow cv. and Cometa, a white cv. Onion stunting caused by Rhizoctonia spp. reduced bulb yields significantly for all three cultivars, and the greater the severity of stunting, the greater the decrease in total and marketable bulb weight as a result of smaller bulbs.

Technical Abstract: Rhizoctonia spp. cause patches of stunted onion plants in onion bulbs crop in the Columbia Basin of Washington and Oregon when onion crops are planted in sandy soils of this semi-arid region following winter cereal cover crops. A herbicide application is used to kill the cereal cover crop, usually just prior to planting onion seed, to avoid the cover crop competing with the onion crop. Plants in the stunted patches do not resume the normal growth and development compared to adjacent healthy plants, resulting in reduced bulb size in the patches. Field surveys were carried out to assess the effects of onion stunting caused by Rhizoctonia spp. on the yield of each of three onion cultivars grown in the semi-arid Columbia Basin of Oregon and Washington in grower-cooperator, center-pivot irrigated fields. Onion cultivars evaluated were: Mercury, a red cv. seeded in four rows per 44 in. wide bed on 13 Mar in a field near Paterson, WA; Tamara, a yellow cv. planted in two double-rows per 40 in. wide bed on 16 Mar in a field near Boardman, OR; and Cometa, a white cv. planted in two double-rows per 34 in. wide bed on 8 Apr in a field near Pasco, WA. Each field was maintained by a different grower-cooperator. Patches of onion plants with different severity of stunting were selected in each field at the five to seven true-leaf growth stages. The severity of stunting was rated using a 1 to 3 scale where: 1 = a majority of the plants in the patch were stunted < 33% relative to adjacent, healthy plants outside the patch; 2 = a majority of plants were stunted 33 to 66%; and 3 = a majority of plants were stunted > 66%. Five patches were selected per field for each of the three severity ratings. Each patch was > 5 ft. long and at least two beds wide. Manual harvest of each cultivar was done at maturity: 1 Aug for Mercury, 27 Aug for Tamara, and 21 Aug for Cometa, by harvesting bulbs from 5 ft. of each of two double rows per patch. Bulbs also were harvested from an equivalent area of healthy plants adjacent to each patch. Bulbs from each patch and adjacent healthy area were bagged separately and graded by size: colossal (> 4.00 in. diameter), jumbo (3.00-4.00 in.), medium (2.25-3.00 in.), prepack (< 2.25 in.), and culled (non-marketable). The total number of bulbs in each size category was counted, and the total bulb weight per size category measured. All sizes, except culled bulbs, were considered marketable. The number and weight of bulbs in each size category was calculated as a percentage of the bulbs harvested in a plot. The reduction in number and weight of bulbs in each size category was then computed as a percentage of the number and weight of bulbs of each size harvested from healthy plants adjacent to the patch. The relationship between severity of onion stunting and percentage reduction in yield for each cultivar was calculated using correlation and regression analyses in SigmaPlot (Version 10; Systat Software Inc., San Jose, CA). The number of bulbs harvested within stunted patches did not differ significantly from the number of bulbs harvested from adjacent, healthy plants in each of the three fields evaluated (data not shown), i.e., stunting caused by Rhizoctonia did not affect plant stands. However, bulb size was affected by onion stunting, with a greater impact on bulb size the more severely the plants were stunted (Table 1). Colossal bulbs were present only in healthy areas sampled from the fields planted with Mercury or Tamara, but not in healthy areas in the Cometa field or in any of the patches for any of the cultivars (Table 1). In the healthy areas sampled from the Mercury and Tamara fields, a majority of the bulbs were jumbo size, followed by medium size bulbs, with few or no prepack and culled bulbs. In healthy areas of the Cometa field, a majority of bulbs were medium and jumbo. As the stunting severity increased, the num