Malheur Experiment Station Information for Sustainable Agriculture

PERFORMANCE OF ONION VARIETIES IN A FIELD WITH HIGH

IRIS YELLOW SPOT VIRUS PRESENCE

Clinton C. Shock, Erik B. G. Feibert, and Lamont D. Saunders

Malheur Experiment Station

 Oregon State University

Ontario, OR, 2009

Introduction

Onion plants infected with iris yellow spot virus (IYSV) can progressively lose leaf area, resulting in reduced yield and reduced bulb size. The IYSV is transmitted by onion thrips (Thrips tabaci).  The incidence of IYSV can be increased by inadequate control of onion thrips.  Thrips control has been complicated by their increasing resistance to pyrethroid and organophosphate insecticides. 

A certain degree of varietal tolerance to thrips and IYSV is suggested by the results of the yearly onion variety trials conducted at the Malheur Experiment Station (Shock et al. 2007).  The severity of IYSV infestation in these trials is often low or modest and varies from year to year.  A severe IYSV infestation has occurred consistently each year in onion fields in an area approximately 10 miles to the east of the experiment station.  A subset of the onion varieties in the station trial was evaluated in a commercial onion field in the area of high IYSV severity in 2009.

Materials and Methods

Onion seed of 12 varieties was planted in double rows spaced 3 inches apart at 9 seeds/ft of single row on April 8.  Each double row was planted on beds spaced 22 inches apart. Planting was done with customized John Deere Flexi Planter units equipped with disc openers.  On June 2, the seedlings were hand thinned, as closely as possible, to a plant population of 2 plants/ft of single row (6-inch spacing between individual onion plants equivalent to 95,000 plants/ac).   

On July 31 and August 24, onions in each plot were evaluated subjectively for severity of symptoms of iris yellow spot virus (IYSV).  Twenty consecutive plants in a row in each plot were rated on a scale of 0 to 5, where 0 = no symptoms, 1 = 1 to 25 percent of foliage diseased, 2 = 26 to 50 percent of foliage diseased, 3 = 51 to 75 percent of foliage diseased, 4 = 76 to 99 percent of foliage diseased, and 5 = 100 percent of foliage diseased.  Onions from 23 ft of the middle 2 rows in each plot were topped by hand and bagged on August 24.

The onions were graded on September 10.  During grading, bulbs were separated according to quality: bulbs without blemishes (No. 1s), split bulbs (No. 2s), neck rot (bulbs infected with the fungus Botrytis allii in the neck or side), plate rot (bulbs infected with the fungus Fusarium oxysporum), and black mold (bulbs infected with the fungus Aspergillus niger).  The No. 1 bulbs were graded according to diameter: small (<2.25 inches), medium (2.25-3 inches), jumbo (3-4 inches), colossal (4-4.25 inches), and supercolossal (>4.25 inches).  Bulb counts per 50 lb of supercolossal onions were determined for each plot of every variety by weighing and counting all supercolossal bulbs during grading.

After grading, bulbs from each plot were rated for single centers.  Twenty-five consecutive onions ranging in diameter from 3.5 to 4.25 inches were rated.  The onions were cut equatorially through the bulb middle and, if multiple centered, the long axis of the inside diameter of the first single ring was measured.  These multiple-centered onions were ranked according to the inside diameter of the first single ring: small had diameters less than 1.5 inches, medium had diameters from 1.5 to 2.25 inches, and large had diameters greater than 2.25 inches.  Onions were considered "functionally single centered" for processing if they were single centered or had a small multiple center.

Results and Discussion

Bulb yields were low over all varieties, probably due at least in part to late planting and heavy virus presence.  Varieties ‘Charismatic’, ‘Swale’, and ‘Affirmed’ were among those with the highest total and marketable yield (Table 1).  ‘Arcero’, ‘Joaquin’, ‘OLYX06-25’, and ‘Granero’ were among the varieties with the highest percentage of single-centered bulbs (Table 2).

‘The Rock’, Charismatic, and Joaquin were among those with the lowest severity of symptoms of IYSV (Table 2). 

References

Shock, C.C., E.B.G. Feibert, L.D. Saunders, L. Jensen, and K. Mohan. 2007. 2006 Onion Variety Trials. Oregon State University Agricultural Experiment Station Special Report 1075:33-42.

Acknowledgements

This project was partially funded by the Western Sustainable Agricultural Research and Education Program, the Idaho-Eastern Oregon Onion Committee, cooperating onion seed companies, Oregon State University, University of Idaho, and Washington State University.

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Last updated  Tuesday June 28, 2011.